User's Guide

GEMAdvanced Compositional and GHG Reservoir SimulatorVersion 2009

By Computer Modelling Group Ltd.

This publication and the application described in it are furnished under license exclusively to the licensee, for internal use only, and are subject to a confidentiality agreement. They may be used only in accordance with the terms and conditions of that agreement. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic, mechanical, or otherwise, including photocopying, recording, or by any information storage/retrieval system, to any party other than the licensee, without the written permission of Computer Modelling Group. The information in this publication is believed to be accurate in all respects. However, Computer Modelling Group makes no warranty as to accuracy or suitability, and does not assume responsibility for any consequences resulting from the use thereof. The information contained herein is subject to change without notice.

Copyright 1987-2009 Computer Modelling Group Ltd. All rights reserved.

The license management portion of this program is based on: Reprise License Manager (RLM) Copyright (C) 2006-2009, Reprise Software, Inc GEM uses Intel(R) Compilers. GEM, CMG, and Computer Modelling Group are registered trademarks of Computer Modelling Group Ltd. All other trademarks are the property of their respective owners. Computer Modelling Group Ltd. Office #150, 3553 - 31 Street N.W. Calgary, Alberta Canada T2L 2K7

Tel: (403) 531-1300

Fax: (403) 289-8502

E-mail: cmgl@cmgl.ca

PrefaceGEM is CMG's advanced general equation-of-state compositional simulator which includes options such as equation-of-state, dual porosity, CO2, miscible gases, volatile oil, gas condensate, horizontal wells, well management, complex phase behavior and many more. GEM was developed to simulate compositional effects of reservoir fluid during primary and enhanced oil recovery processes. This User's Guide presents the important mechanisms which occur during gas injection processes and the equations that describe those mechanisms. The techniques used in GEM to solve these equations are also described. This User's Guide is aimed at reservoir engineers who want to use GEM to design gas injection processes for enhanced oil recovery. It requires some basic knowledge of reservoir engineering and some rudimentary exposure to reservoir simulation. This User's Guide provides a step-by-step procedure for preparation of input data for this program. A tutorial section is provided as well as a set of appendices. Every attempt has been made in the preparation of this User's Guide to provide the user with all the necessary details. If questions arise, please contact: Computer Modelling Group Ltd.#150, 3553 31 Street N.W. Calgary, Canada T2L 2K7 Telephone: (403) 531-1300 Fax: (403) 289-8502 E-mail: cmgl@cmgl.ca

Confidentiality: All components of CMG technology including software and related documentation are protected by copyright, trademark and secrecy. CMG technology can be used only as permitted by your license from CMG. By the license, you have agreed to keep all CMG technology confidential and not disclose it to any third party. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic, mechanical, or otherwise, including photocopying, recording, or by any information storage/retrieval system, to any party other than the licensee, without the written permission of Computer Modelling Group. Corrections/Errors: CMG ENDEAVORS TO PRODUCE TECHNOLOGY OF THE HIGHESTQUALITY; NEVERTHELESS ERRORS OR DEFICIENCIES IN SUCH TECHNOLOGY ARE INEVITABLE. IF YOU FIND AN ERROR OR DEFICIENCY, YOU ARE REQUESTED TO PROVIDE DETAILS OF IT AND ILLUSTRATIVE DATA SET(S) TO CMG SUFFICIENT TO PERMIT CMG TO REPRODUCE THE ERROR OR DEFICIENCY. CMG SHALL ENDEAVOR TO REMEDY A DEFICIENCY IN A TIMELY MANNER AND SHALL PERIODICALLY REPORT TO YOU AS TO THE STEPS BEING TAKEN TO REMEDY THE DEFICIENCY. THE RESPONSE TIME FOR A DEFICIENCY MUST BE PRIORITIZED FOR THEIR GENERAL APPLICATION TO CMG MEMBERS AND WHETHER THEY FORM PART OF A CMG PROGRAM. CMG DOES NOT WARRANT THAT DEFICIENCIES WILL BE REMEDIED.

Limited Liability: CMG does not warrant the accuracy or usefulness of the technology and software - Refer to your license.

Contents1. Introduction 1Important Changes in GEM 2009.10 ............................................................................1 Important Changes in GEM 2008.10 ............................................................................5 Important Changes in GEM 2007.10 ..........................................................................10 Important Changes in GEM 2006.10 ..........................................................................14 Important Changes in GEM 2005.10 ..........................................................................15 Important Changes in GEM 2004.10 ..........................................................................17 Important Changes in GEM 2003.10 ..........................................................................18 Important Changes in GEM 2002.10 ..........................................................................19 Important Changes in GEM 2001.10 ..........................................................................20 Important Changes in GEM 2000.10 ..........................................................................22 Introduction to GEM...................................................................................................23

2. Tutorial

27

Introduction.................................................................................................................27 Data Groups in the Keyword Input System ................................................................28 How to Document Your Data Set ...............................................................................29 How to Do a Restart....................................................................................................30 Controlling Contents of the Output Print File.............................................................31 Controlling Contents of the Simulation Results File ..................................................32 Describing Your Grid System.....................................................................................33 Describing Refined Grid .............................................................................................35 Entering Null Blocks ..................................................................................................36 Using Dual Porosity/Dual Permeability......................................................................37 Using the Aquifer Option ...........................................................................................38 Problems with Small Timesteps or Long Execution Times........................................39 Simulating a Single Phase Reservoir ..........................................................................42 Horizontal Wells .........................................................................................................43 Vertical Equilibrium Calculation................................................................................44 Defining Wells............................................................................................................46 How to Shut In a Well and Reopen It .........................................................................47 Using the Wellbore Model..........................................................................................49 Operating and Monitoring Constraints .......................................................................50 Entering Well Indices .................................................................................................52 Stopping a Simulation Run .........................................................................................53User's Guide GEM Contents i

Guidelines for Setting Up Well Data.......................................................................... 54 Recurrent Data from Other Sections .......................................................................... 56 Parallel GEM.............................................................................................................. 57 Saline Water Injection ................................................................................................ 59 Using the Oil Wet Option........................................................................................... 60 Modelling Non Darcy Flow in Hydraulic Fractures Accurately Using a Grid Based Approach .................................................................................................... 66

3. Keyword Data Entry System

73

Introduction to Keyword System................................................................................ 73 Comments (Optional) ................................................................................................. 78 Blank Lines (Optional)............................................................................................... 79 Scan Mode for Checking Errors ................................................................................. 80 Include Files (Optional).............................................................................................. 81 Controlling Data File Listing (Optional).................................................................... 82 Changing the Comment Indicator (Optional)............................................................. 83 Changing the Keywords by Using Translate Rules (Optional) .................................. 84 User Block Address.................................................................................................... 86 Input of Grid Property Arrays .................................................................................... 87 Entering Matrix Grid Properties................................................................................. 89 Entering Fracture Grid Properties .............................................................................. 90 Entering Refined Grid Properties ............................................................................... 91 J and K Direction Data from I Direction .................................................................... 92 Constant Value Arrays ............................................................................................... 93 Array Input in IJK Notation ....................................................................................... 94 Array Input of Values that Vary in the I Direction..................................................... 96 Array Input of Values that Vary in the J Direction .................................................... 97 Array Input of Values that Vary in the K Direction ................................................... 98 Values that Vary for Most or All Grid Blocks ........................................................... 99 Values Stored in Binary Form.................................................................................. 100 Modifying Array Data (Conditional)........................................................................ 102 Interpolating Table Data (Optional) ......................................................................... 105

