Fundamentals of Formation Testing

Fundamentals ofFormation Testing

Schlumberger225 Schlumberger DriveSugar Land, Texas 77478

Produced by Schlumberger Marketing Communications.

© 2006 Schlumberger. All rights reserved.

No part of this book may be reproduced, stored in a retrieval system, or transcribed in any form or by any means, electronic or mechanical, including photocopying and recording, without the prior written permission of the publisher. While the information presented herein is believed to be accurate, it is provided as is without express or implied warranty.

06-FE-014

An asterisk (*) is used throughout this document to denote a mark of Schlumberger.

Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiOverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Well testing applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Formation pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Permeability and skin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Formation fluid characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Reservoir characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Well testing methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Openhole and cased hole, no completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Wireline testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Production or injection test with completion string in place . . . . . . . . . . . . . . . . . . . . . . 2

Well testing objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Exploration and appraisal well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Development well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Production and injection well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Productivity well tests versus descriptive reservoir tests . . . . . . . . . . . . . . . . . . . . . . . . . . 6Goals of well test interpretations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Reservoir pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Steady-state radial flow in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Transient flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Dynamic Properties of Reservoir Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Sandstones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Diagenesis and secondary porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Natural fracturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Dissolution of limestone by leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Dolomitization of limestone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Diagenesis and porosity degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Absolute permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Effective permeabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Surface tension and wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Capillary pressure and saturation profiles in the reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Capillary pressure and capillary rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Fundamentals of Formation Testing n Contents iii

Saturation profiles in reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Irreducible water saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Displacement capillary pressure and FWL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Drainage and imbibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Capillary pressure and wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Quantifying relative permeabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Dynamic Properties of Produced Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Components of hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Classification of reservoir fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Phase behavior of single-component systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Phase behavior of multiple-component systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Phase behavior of reservoir fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Nonvolatile oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Volatile oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Condensate gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Wet gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Dry gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

PVT properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Data sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Gas compressibility factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36EOS: Basics of understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Solution GOR and bubblepoint pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38FVFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Fluid densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Viscosities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Compressibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Modeling fluid behavior and PVT properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39PVT correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Pressure Sensors Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Mechanical pressure sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Capacitance pressure sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Strain pressure sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Bonded wire sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Thin film sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Sapphire sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Quartz pressure sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Hewlett-Packard design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Quartzdyne design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Crystal Quartz Gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Paroscientific design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Quartztronics design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Introduction to metrology of pressure transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Static parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

iv

Dynamic parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Transient response during temperature variation . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Transient response during pressure variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Dynamic response during pressure and temperature shock . . . . . . . . . . . . . . . . . 52Dynamic temperature correction on the pressure measurement . . . . . . . . . . . . 53

Calibration and evaluation tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Well Testing Operations and Data Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Openhole wireline testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Pressure profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Permeability anisotropy profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Miniproduction tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Representative fluid sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Modular reservoir power cartridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Modular reservoir hydraulics module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Modular reservoir probe single module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Modular reservoir sample chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Modular reservoir packer dual module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Flow-control module MRCF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Modular reservoir packer module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Modular reservoir pumpout module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Modular reservoir fluid analyzer module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Modular reservoir multisample module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Cased hole wireline testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Drillstem testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Purpose of a DST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Pressure-controlled tester valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68Intelligent Remote Implementation System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69Universal Pressure Platform and UNIGAGE recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72DataLatch recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74DST sample chambers and carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77Other DST string configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Simultaneous perforating and testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78MWP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78Well tests for TCP and MWP operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Well tests in production and injection wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Control of wellbore effects while testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81Testing in pumping wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Permanent pressure measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Test Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85New wells: DST or wireline testing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85Production and injection wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86Workover wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86SRO versus downhole recording . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87Pressure gauge metrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87Fluid sampling options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88Operational constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88Interpretation requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Fundamentals of Formation Testing n Contents v