4. Input/Output Control

107

Notes on Input/Output Control................................................................................. 107 Command-Line Arguments (Optional) .................................................................... 108 Input/Output File Names (Optional) ........................................................................ 111 Run-Time Dimensioning (Optional) ........................................................................ 116 Interrupt and CTRL-C Control (Optional) ............................................................... 119 Project Main Title (Optional) ................................................................................... 121 Project Second Title (Optional)................................................................................ 122 Project Third Title (Optional)................................................................................... 123 Case Identification (Optional) .................................................................................. 124ii Contents User's Guide GEM

Check Only (Optional)..............................................................................................125 Diary Format (Optional) ...........................................................................................126 Input Data Units (Optional) ......................................................................................128 Data Range Checking (Optional)..............................................................................130 Maximum Number of Error Messages (Optional)....................................................131 Restart Record Writing Frequency (Optional)..........................................................132 Restart Time Step (Optional) ....................................................................................134 Output Printing Frequency (Optional) ......................................................................135 Items in the Output Print File (Optional) ..................................................................137 Selection of Results File Format (Optional) .............................................................142 Selection of Additional Plain Text SR2 Main File (Optional)..................................144 Simulation Results Writing Frequency (Optional) ...................................................145 Items in Simulation Results File (Optional) .............................................................148 CO2 Inventory Output (Optional) .............................................................................158 Well, Aquifer and Sector Summary Tables (Optional).............................................159 Debug Output (Optional) ..........................................................................................160

5. Reservoir Description

163

Notes on Reservoir Description ................................................................................163 Fundamental Grid Definition (Required)..................................................................164 K Direction Index (Optional)....................................................................................171 Convert Cartesian Grid to Corner Point (Optional) ..................................................173 Block Dimensions for the I Direction (Conditional) ................................................174 Block Dimensions for the J Direction (Conditional) ................................................176 Block Dimensions for the K Direction (Conditional)...............................................177 Depth (Conditional) ..................................................................................................178 Depth to the Tops of Grid Blocks (Conditional).......................................................180 Depths to Centre of Pay (Conditional)......................................................................182 Depths to Top of Block (Conditional) ......................................................................184 Grid Tilt Angles (Conditional)..................................................................................186 Corner Point Depths for Corner Point Grids (Conditional) ......................................188 Lateral Corner Point Locations for Corner Point Grids (Conditional) .....................190 Line-Based Corner Point Locations for Corner Point Grids (Conditional) ..............192 Complete Corner Point Locations for Corner Point Grids (Conditional) .................194 Local Refined Grid (Optional)..................................................................................197 Refined Grid Location (Conditional)........................................................................208 Dual Porosity (Optional)...........................................................................................210 Dual Permeability (Optional)....................................................................................211 Dual Porosity Subdomain Method (Optional) ..........................................................212 Assigning Fractional Volumes to Subdomain blocks (Optional) .............................214 Subdomain to Subdomain Flow Transmissibility Multipliers (Optional).................216 Dual Porosity MINC Method (Optional) ..................................................................218 Shape Factor Calculation (Conditional)....................................................................219Contents iii

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Matrix-Fracture Transfer Calculation (Conditional) ................................................ 221 Fracture Spacing (Conditional) ................................................................................ 222 Null Block Indicator (Optional) ............................................................................... 224 Porosity (Required) .................................................................................................. 226 Rock Compressibility ............................................................................................... 228 Pore Volume Modifiers (Optional) .......................................................................... 230 Permeabilities (Required)......................................................................................... 232 Netpay (Optional)..................................................................................................... 234 Net to Gross Multipliers (Optional) ......................................................................... 236 Transmissibility Multipliers (Optional).................................................................... 237 Transmissibility Multipliers for Lower Indexed Cell Blocks (Optional) ................. 240 Transmissibility Multiplier for Matrix-Fracture Flow (Optional) ............................ 242 Faults (Optional)....................................................................................................... 243 Aquifers (Optional) .................................................................................................. 245 Aquifer Pressure Influence (Conditional) ................................................................ 251 Pore Volume Cut-Off Threshold (Optional) ............................................................ 253 Pinch Out Array (Optional)...................................................................................... 254 Pinchout Tolerance (Optional) ................................................................................. 256 Corner Point Tolerance (Optional)........................................................................... 258 Sectors (Optional)..................................................................................................... 259 Sector Array (Optional)............................................................................................ 261 Sector Assignment via Names and an Array (Optional) .......................................... 262 Special Connections (Optional)................................................................................ 264 Fault Array (Optional).............................................................................................. 266 Compaction/Dilation Rock Type (Optional) ............................................................ 268 Compaction/Dilation Rock Compressibility (Optional)........................................... 271 Compaction Rock Table (Optional) ......................................................................... 272 Compaction Hysteresis Rock Table (Optional)........................................................ 274 Compaction Irreversibility Flag (Optional).............................................................. 277 Dilation Rock Table in Elastic Zone (Optional)....................................................... 278 Dilation Rock Table in the Plastic Zone (Optional) ................................................. 280 Dilation Rock Table in Unloading Zone (Optional)................................................. 282 Dilation Rock Table in Recompacting Zone (Optional) .......................................... 284 Dilation Rock Table in Reloading Zone (Optional) ................................................. 286 Compaction/Dilation Rock Region (Optional)......................................................... 294 Palmer and Mansoori Parameters (Optional) ........................................................... 300 Fault Transmissibilities (Optional)........................................................................... 306 Cap Rock Leakage Using Wells (Optional) ............................................................. 308

5. Other Reservoir Properties

313

Summary of Other Reservoir Properties .................................................................. 313 Indicate End of Grid Definition (Required) ............................................................. 314 Thermal Expansion Coefficient (Optional) .............................................................. 315iv Contents

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6. Component Properties

317

Notes on Component Properties ...............................................................................317 Fluid Model (Required) ............................................................................................320 Number of Components (Required)..........................................................................321 Component Name (Required) ...................................................................................322 Equation of State Set (Optional) ...............................................................................324 Equation of State Model Type (Optional) ................................................................327 User Component Properties (Conditional)................................................................328 Equation of State Omega Parameters (Optional)......................................................331 User Component Interaction Coefficients (Conditional) ..........................................332 Parameters for Hydrocarbon Interaction Coefficients (Optional).............................333 Critical Property Correlations for User Components (Conditional) .........................334 Volume Shift Parameters (Optional) ........................................................................335 Heating Values (Optional) ........................................................................................337 Equation of State Parameters at Surface Conditions (Optional)...............................340 Reservoir Temperature (Required) ...........................................................................343 Identity of Single Phase Fluid (Optional) .................................................................345 Average Saturation Pressure (Optional) ...................................................................348 Viscosity Correlation Specification (Optional).........................................................349 Parameters for Computing Viscosity of Hydrocarbon Fluids (Optional) .................353 Aqueous Phase Properties (Optional) .......................................................................357 Solubility Data (Optional) ........................................................................................361 Gas-Phase Diffusion in Fractured Reservoirs (Optional) .........................................366 Flash Method Technique (Optional).........................................................................368 Method for Evaluating Entries of the Jacobian Matrix (Optional) ...........................371 Molecular Diffusion (Optional) ................................................................................373 Non-Darcy Gas Flow Effects (Optional) ..................................................................377 Asphaltene Precipitation Model (Optional) ..............................................................379 Solid Adsorption (Optional) .....................................................................................381 Irreversible Asphaltene Model (Optional) ................................................................383 Solid Deposition Model (Optional) ..........................................................................385 Coal Matrix-to-Cleat Modelling Parameters(Optional)............................................388 Gas Adsorption for Water Filled Blocks (Optional).................................................391 Selection of Model Accounting for Coal Fracture Porosity and Absolute Permeability Changes (Optional).........................................................................393 Trace Component (Optional) ....................................................................................394 EOS Cubic Root Selection (Optional) ......................................................................395 H2O Vaporization (Optional) ...................................................................................396 GHG Option - Aqueous Components .......................................................................397 GHG Option - Mineral Components.........................................................................398 GHG Option Reactions ..........................................................................................399 GHG Option Reaction Equilibrium and Rate Parameters .....................................401 GHG Option - Equilibrium-Rate-Annihilation (ERA) Matrix .................................404Contents v