Test schedule and simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89Summary of test types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Reservoir Fluid Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Representativity of reservoir fluid samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Conditions for representativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Level of contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Well conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94The FPE service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Wellsite sample validation and properties estimation . . . . . . . . . . . . . . . . . . . . . . . 94Assessing bottomhole sample validity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95FPE analysis on bottomhole samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96FPE analysis on separator samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Fluid sampling methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Wireline sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Sampling during a DST well test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Surface sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Production well sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

DST and bottomhole sampling of oil reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Preliminary considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Sampling new wells and undepleted reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Sampling depleted reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Sampling high-volatility oil reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Sampling near-critical fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

DST and bottomhole sampling of gas reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Preliminary considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Bottomhole versus surface sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Sampling procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Sampling new wells and undepleted reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Sampling depleted reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Surface sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103Wireline tester flowline fluid resistivity and optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Flowline resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103Optical density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105Fluid coloration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Gas detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Holdup indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Wireline tester real-time fluid identification and contamination monitoring . . . . . . . . . . . . 111Introduction and summary of methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Oil-base filtrate contamination monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Direct sample methane detection: the LFA tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114In-situ determination of PVT properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Advanced wireline sampling techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Dual-packer fluid sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Pumpout module performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Minimum permeability requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Pumpout time estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Flowline and pump volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

vi

Controlled drawdown sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120Low-shock sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Charged-chamber sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

Control of asphaltenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Heavy oil sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Gas condensate sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Water sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Guard probe sampling and fluid flow modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Static Pressure Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Determination of static reservoir pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Wireline tester pretests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Supercharging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126Static pressure from buildup tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127Reservoir pressure from limit tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

Drawdown mobility from wireline testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128Drawdown mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128Radius of investigation for drawdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129Drawdown permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129Buildup mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Pressure-depth plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Reservoir fluid density from gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Virgin reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133Developed reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Pressure probes in tandem or triplex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136Pressure profiles in horizontal wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Effect of capillary pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

Applications of static pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148Depth datum of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148Reservoir pressure in producing fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148IPR, PI and AOF potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149Selective Inflow Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151Transient Testing Interpretation Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

Interpretation methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153IARF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153Line source solution to the diffusivity equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154Wellbore storage and skin effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

Wellbore storage effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155Skin effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155Combined WBS and skin effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

Log-log plots and type-curve analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157Semilog approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157Log-log approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157Pressure derivative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158Attributes of the log-log plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158Type-curve matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159Succession of events detected during a well test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

Interpretation of drawdown tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162Validity of interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

Fundamentals of Formation Testing n Contents vii

Miller, Dyes, Hutchinson plot and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162Interpretation of buildup tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Drawdown versus buildup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Horner plot and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Multirate superposition plot and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

Changing WBS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164Convolution and deconvolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

The convolution integral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166Rate normalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168Deconvolution of afterflow rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Boundary conditions and reservoir flow models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171Inner boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Fractured well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171Partially completed well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172Horizontal well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

Reservoir flow models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172Dual-porosity reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172Double-permeability reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173Composite reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

Outer boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173No-flow boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173Constant pressure boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173Mixed boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174Numerical simulation of boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174Specialized plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

Interpretation of gas well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175Pseudopressure and pseudotime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

Pseudoskin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

Advanced Applications of Transient Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177Buildup mobilities from wireline testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Spherical and radial derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177Buildup mobilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177Radius of investigation of buildup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

Multiple well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180Horizontal interference tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180Pulse tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

Conventional vertical interference tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184Anisotropic permeability determination using wireline testers . . . . . . . . . . . . . . . . . . . . . . . . . . 186

Probe configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186Rate of flow through the sink probe: “superflow” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186Flow regime identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188Estimation of mobilities and storativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188Model verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192Influence of invasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

MPTs with wireline packer testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197IPTTs with wireline packer testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199Multilayer tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

Conventional testing approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

viii

LRT overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205Interpretation of the LRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

Complexity of the interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206Model identification and initial estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207History match . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

LRT example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207Horizontal well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

Flow regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211Use of simultaneous pressure and flow measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

Tests in naturally fractured reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215Conventional tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215Openhole wireline tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

Multipoint gas well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219Multipoint well testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219Flow-after-flow tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219Isochronal tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219Modified isochronal tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219Interpretation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

Impulse tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223Impulse testing theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223Surface flow impulse tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223Closed chamber impulse tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225Slug tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

Other specialized tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227Reservoir limit tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227Injection well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231Rod-pumping well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234Testing Interpretation Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237Conventional well test interpretation software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

Data loading and editing facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237Quality-control facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238Fluids and PVT module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238Basic interpretation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238Test design module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239Advanced interpretation functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

Changing well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240Average reservoir pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240Modeling of the variable downhole flow rate during drawdown periods . . . . 240Multilayer testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240Inflow performance analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241Interpretation of multipoint gas well tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Modeling of multiple wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241Artificial intelligence and advanced regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241Interaction with a well flow model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

Wireline test interpretation software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

Fundamentals of Formation Testing n Contents ix

Software used by Schlumberger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Rock stress determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Operating technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243Filtration test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244Fracturing and shut-in decline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244Reopening test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244Flowback/pressure rebound test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Combined answers from wireline testers and NMR logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247Estimating the capillary pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247Correlating with MDT permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249Identifying reservoir fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250Selecting sampling depths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

Roman symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253Greek symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

Marks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261Unit Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

x

Cost-effective formation testing is a vital part of modern oil and gas operations. As productioncompanies pursue reserves in high-cost, high-risk environments, formation testing plays anincreasingly prominent role in decision making. Complete understanding of reservoir propertiesand fluids is crucial from the first exploratory well to wells drilled for enhanced recovery in anyfield.

Fundamentals of Formation Testing is the first of a series of Schlumberger reference booksproduced for current and future oilfield technical professionals. This document illustratesstate-of-the-art technologies with field examples from well testing applications to test design,sampling techniques, operations, and interpretation. We trust this information will reinforce thevalue of formation testing.

Sami IskanderPresident, WirelineClamart, FranceMarch 2006

Fundamentals of Formation Testing n Foreword xi

Foreword

Fundamentals of Formation Testing n Overview 1

Overview

IntroductionThe general formation evaluation workflow consists ofdelineating the reservoir using seismic information andwell-to-well correlations, evaluating the volume ofreserves, and then determining the fluids present andthe ability to produce them. Logs provide initial infor-mation on the fluid type and producibility. Testing pro-vides confirmation, detailed fluid properties, accuratepressure measurements and production evaluation.Formation testing is the final evaluation step before thewell is put into production and provides essential infor-mation to design the well completion and productionfacilities.

Two different technologies can be used for testing:n Wireline formation testing uses a sonde that can be

positioned at a selected depth in the formation to pro-vide accurate measurements of pressure and fluidtype but limited production data.

n Well testing uses a packer lowered on drillpipe ortubing. The tested interval is not precisely definedand downhole measurements are limited, but thevolume of fluid produced enables complete evalua-tion of production potential.

This book will describe the advantages and limita-tions of both formation testing techniques and how theycomplement each other.

Well testing applicationsThere are many applications of well testing, but they aregrouped into four fundamental classes.

Formation pressure measurementThis class of application uses the direct static formationpressure measurement. It includesn static pressure measurement and depletion

determination n determination of the inflow performance and pro-

ductivity index (PI) of the reservoir and, in gas wells,the absolute open flow (AOF) potential of the reservoir

n determination of reservoir fluid density from gradientsn determination of reservoir fluid contactsn identification of reservoir vertical permeability

barriersn identification of vertical flow through layered

sequences in developed reservoirsn numerical reservoir simulation applications.

Permeability and skinThe pressure and associated measurements (e.g., down-hole flow) are interpreted to yield reservoir dynamicparameters relevant to fluid flow, such as formation per-meability, and any occurrence of skin (e.g., formationdamage) that would impair the flow. The measurementswill help determinen reservoir permeabilityn well deliverabilityn a damaged or stimulated well conditionn vertical rock permeabilityn the efficiency of stimulation treatments.