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GHG Option - Aqueous Phase Model (Optional) .................................................... 405 GHG Option Factors for CO2 Inventory (Optional) ............................................. 406 GHG Option - Permeability Change Versus Porosity (Optional) ............................ 407

7. Rock-Fluid Data

409

Notes on Rock-Fluid Data........................................................................................ 409 Start of Rock-Fluid Property Input (Required) ........................................................ 413 Tabular Relative Permeabilities ............................................................................... 414 Water-Oil Relative Permeability Table (Conditional).............................................. 416 Liquid-Gas Relative Permeability Table (Conditional)............................................ 419 Rock Compressibility for Each Rock Type (Optional) ............................................ 423 Hysteresis Parameters (Optional) ............................................................................. 424 WAG Relative Permeability Hysteresis Modelling (Optional)................................ 428 Rock Type (Optional)............................................................................................... 434 Trapped Oil Saturation (Optional)............................................................................ 435 Interfacial Tension Effects on Relative Permeability (Optional) ............................. 436 Velocity Dependent Relative Permeability Option (Optional)................................. 438 Single Phase Gas Relative Permeability Flag (Optional)......................................... 443 Method for Evaluating 3-Phase Kro (Optional) ....................................................... 444 Saturation Endpoints for each Grid Block (Optional) .............................................. 447 Leverett J Function (Optional) ................................................................................. 452 Maximum Relative Permeability/Capillary Pressure Values for Each Grid Block (Optional).................................................................................................. 455 Water-Oil Capillary Pressure (J Function) Shift for Each Grid Block (Optional) ...... 457 Multicomponent Adsorption from the Gas Phase (Optional)................................... 458 Multicomponent Adsorption from the Gas Phase Tabular Adsorption Data Entry (Optional) .................................................................................................. 460 Velocity-Dependent Dispersion (Optional).............................................................. 465 Coal Bed Methane Features (Optional).................................................................... 467 Subdomain Reinfiltration (Optional)........................................................................ 470 Single-Phase Relative Permeability Interpolation (Optional) .................................. 472 Non-Darcy Flow in Reservoir (Optional)................................................................. 474 Non-Darcy Coefficient Correction Factor (Conditional) ......................................... 478

8. Initial Conditions

479

Notes on Initial Conditions....................................................................................... 479 Initial Conditions Identifier (Required).................................................................... 485 User Input or Vertical Equilibrium Selection (Required) ........................................ 486 Number of Initialization Regions (Optional) ........................................................... 490 Specification of Initialization Regions (Optional) ................................................... 491 Oil Zone and Gas Cap Compositions (Conditional)................................................. 492 Initial Reservoir Fluid Properties (Conditional)....................................................... 495 Initial Reservoir Temperature (Optional)................................................................. 497vi Contents User's Guide GEM

Reference Depth and Reference Pressure (Conditional)...........................................498 Depth to Water-Oil, Gas-Oil, and Water-Gas Contacts (Conditional) .....................499 Water Saturation Below Water-Oil Contact (Optional)............................................502 Initial Reservoir Fluid Formation Volume Factors (Optional) .................................503 Separator for Initial Fluid-In-Place Calculations (Optional) ....................................504 Specification of Non-EOS Stream Density Calculations (Optional) ........................510 Standard Pressure, Temperature, and EOS Set (Optional) .......................................515 Table Defining a Gas Plant (Optional) .....................................................................517 Critical Depth (Conditional) .....................................................................................521 Variation of Composition with Depth (Conditional) ................................................522 Datum Depth Specification (Optional) .....................................................................524 Specification of Capillary Pressure Values at Contacts (Optional) ..........................526 Water Saturation Used with Block_Center Initialization (Conditional) ...................529 Specification of Method for Computing Oil Saturations in Gas Zone......................531 Critical Temperature Multiplier (Optional) ..............................................................533 Initial Aqueous/Mineral Concentrations .......................................................................535

9. Numerical Methods Control

537

Notes on Numerical Methods Control ......................................................................537 Numerical Methods Control Identifier (Optional) ....................................................539 Maximum Time Steps (Optional) .............................................................................540 Maximum CPU Seconds (Optional) .........................................................................541 Maximum and Minimum Time Step Size (Optional) ...............................................542 Normal Variation in Variables Per Time Step (Optional) ........................................543 Adaptive-Implicit Switching (Optional) ...................................................................546 Convergence of Newton's Method (Optional) ..........................................................549 Maximum Newton Cycles for Constraint Equation Residual Checking (Optional).....552 Maximum Newtonian Cycles (Optional)..................................................................553 Underrelaxation Option (Optional)...........................................................................554 Convergence Tolerance for Linear Solver (Optional) ..............................................555 Orthogonalization (Optional)....................................................................................556 Solver Equation Ordering (Optional)........................................................................557 Solver Factorization Degree (Optional)....................................................................558 Pivot Stabilization (Optional) ...................................................................................559 Modified ILU Factorization (Optional) ....................................................................560 Maximum Iterations (Optional) ................................................................................561 Numerical Dispersion Control (Optional) ................................................................562 AIMSOL/PARASOL (Optional) ..............................................................................563 Number of Parasol Classes (Optional)......................................................................564 Red-Black Ordering Check for Parasol (Optional)...................................................565 Factorization Degree Within Parasol Classes (Optional) .........................................566 Factorization Degree Between Parasol Classes (Optional).......................................567 Parasol Class Partitioning Pattern (Optional) ...........................................................568Contents vii

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Parallel Equation and Residual Building (Optional) ................................................ 573 Parallel Jacobian and Residual Building based on Parasol Classes (Optional) ....... 575 Number of Threads (Optional) ................................................................................. 576

10. Geomechanics

577

Summary of Geomechanical Model......................................................................... 577 Geomechanical Model Identifier (Optional) ............................................................ 584 3D Finite Element .................................................................................................... 585 Plane Strain Option .................................................................................................. 586 Deformation Rock Type........................................................................................... 587 Plastic Model Formation Properties................................................................................. 588 Yield Criterion.......................................................................................................... 590 Cap Model ................................................................................................................ 592 Cap Model 1 ............................................................................................................. 593 Nonlinear Constitutive Model .................................................................................. 597 Nonlinear Elastic Constitutive Model 1 ................................................................... 598 Nonlinear Elastic Constitutive Model 2 ................................................................... 600 Creep Model ............................................................................................................. 607 Creep Model 1, 2 ...................................................................................................... 609 Generalized Plasticity Model ................................................................................... 613 Single Surface Failure Model................................................................................... 617 Thermal Expansion Coefficient................................................................................ 623 Matrix Permeability Option...................................................................................... 624 Barton-Bandis Fracture Permeability ....................................................................... 627 Fracture Direction..................................................................................................... 631 Dilation Relative Permeabilities............................................................................... 632 Other Dilation Properties.......................................................................................... 634 Well Radius .............................................................................................................. 636 Stiffness Matrix Calculation Option......................................................................... 637 Deformation Solution Control.................................................................................. 638 Geomechanics AIMSOL Control ............................................................................. 641 Dimension Over-Rides (Optional) ........................................................................... 644 Initial Stress Distribution (2D) ................................................................................. 645 Initial Stress Distribution (3D) ................................................................................. 648 Geomechanical Reference Block ............................................................................. 653 Prescribed Boundary Conditions (2D) ..................................................................... 654 Prescribed Boundary Conditions (3D) ..................................................................... 660 Point Loads (2D) ...................................................................................................... 664 Point Loads (3D) ...................................................................................................... 668 Distributed Edge Loads (2D) ................................................................................... 670 Distributed Surface Loads (3D) ............................................................................... 674 Gravity Loads (2D) .................................................................................................. 680 Gravity Loads (3D) .................................................................................................. 682viii Contents

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Fixed Null Block.......................................................................................................684 Fixed Cap Rock ........................................................................................................687 Geomechanics Domain .............................................................................................688 Pressure Boundary Domain ............................................................................... 690 Coupling Options......................................................................................................693 Geomechanical Coupling Factor...............................................................................696 Pressure Tolerance Multiplier...................................................................................697 Coupling Update Times ............................................................................................698 Porosity Calibration ..................................................................................................700 Iterative Coupling to Fluid Flow...............................................................................702 Boundary Stress Unloading ......................................................................................704

11. Well and Recurrent Data

705

Notes on Well and Recurrent Data for the 2009.10 Release.....................................705 New Keywords and Options for the 2008.10 Release ..............................................706 Notes on Well and Recurrent Data for the 2007.10 Release.....................................707 Notes on Well and Recurrent Data for the 2006.10 Release.....................................710 Notes on Well and Recurrent Data for the 2005.10 Release.....................................711 Notes on Well and Recurrent Data for the 2004.10 Release.....................................712 Notes on Well and Recurrent Data for the 2003.10 Release.....................................713 Notes on Well and Recurrent Data for the 2002.10 Release.....................................714 Notes on Well and Recurrent Data for the 2001.10 Release.....................................715 Notes on Well and Recurrent Data for the 2000.10 Release.....................................716 Placement of Keywords in the Well and Recurrent Data .........................................717 Note on Recycling and Well Stream Compositional Monitoring .............................719 Miscellaneous Notes .................................................................................................720 Well Management and Group Control......................................................................722 Specifying the Well and Group Control Hierarchy ..................................................724 Notes on Group Well Control ...................................................................................725 Limitations of the Well Management and Group Control Module...........................729 Well and Recurrent Data Identifier (Required).........................................................731 Well Change Date (Conditional) ..............................................................................732 Well Change Time (Conditional)..............................................................................733 Well Change First Time Step Size (Optional) ..........................................................734 Maximum and Minimum Time Step Size (Optional) ...............................................735 Setting Grid Blocks to Implicit or Explicit (Optional) .............................................736 Setting Well Blocks and Neighbours to Implicit (Optional).....................................738 Set Frequency of Initialization of Bottom-Hole Pressure (Optional) .......................740 Group Identification (Optional) ................................................................................743 Define Reporting Group (Optional)..........................................................................747 Well Identification (Required)..................................................................................749 Well Type Definition (Required)..............................................................................751 Shut in Wells above Formation (Optional) ...............................................................753Contents ix

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Well Status Definition (Optional) ............................................................................ 755 Shut and Reopen a List of Wells (Optional) ............................................................ 758 Tubing Data for Injectors (Conditional)................................................................... 760 Tubing Data for Producers (Conditional)................................................................. 763 Set Application Mode of Correlation for Well Head Pressure (Optional) ............... 768 Set Number of Points for WHP Root Search (Optional).......................................... 770 Composition of Injected Fluid (Conditional) ........................................................... 772 Well Operating Constraints (Required).................................................................... 774 Monitored Well Constraints (Optional).................................................................... 784 Alter Primary Well Operating Constraint Value (Optional) .................................... 791 Alter Well Constraint Value (Optional) ................................................................... 793 Well Head Method (Optional).................................................................................. 797 Well Head Iteration (Optional)................................................................................. 799 Simultaneous BHP and Well Head Iteration (Optional) .......................................... 801 Perforations in Inactive Blocks (Optional)............................................................... 803 Well Backflow Model (Optional)............................................................................. 805 Data for Workover Action for Wells (Optional) ...................................................... 807 Resetting Well Operating Constraint after Value Change (Optional) ...................... 810 Gas Lift Option (Optional) ....................................................................................... 812 Gas Lift Control (Optional)...................................................................................... 815 Gas Lift Optimization (Optional) ............................................................................. 817 Well Separators (Conditional).................................................................................. 820 Specification of Non-EOS Stream Density Calculations (Optional)........................ 826 Table Defining Gas Plant (Optional)........................................................................ 831 Standard Pressure, Temperature, and EOS Set (Optional)....................................... 835 Well Element Geometry (Conditional) .................................................................... 837 Location of Well Completions (Conditional)........................................................... 840 Location of Vertical Well Completions (Conditional) ............................................. 849 Location of Vertical Well Completions for Hybrid Refined Grids (Conditional)........ 855 Geometric Data for Deviated Well Completions (Conditional)............................... 862 Simplified Geometric Data for Deviated Well Completions (Conditional) ............. 866 Special Relative Permeability Data for Well Completions (Conditional)................ 869 Pressure Gradients for Calculation of Pressure Differences Between Completions (Conditional) ........................................................................................................ 874 User-Specified Reference Depth for Well BHP (Optional) ..................................... 877 User-Specified Pressure Gradient For Reference Depth for Well BHP (Optional)...... 879 Gas Recycling Option (Optional)............................................................................. 881 Group Production Constraints (Optional) ................................................................ 883 Group Injection Constraints (Optional).................................................................... 888 Fuel Consumption Fraction for Group Gas Recycling (Optional) ........................... 898 Fuel Component Mask for Group Gas Recycling (Optional)................................... 900 Maximum Fuel Consumption Rate for Group Gas Recycling (Optional)................ 902 Make-up Gas Composition for Group Gas Recycling (Optional)............................ 904 Maximum Make-up Gas Rate for Group Gas Recycling (Optional)........................ 906x Contents User's Guide GEM

Gas Make-up Target for Group Gas Recycling (Optional).......................................908 Gas Producing Group for Group Recycling (Optional) ............................................910 Recycled Gas Component Mask for Group Gas Recycling (Optional) ....................912 Maximum Re-injection Rate for Group Gas Recycling (Optional) ..........................914 Sales Rate Fraction for Group Gas Recycling (Optional).........................................916 Sales Component Mask for Group Gas Recycling (Optional)..................................918 Maximum Sales Rate for Group Gas Recycling (Optional) .....................................920 Maximum Water Make-up Rate for Group Water Recycling (Optional) .................922 Water Make-up Target for Group Water Recycling (Optional)................................924 Water Producing Group for Group Recycling (Optional) ........................................926 Maximum Re-injection Rate for Group Water Recycling (Optional) ......................928 Monitored Group Constraints (Optional)..................................................................930 Defining Group Production or Injection as Going Through a Manifold (Optional) .....935 Pressure-Constraint Translation for Manifolds (Optional) .......................................937 Specification of Hydraulics Tables for Calculating Pressure Difference Between Manifold and Surface (Optional) ...........................................................................939 Manifold Depth for Calculating Pressure Difference Between Manifold and Surface (Optional) ................................................................................................941 Group Artificial Lift Quantity Value (Optional).......................................................943 Well Artificial Lift Quantity Value (Optional) .........................................................945 Priority List for Automatic Drilling of Wells (Optional)..........................................947 Group Apportionment Options (Optional)................................................................949 Apportionment Method for Meeting Group Targets (Optional) ...............................951 Priority Formulae for Apportionment (Conditional) ................................................955 Guide Rates for Groups or Wells (Optional) ............................................................959 Flag for Specifying Groups or Wells Not Under Group Control (Optional) ............962 Well/Group On-time Fraction (Optional) .................................................................964 Sets/Alters a Wells Phase Productivity (Optional)..................................................968 Hydraulic Pressure Table (Conditional) ...................................................................973 Allow a Set of Keywords to be Processed When a Specified Condition (Trigger) is Satisfied (Optional) ...........................................................................................981 Alter Well Constraint Value via a Multiplier (Optional) ..........................................999 Group Production Constraints Multiplier (Optional)..............................................1003 Group Injection Constraints Multipliers (Optional)................................................1006 Allow a Cycling Group to be Defined (Optional)...................................................1010 Allow Cycling Group Data to be Revised (Optional).............................................1021 Allow Cycling Group Data to be Revised (Optional).............................................1023 Select Cycling Group Cycle Part to Start and End Cycling Group Control (Optional)...........................................................................................................1025 Declaring Cap Rock Leakage Wells (Optional) .....................................................1027 Salinity of Injected Water (Optional) .....................................................................1029 Dynamic Grid Amalgamation Control (Optional) ..................................................1031 Terminate Simulation (Required) ...........................................................................1037