Formation fluid characterizationThe essence of formation testing is flowing the well,which presents the unique opportunity to recover sam-ples of the reservoir fluid. It enablesn collecting representative reservoir samplesn characterizing the fluid composition, its phase

behavior and its pressure-volume-temperature(PVT) properties.

Reservoir characterizationThe pressure response during a well test provides thecharacteristic signature of reservoir fluid flow eventsthat will be interpreted in terms of boundaries, hetero-geneities and reservoir volume. It enablesn determining the total reservoir pore volume con-

nected to the tested welln determining the average reservoir pressuren determining reservoir boundary conditions such

as impermeable barriers and constant pressure conditions

n characterizing reservoir heterogeneities such as layered systems and natural fractures

n quantifying vertical and horizontal reservoir communications.

Well testing methodsThe three primary ways to test a well are covered in thefollowing sections.

Openhole and cased hole, no completionConventional deliverability tests, involving extensive sur-face and downhole equipment, are designed to simulatethe production characteristics of new wells. Figure 1shows a typical surface offshore layout for an explorationwell test and a sketch of the drillstem test (DST) stringof downhole testing tools.

Wireline testingWireline tests are performed mostly in open hole using acable-operated formation tester and sampling toolanchored at depth while reservoir communication isestablished through one or more pressure and samplingprobes. Figure 2 shows typical configurations for testingand sampling with the MDT* Modular FormationDynamics Tester tool.

Production or injection test with completion string in placeProduction and injection well tests, performed usingproduction logging tools, are conducted to obtain pres-sure and optional flow measurements. Figure 3 shows asketch of a basic version of the PS Platform* new-gener-ation production services platform, equipped with a gasholdup sensor.

During a well test, a particular flow rate schedule isapplied to the targeted reservoir, using surface or down-hole flow control equipment (in the case of conventionaltesting and production or injection well testing) or asoftware-selected drawdown routine (in the case of wire-line formation testing). The resulting pressure changesand the flow rates (surface and optionally downhole) arerecorded versus time, typically either in the same well orprobe, or in a nearby well or probe during interferencetests. From the measured pressure response, and frompredictions of how reservoir properties influence thisresponse, one can infer the values of these properties,which include permeability, skin factor and other para-meters. A particular aspect of well testing is formationfluid sampling, which is one of the main reasons wellsare tested.

Well tests are quite varied in nature. Unlike loggingruns, which consist of recordings of static formationproperties such as density and resistivity, well tests mustbe designed before they are executed. Effective welltesting must obey the design-execute-evaluate cycle,where the operations are first designed to target themeasurement of specific parameters, then executed inthe field, then evaluated—the evaluation expectedlyyielding the values of the targeted parameters. If theobjectives are not met, the evaluation feeds back intothe design of future tests—sometimes also into thedesign of the test being performed—and the processcontinues through the design-execute-evaluate cycle tooptimize the results versus the expectations.

Because well tests can be designed to achieve manyobjectives under highly varied environmental and reser-voir conditions, implementation in the field can be per-formed in a number of ways, using many different hard-ware configurations and an extensive suite ofinterpretation methods.

This chapter will discuss the objectives of well testing(including the hardware and the measurement sensorsused), testing data acquisition, interpretation, applica-tions and software. A separate chapter will discuss for-mation fluid sampling.

2

Fundamentals of Formation Testing n Overview 3

Figure 1. Typical offshore layout for an exploration well test.