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Contents xi

Appendix A

1039

Theoretical Outline................................................................................................. 1039 Well Model............................................................................................................. 1043 Figure A-1: Well Fraction and Geometrical Factor for Various Common Geometries......................................................................................................... 1045 Figure A-2: Multiblock Well Completion............................................................. 1046 Wellbore Model...................................................................................................... 1047 Aquifer Model ........................................................................................................ 1048 Table A-1: Built in Aquifer Influx Function ......................................................... 1050 Solubility Model..................................................................................................... 1051 Dual Porosity / Dual Permeability Model .............................................................. 1053

Appendix B

1057

Figure B-1: Numbering of the Grid System .......................................................... 1057 Figure B-2: Cylindrical Reservoir ......................................................................... 1058 Figure B-3: Calculation of Gravity Components for a Tilted Grid System .......... 1059 Figure B-4: Radial (Cylindrical) Coordinates ....................................................... 1060 Figure B-5a: 2-Dimensional Variable Thickness Grid System (8x1x3) ............... 1061 Figure B-5b: 3-Dimensional Variable Thickness Grid System (8x3x2) ............... 1062 Figure B-6: Discretized Reservoir with Faults ...................................................... 1063 Figure B-7: Relative Permeability Curves ............................................................ 1064 Figure B-8: Recycling Option in GEM ................................................................. 1065

Appendix C

1067

References .............................................................................................................. 1067

Appendix D

1071

Coupling GEM with Surface Network Models ...................................................... 1071 General Considerations and Guidelines .................................................... 1071 Preparation of the GEM Data Set.............................................................. 1073 Multiple Reservoirs ................................................................................... 1076 Coupling with GAP ................................................................................... 1077 Coupling with FORGAS ........................................................................... 1087 GEM Results Accessible from RESOLVE Script.................................................. 1093 Total Numbers........................................................................................... 1093 Well Variables........................................................................................... 1093 Group Variables ........................................................................................ 1094 Sector Variables ........................................................................................ 1095

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Appendix E

1097

Compositional Simulation for Sequestration of CO2 and Other Greenhouse Gases in Saline Aquifers....................................................................................1097 Thermodynamic Equilibrium..................................................................................1098 H2O Vaporization....................................................................................................1102 Gas and Aqueous Phase Properties.........................................................................1104 Reaction Stoichiometry...........................................................................................1105 Chemical Equilibrium.............................................................................................1106 Mineral Dissolution and Precipitation Reactions....................................................1107 Component Material Balance Equation ..................................................................1109 Elimination of the Chemical Equilibrium Reaction Rates......................................1110 Solution Method .....................................................................................................1112 References...............................................................................................................1114

Keyword Index

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User's Guide GEM

Contents xiii

Introduction

Important Changes in GEM 2009.10SIMULATOR CHANGES Geomechanics Coupling One- and two-way (explicit as well as iterative) coupling with CMGs Geomechanics module has been implemented into GEM and is being released as a Beta feature. The geomechanics module handles different constitutive models like, linear and non-linear elastic, generalized plasticity, elasto-plastic, single surface, and creep models. For the elastoplastic model the yield criteria can be specified via Mohr-Columb or Drucker-Prager that are suitable for the description of geologic material. There is also an isotropic strain hardening and softening option. The behavior of cyclic loading and unloading that occurs during cyclical injection and production can be modeled. A cap model is also available for yield criteria of elasto-plastic material. For Non-linear elasticity, hypoelastic and hyperelastic constitutive models are available. Associated or non-associated flow rules can be used for Mohr-Columb, Drucker-Prager, single surface, or generalized plasticity models. The above constitutive models can be used in 2D or 3D problems in the Cartesian, cornerpoint, or axissymmetric grid. Finite element method is used in the geomechanics module to compute the geomechanical responses. The user can specify a variety of boundary conditions in 2D and 3D on nodal points of the finite element. The geomechanics module solves for the force equilibrium of the formation and calculates volumetric dilation/compression as a result of both elastic and plastic straining. The coupling options allow porosity to be optionally dependent on deformation. It can be a function of pressure temperature or volumetric strain; or a function of pressure temperature and total mean stress. The change in matrix and fracture permeability due to geomechanical effects can be computed by a number of algorithms including the ones by Li and Chalaturnyk; and Barton-Bandis. A large number of possible geomechanics responses are available for viewing in CMGs graphical post processor Results, and also available to the text .out and .geo files. The user can view grids in Results that deform with time, but there are some restrictions for displaying 2D radial grids. A new data section GEOMECHANICS has been added which precedes WELLS AND RECURRENT DATA section in the GEM input data file to facilitate entry of geomechanics related keywords. Please see the chapter on geomechanical features for specific keywords.

User's Guide GEM

Introduction 1

The coupled geomechanic modeling may be useful for a variety of situations, like determination of leakage through points of weakness in the cap rock during GHG sequestration, effect of rock deformation and porosity change during CBM/ECBM simulation, solid deposition in asphaltene modeling, and mineral dissolution and precipitation during geochemical processes, and others. Improved Parallel Performance A new method has been implemented for parallel equation and residual building using a method of domain decomposition based on Parasol classes. This method is expected to give better scalability on computers with larger number of processors. The method is activated by using *JACPAR *ON in the Numerical Methods section, or by specifying jacpar on the command line. In order to use *JACPAR, the parallel computing licensing feature must be active. Oil-Wet Rock-Fluid Model The oil-wet rock fluid model modifies both the initialization and flow calculations in GEM. See the OILWET subkeyword of the RPT keyword for details. The oil wet option initialization assumes that the reservoir was initially water wet and has undergone a wettability alteration to become an oil wet reservoir. This assumption has a significant impact on how fluids are placed during reservoir initialization. In the oil wet model, three phase flow is calculated assuming water is the intermediate phase. Therefore Kro is a function of oil saturation, Krg is a function of gas saturation, while Krw is a function of both oil and gas saturation (through Krwo and Krwg). The normal three phase flow models (e.g. Stone 1, Stone 2, segregated, etc) are used to determine water relative permeability, not oil permeability. Non-Darcy Flow Model Enhancements Direct input of the parameters used in the calculation of the non-Darcy Beta factor can now be specified with the NONDARCY GENERAL keyword. A new array keyword NDARCYCOR has been implemented to allow input of a multiplier for the Beta parameter used in non-darcy flow calculations. These features are useful for explicit modelling of nonDarcy flow in fractures. In previous releases, the non-Darcy model specification was done in the Fluid model section. Beginning with this release, the NONDARCY keyword will be read in the Rock-Fluid section along with the NDARCYCOR keyword. The old format for the NONDARCY keyword in the Fluid model section is obsolescent but will continue to be read for backward compatibility. Constraint Equation Residual Convergence The constraint equation residual checking has been implemented to reduce the sum of square residuals of constraint equations based on user specified tolerance and user specified maximum number of Newton iteration for which the checking is required. It can be invoked via *CONVERGE sub-keywords, *CEQVLE, *CEQAQU, *CEQADS, , etc. The number of iterations for checking can be specified via, *NCHEK-CEQ. The option is especially useful for GHG sequestration, CBM modeling, etc, where use of keywords, *ITERMIN or *DTMAX could be minimized.

2 Introduction

User's Guide GEM

Combined Stability and Threshold Criterion of Adaptive Implicit Switching A combined stability (CFL) and threshold criterion has been implemented for adaptive implicit switching. While switching based on CFL is applied at the timestep level, the switching with saturation/composition threshold can be done at timestep on Newton iteration level. The option is available via *AIM *STAB *AND-THRESH in the NUMERICAL METHODS section, and is being released as a Beta feature. Diary Output Redesign The Gem log file has been redesigned to provide more useful and flexible information to the user: The average material balance error, and cumulative solver failures are printed at the end of each time step for *CHANGES format. The width of user block address is dynamically expanded for large size models to accommodate i,j,k triplets for large models for the *CHANGES format. Convergence status flags can be optionally printed with changes format to detect convergence problems. Material balance error and status of wells is printed at the end of each time step for *WELL-INFO format. The header is repeated after a page-full of information for easy identification of the printed information on the log file.

*PTUBE1/*ITUBE1 Table Generation in Builder The new PTUBE1/ITUBE1 well head pressure tables are supported by the well head pressure calculator program. This now includes generation of injection tables as well as production tables which incorporate gas lift. GHG and Aqueous Phase Modelling Improvements A number of improvements to the models for gas solubility and aqueous phase property modelling have been implemented. These included fixes to the *INCOMP *AQUEOUS feature, improvements to water vaporization modelling, allowing use of SORM with the OGW flash, improved reporting of aqueous and mineral component, improved salting-out calculations, and other bug fixes. WELL MANAGEMENT CHANGES *ITUBE1/*PTUBE1 PTUBE has been replaced by the more flexible PTUBE1, in addition ITUBE1 allows for the definition of injection tables using CMG standard well head pressure input table format. *PTUBE tables are still supported. *GCONI Stream *PMAINT *PMMAXR d1 d2 Introduces the maximum surface rate of the specified injection stream that is available for the pressure maintenance. It is the summation of two sources: a fraction (d1) of the total produced (from the same group) and a make-up rate (d2). The values must be non-negative real numbers. The fraction d1 is nondimensional and defaulted to 0. The make-up rate d2 is (m3/day | bbl/day | cm3/min) for water and (m3/day | scf/day | cm3/min) for gas or solvent, and is defaulted to 1.0e+20.User's Guide GEM Introduction 3

*GCONPMULT *RECYCLE This subkeyword specifies a recycling production target. This indicates that the production wells connected to this group produce such that the phase injected by the injection wells connected to this group as specified by *GAS, *WATER or *SOLVENT is reproduced (recycled) out of the reservoir. *TARGET *WTG / *BHW The *WTG subkeyword for surface wet gas rate and *BHW subkeyword for reservoir water rate are now available for setting well constraints with the *TARGET keyword. *PERFRG keyword removed This keyword is no longer required as the *PERF keyword can be used instead.

4 Introduction

User's Guide GEM

Important Changes in GEM 2008.10SIMULATOR CHANGES SUBDOMAIN, MINC and SUBDOMAIN-Dual Permeability-Reinfiltration New features have been added to GEM for modelling dual-continua (matrix/fracture) processes: The standard SUBDOMAIN model is an extension of the DUALPOR model which divides the matrix vertically to allow simulation of fluid segregation due to gravity within each matrix block. The MINC (multiple-interacting-continua) model is an extension of the DUALPOR model which divides the matrix into multiple nested rings. This allows simulation of fluid transport from the center of the matrix block through each nested ring before reaching the fracture system. The SUBDOMAIN model can be extended to include gravity drainage between subdomains via the TRANSD keyword. This allows fluid transport from the bottom of one subdomain into the top subdomain of the block below it without having to flow into the fracture first. This provides a hybrid of subdomain and dual-permeability modelling capabilities. This behavior can be further modified with the SD_REINF keyword, which will scale the capillary pressure in the bottom subdomain of a block and allow the flow to be modelled as a combination of gravity drainage and reinfiltration. See the keywords *MINC, *SUBDOMAIN, *FRACVOL and *TRANSD in the Reservoir Description section and *SD_REINF in the Rock-Fluid section. Templates gmfrr004 through gmfrr007 show example applications of these new features. Hysteresis for WAG Floods Special hysteresis options have been implemented for modelling important rock-fluid interactions occurring during Water Alternating Gas injection processes. The WAG hysteresis functions may be applied to water and or gas phases. In this model, the relative permeability of a phase is dependent on its own saturation history, as well as the saturation history of the other phase. This allows modelling the decrease in injectivity observed with repeated WAG cycles. The Land and Carlson model is used for gas phase hysteresis, and a 3-phase relative permeability table is entered for water. See the HYS_MODEL keyword in the Rock-Fluid section, and template file gmsmo034.dat. GEM-GHG (Greenhouse Gas) Modelling Prior to the 2008.10 release of GEM, a standalone special executable called GEM-GHG was available for modelling aqueous phase chemical reactions and mineral precipitation/dissolution. In this release the GHG calculation modules are available with the standard version of GEM. As a result, the options for aqueous property modelling in GEM have been enhanced substantially, even when GHG reactions are not being used, as described in the following sections. The models with GHG options can now be run using parallel processing feature of GEM which was not supported by the standalone GEM-GHG executable.

User's Guide GEM

Introduction 5

There are many new keywords associated with the GEM-GHG modules. In the Component Properties section, see the headings: GHG Option Aqueous Components, GHG Option Mineral Component, GHG Option Reactions, GHG Option Reaction Equilibrium and Rate Parameters, GHG Option - Aqueous Phase Model and GHG Option Factors for CO2 Inventory. In the Initial Conditions section, see the entry titled Initial Aqueous/Mineral Concentrations. In Recurrent Data, the composition of a water injection can now be specified with the keywords *INCOMP *AQUEOUS. In addition, new subkeywords of *OUTPRN and *OUTSRF are available in the I/O Control section, along with the new keyword *INVENTORY-CO2, to display the results of the GHG calculations. Template data sets are available in the tpl/ghg folder. Aqueous Phase Properties Keywords DENWS and RHOWS have been implemented to allow specification of mass or molar water density for surface calculations. Previously, GEM did not accept user input for these quantities. Mass density will be converted to molar density for use in GEM. Molar density is used to convert between molar and volumetric rates for surface water injection and production calculations. Aqueous density can now be calculated from the Rowe and Chou correlation, which is a function of pressure, temperature and salinity, in addition to the usual linear model. See the keyword *AQUEOUS-DENSITY in the Component Properties section. Aqueous viscosity can now be calculated as a function of pressure, temperature and salinity using the Kestin correlation, activated with the *AQUEOUS-VISCOSITY keyword. A constant brine salinity can be entered with the *SALINITY keyword. More detailed modelling of aqueous ion concentrations is available with the Greenhouse Gas (GEM-GHG) options. An example is given in template file gmsmo037. Solubility of Gases in the Aqueous Phase Accurate models for the Henrys constants of CO2, N2, H2S and CH4 have been implemented, taking into account pressure, temperature and salinity (salting-out coefficient). These models are activated with the keywords *HENRY-CORR-CO2, *HENRY-CORR-N2, *HENRYCORR-H2S and *HENRY-CORR-C1. The existing aqueous phase solubility models are still available in GEM. Water Vaporization Vaporization of water may be modelled using the *OGW_FLASH keyword. In this case, H2O must be included as a component in the EOS set, as indicated by the H2O_INCLUDED keyword. These keywords are described in the Component Properties section.

6 Introduction

User's Guide GEM

Saline Water Injection Gem version 2008.10 allows injection of aqueous components into the reservoir through use of keyword *INCOMP *AQUEOUS in the Well and Recurrent data section provided the model is set up to handle aqueous components. The definition of aqueous components is done in the Component Properties section and the specification of concentration of aqueous components is done in the Initialization section. Please refer to keywords *NC-AQUEOUS, *COMPNAMEAQUEOUS etc. in the Component Properties section and keyword *MOLALITY-AQUEOUS etc. in the Initialization section. There is a description in the Tutorial section for Saline Water injection. Temperature-Dependent Asphaltene Precipitation The asphaltene precipitation model has been enhanced to include temperature-dependent terms. This allows the asphaltene onset pressure to vary with temperature, and the amount of asphaltene precipitate may be a function of temperature, pressure and composition. WinProp version 2008.10 may be used to generate the temperature-dependent parameters, given one or more onset pressures at temperatures different than the reference temperature. See Component Properties section keyword *SOLIDTHERMAL and *SOLID_TREF. An example is shown in template file gmspr018.dat. Single-Phase Identification Component Properties keyword *PHASEID has a new subkeyword *TCMIX, indicating that single phase identities will be determined by comparing the block temperature to the pseudocritical temperature of the mixture calculated using the Li critical temperature mixing rule (Li, 1971): If block temperature is greater than or equal to the pseudo-critical temperature, the phase is identified as a gas, otherwise it is identified as an oil. In the Initial Conditions section, the keyword TCMULT can be used to tune the critical temperature correlation on a region by region basis. In the Rock-Fluid section, the keyword TRLIMS can be used to introduce reduced temperature limits which will be used together with the pseudo-critical temperature of a single hydrocarbon phase to generate a relative permeability interpolating function. This function is used to calculate a water-hydrocarbon relative permeability by interpolating between the water-oil and gas-liquid relative permeability curves. An example is shown in template file gmspr019.dat. Modelling of Cap Rock Leakage via Special Well Controls A method has been implemented in GEM to model the flow of fluids, primarily gas, from the reservoir through the cap rock once the pressure difference across the seal exceeds a specified threshold value. This cap rock leakage occurs through wells defined with special controls, situated in blocks where the cap rock properties have been defined using the CROCKTYPE keyword. See the keywords WEL_OPN_THR and WEL_OPN_MLT in the Reservoir Description section and CRL_WELLS in the Recurrent Data section. An example is shown in template file gmsmo035.dat.

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Introduction 7

Improvements in GEM-Surface Network linked models A number of enhancements have been implemented, including: passing phase gravities and non-hydrocarbon concentrations to allow GEM to run with the surface network in black-oil mode (compositional tracking mode), on-time fraction exchange between GEM and surface network, and the ability to submit jobs using SSH. Appendix D of the Users Guide has detailed documentation on all aspects of GEM-surface network modelling. Well Layer Output Unlike versions prior to 2008, well layer output is not written to SR2 unless requested via *OUTSRF *WELL *LAYER and associated subkeywords. The default is *OUTSRF *WELL *LAYER *NONE. If no subkeyword follows after *OUTSRF *WELL *LAYER, then *NONE is assumed. Well and block pressures are written under both *OUTSRF *WELL *LAYER *ALL and *OUTSRF *WELL *LAYER *ALL *DOWNHOLE. Command Line Options Two new command line options are described in the I/O Control section: -log log_path is used to identify a path to where the log file should be written, and file_no_replace will cause the simulation to stop with a fatal error if an attempt is made to overwrite any existing output file, including the log file. WELL MANAGEMENT CHANGES Cycling Groups Allow the user to specify data for EOR/IOR processes which rely on alternating production and/or injection cycles. Generally multiple cycles are required and each cycle consists of a number of well defined parts. A typical example is water alternating gas (WAG) injection. *GCONCYCLE_START allows the user to begin to specify data for EOR/IOR processes which rely on alternating production and/or injection cycles. *GCONCYCR_START allows the user to revise data specified earlier with *GCONCYCLE_START. The revisions will be applied immediately. *GCONCYCLE allows the user to turn on or off group cycling control specified earlier with *GCONCYCLE_START. The action indicated with GCONCYCLE takes effect immediately. *CYCPRT_START allows the user to select the cycle starting part. CYCPRT_END allows the user to select the cycle ending part. Group cycling control must be specified earlier with *GCONCYCLE_START. The action indicated with CYCPRT_START and/or CYCPRT_END take effect immediately. GAPPOR Stream Specification The group control may direct a group to switch target (value and stream) if there is a maximum constraint (*GCONP *MAX) being violated. *AUTODRILL followed by the stream identifier ensures that auto-drillable wells can be opened only when the group is apportioned for the desired stream.

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User's Guide GEM

Group Constraint Monitor MINREC and Action SHUTCYCINJ The keyword MINREC specifies that if the total group gas rate available for recycling falls below a specified value, then a remedial action will be taken. This remedial action may be one of *STOP, *SHUTALL or the new action *SHUTCYCINJ. *SHUTCYCINJ indicates that all currently open gas cycling injectors in the group should be shut.

User's Guide GEM

Introduction 9

Important Changes in GEM 2007.10SIMULATOR CHANGES Improvements to the GEM-GAP Interface For surface-network linked runs, GEM can now be made to control certain aspects of a linked wells operating condition. Specifically for linked wells: 1. Any well monitor defined via the *MONITOR keyword is allowed. 2. Keywords that control well and layer operations within a *TRIGGER are allowed. 3. GEM or the surface-network can shut-in a well. The constraint handling capability of surface-network linked runs in GEM has also been upgraded to permit STO, STG, STW, or STL constraints. Please see Appendix-D of this guide for details. For GEM runs coupled with GAP, a portion of the GEM simulation results are now available for scripting. This allows RESOLVE to access variables other than IPR tables and can be used by GAP to control the network. A Resolve script can be written to do calculations using results from both GEM and GAP. Accessible GEM results are also reported in the Resolve Reporting window. Examples of processes that can be handled with scripting include voidage replacement on a localized basis, conditional gas recycling, and preferential withdrawal from certain areas of the reservoir. Leverett J Function A Leverett J Function option has been implemented in GEM. This option allows the user to replace water-oil and/or gas-liquid capillary pressures with Leverett J functions. Capillary pressures are calculated from the J function tables, input surface tension, and account for the varying permeability and porosity of each block. See the keyword *JFUNC in the Rock-Fluid section. Functionality identical to IMEX has been implemented, including input of surface tension arrays (*SRFTNW and SRFTNG) and endpoint scaling (*JFWMAX and *JFGMAX). Also, a technique for modeling tilted water oil contacts is available using PCOW_SHIFT or JFW_SHIFT. Initial Temperature Array Reservoir temperature can be specified for each block using the array keyword *TEMPER in the Initial Conditions section. This provides complete flexibility in the specification of temperature variation throughout the reservoir. Specification of Cyclic Group Controls A new system has been developed to allow the user to specify data for EOR/IOR processes which rely on alternating production and/or injection cycles. Generally multiple cycles are required and each cycle consists of a number of well defined parts. Examples include water alternating gas injection (WAG) or cyclic steam injection (SSI). A number of criteria are available to stipulate the duration of each cycle part.10 Introduction User's Guide GEM

As the keywords for these controls are not yet supported in the Builder interface, this feature is released on a Beta basis. The documentation for the keywords and a template data set are found in the \CMG\GEM\2007.10\doc folder. Speed-up Some additional code parallelization and code modification for work-saving has resulted in significant speed-up of GEM runs, particularly for models with large numbers of well completions. Change of Default for Relative Permeability Scaling Algorithm Beginning with the 2005.XX releases of GEM, an 8-point scaling method was available by specifying *RPT *SCALING-NEW. However, the older 4-point scaling method was still the default. In the 2007.10 release, the 8-point scaling method becomes the default. The 4-point method is available by specifying *RPT *SCALING-OLD. Matrix-Fracture Transmissibility Multiplier New keyword *TRANSMF allows you to modify the transmissibility between a fracture block and its co-located matrix block. Binary Data Reading New grid_array read_option *BINARY_DATA allows you to acquire, store and transfer data in binary form. This subkeyword was in v2006.10 but did not appear in the Users Guide. WELL MANAGEMENT CHANGES Shut in Wells above Formation *MODELSHUT indicates that the listed wells identified by well_list will be closed above the formation with crossflow modelling of the constituent (open) layers, whenever the wells are shut in due to a well operation. Two New Drawdown Constraint Types Well control keyword *OPERATE has two new subkeywords: *DWA defines the maximum drawdown within all open layers,

Pd = max (Pblock l Pwelll )l, open

which corresponds to the notion of imposing the maximum drawdown constraint to avoid formation damage. *DWB specifies the average drawdown for all open layers:

Pd =

PI l (Pblock l Pwelll ) PI ll, open l,open

weighted by the total product/injectivity (PI) at the reservoir condition.

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Introduction 11

Monitoring Well Head Pressure Stability A real well can operate only above a certain rate at which the bottom-hole pressure remains in the stable region of the well-head pressure curve (a region in which the curve of WHP vs. BHP has a positive slope). New *MONITOR subkeyword *WHYSTAB allows you to monitor hydraulic stability for a well. Voidage Replacement Production Target (*GCONP *VREP) This indicates that the production wells connected to this group produce an amount of the bottom-hole fluid in proportion to the total bottom-hole fluid injected into the reservoir by the injection wells connected to this group. Recycling Production Target (*GCONP *RECYCLE) This indicates that the production wells connected to this group produce such that the phase injected by the injection wells connected to this group as specified by *GAS or *WATER is reproduced (recycled) out of the reservoir. Sector Pressure Maintenance Production or Injection Target New *GCONP and *GCONI subkeyword *PMAINT specifies that the group production / injection rates shall be adjusted so as to maintain the hydrocarbon volume weighted average pressure in a particular region/sector (*PMSECT) at a desired level (*PMTARG). Shutting in Multiple Wells to Meet Production Targets/Monitors (Subkeyword *SHUTMOWS) Action subkeyword indicating that if a maximum stock tank rate is exceeded for a group, then a list of prioritized most offending wells (MOWS the wells with the higher rates of the named surface stream) should be shut. Voidage Replacement Injection Targets including Makeup of Gas or Water (GCONI *VREP *GMKUP/*WMKUP) A make-up stream can be added with *GMKUP or *WMKUP to meet a total voidage replacement fraction for each group under voidage replacement. New Apportionment Methods for Meeting Group Targets *APPOR-METHOD defines how to distribute a group target within its pool the collection of all the contributing wells or groups. The pool for a targeted group consists of all its child groups and the wells directly attached. . CMGs multi-level well management module currently supports the following methods: Instantaneous potentials (*IP, default) Guide rates (*GUIDE) Internally generated guide rates (*INGUIDE) Priority ranking (*PRIOR)

12 Introduction

User's Guide GEM

*PRIOR-FORM defines the priority formulae and numerical control parameters for the priority ranking apportionment method set by *APPOR-METHOD *PRIOR to meet group targets. On-Time Factors Applying to Groups *ON-TIME specifies the fraction of time during which a well or group operates. Please see the *ON-TIME keyword for a detailed description of how *ON-TIME operates with Groups.

User's Guide GEM

Introduction 13

Important Changes in GEM 2006.101. It is now possible to define conditions (triggers) under which a sequence of recurrent data lines (actions) will be read and executed. In this way it is possible to get a very wide range of actions taken under specified conditions. Please see the manual page for the TRIGGER keyword in the Well and Recurrent Data section of this manual. 2. Some restrictions upon the ordering and timing of well data have been relaxed. Please see the introductory pages of the Well and Recurrent Data section of this manual for more information. 3. A set of wells can be shut and reopened with a single statement in a way that does not disturb the open/shut pattern of wells that existed when the collective shut command was issued. Please see the manual pages for the WLISTSHUT and WLISTOPEN keywords in the Well and Recurrent Data section of this manual. 4. GEM can now be coupled to Petroleum Experts RESOLVE/GAP or Neotechnology Consultants FGI/FORGAS surface network models. The link to GAP is through an interface dll (GemGapLink.dll) written by CMG, while FORGAS is linked via an interface (FGI) written by Neotechnology Consultants. These dlls take care of the coupling between GEM and the surface network programs and no additional keyword is required in the GEM input file. The communication between the simulator and surface network model is made through ASCII signal and data files. These files are automatically created and continuously updated by the coupled system. 5. Special care is required while preparing the GEM data file for coupling with the network programs. All the linked wells should be defined at the first *DATE/*TIME keyword in the Wells and Recurrent section of the GEM data file. The constraints and operation of these wells are controlled by the network program and not by GEM. Group production and injection controls (*GCONP and *GCONI) should not be used for groups containing wells controlled by RESOLVE or FGI. Please see the manual pages in Appendix D: Coupling GEM with Surface Network Models, of this manual for further details. 6. Streamline output is enabled for display in Results. This is enabled with the keywords OUTSRF GRID STRMLN. More details can be found in the I/O Control section of this manual.

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User's Guide GEM

Important Changes in GEM 2005.101. The group well management facility now can model undersea manifolds and the pressure drops between these manifolds and the surface. Both production and injection manifolds can be modeled; the manifolds are represented as groups of wells running with the same manifold (tubing head) pressure. For more information see the documentation of the MANIFOLD, GCONP, GCONI, PCONMANIFOLD, GPTABLE, GPHYDDEP, GROUPALQ, WELLALQ, and IWELLBORE keywords in the Well and Recurrent Data section of this manual. 2. GEM now is able to read keywords that scale all eight relative permeability endpoints: see descriptions of *SWCRIT, *SGCRIT, *SORW, *SORG, *SWCON, *SGCON, *SOIRW, and *SLCON. GEM can also modify relative permeability table values as well; see descriptions of *KRWIRO, *KROCW, *KRGCL, *KROGCL, *PCWMAX, and *PCGMAX. Also, hysteresis effects are allowed for Pcwo, Pcgo, krg and krow. *SCALING-NEW must be stated to specify the treatment of relative permeability tables using this new system (see the Rock-Fluid section of this manual), while *SCALING-OLD can be specified to use the old system (as was used in versions GEM 2004.xx and earlier). If nothing is specified, then *SCALING-OLD is the default. Any use of any of the new keywords (those other than *SWCRIT, *SGCRIT, *SORW, *SORG) must be accompanied by use of *SCALING-NEW. Also *KRPERF (see Recurrent Data section) can now be used in GEM, provided that *SCALING-NEW has been specified. 3. GEM can now be run in parallel using shared-memory multi-processor computers. To invoke the parallel solver and use other parallel features, specify *SOLVER *PARASOL and *DPLANES 4 in the *NUMERICAL section of the data set. The value following *DPLANES does not affect GEM (although it does affect IMEX and STARS when used for those simulators), but the same keyword has been used to make data input similar for the thre