Fullbore test string

1. Firing head 2. Perforated tail pipe 3. Fullbore recorder carrier 4. PosiTest* retrievable compression packer 5. Pressure transfer sub 6. Safety joint 7. Hydraulic jar 8. Fullbore recorder carrier 9. Hydrostatic reference tool 10. Fullbore PCT* Pressure Controlled Tester 11. Single-ball safety valve 12. Single-shot hydrostatic overpressure reverse tool 13. DataLatch* electrical wireline downhole recorder/transmitter 14. Multiple ID reversing valve 15. Drill collar 16. Slip joints

Subsea safety equipment

17. E-Z Tree* retrievable well control tree with glycol injection system 18. Retainer valve 19. Deep-sea hydraulic control pod 20. Lubricator valve

Surface equipment

21. Hose bundle 22. E-Z Tree control unit and glycol injection pump 23. Flowhead 24. Flowhead safety valve 25. Wireline wellhead equipment 26. Logging skid unit offshore wireline unit with COMPUTEST* wellsite computer equipment 27. Data acquisition units 28. Emergency shutdown system 29. Data header 30. Choke manifold 31. Heater and steam exchanger 32. Three-phase separator 33. Oil manifold 34. Surge tank 35. Transfer pump 36. Air compressor 37. Gas manifold 38. Supporting boom 39. Burner

*Mark of Schlumberger

Water Oil

Gas Well stream

1234

5

6

789

101112

13

141516

17

18

19

20

21

222324

2527

28

29

3938

37

35

36

34

31

32

33

30

26

4

Figure 2. Typical MDT configurations for formation testing and sampling.

Electric

Sample

Packer

Electric

Hydraulic

Single probe

Electric

Sample

Hydraulic

Single probe

Electric

Sample

OFA* OpticalFluid Analyzermodule

Hydraulic

Single probe

Electric

Sample

Multisample

Single probe

OFA module

PumpoutSample Pumpout Pumpout Pumpout

Hydraulic

a b c d e

Figure 3. Sketch of a basic PS Platform tool for production logging and testing in production and injection wells.

25.4 ft[7.72 m]

8.3 ft[2.52 m]

4.8 ft[1.45 m]

7.1 ft[2.18 m]

5.2 ft[1.59 m]

Basic measurement sondeTelemetry, gamma ray,casing collar locator, pressure, temperature

GradiomanometerDensity, deviation

Caliper and flow imaging toolVelocity, X-Y caliper,

water holdup,water-hydrocarbon

bubble count,relative bearing

GHOST* toolGas holdup,

gas-liquid bubble count,one-arm caliper, relative bearing

Well testing objectivesWell tests are conducted at all stages in the life of a reser-voir: exploration, development, production and injection.At each of these stages, tests are performed with setobjectives, using specific hardware and design options.

Exploration and appraisal well testsAt the exploration stage, tests are performed both withan openhole wireline tester, to measure pressures andcollect fluid samples, and with the drillstring, to simu-late production at the time of the completion.

A wireline tester is used to measure the static pres-sures of all the permeable layers of interest. Pressureversus depth plots help establish the formation fluid gra-dients and identify the fluid contacts in the reservoir.The MDT tool can also be used with a packer module toperform a smaller-scale production test.

A conventional DST is necessary because at the explo-ration stage the operator needs to know whether thediscovery is of commercial value. The objectives are toestablish a production rate and the volume of the reser-voir. If production rates are low, the operator needs toknow if it is because of poor reservoir deliverability orbecause of a high skin factor. In the latter case, flowrates could be increased if the skin were removed, andthe well would have better potential. If the volume ofhydrocarbons in place, inferred from the initial test, wastoo small, the discovery would lose its economic poten-tial even if production rates were high.

A primary reason for testing exploration wells is torecover a fluid sample. This is the best stage at which tocharacterize the reservoir fluid, because the reservoir isin a virgin state and no fluid has been produced. In laterstages of a reservoir’s life, the fluid composition changesand the surface fluid composition may not exactly matchthe downhole fluid composition. (This is especially truein two-phase situations such as condensate reservoirs.)Because the fluid sampling program in an explorationwell is particularly important, it will feature both a wire-line sampling program (including the collection of PVT-quality samples) and larger volumes produced duringthe DST operation.

Development well testsDuring the development phase, the test objectives aredifferent from those of the exploration and appraisalstages. Presumably the reservoir deliverability has beenassessed and the reservoir fluid has been characterized.The operator now needs to better understand thehydraulic communications in order to relate the reser-voir characterization to the geological model.

Formation testing at this stage predominantly con-sists of openhole wireline pressure testing. The empha-sis is on static reservoir pressures, which are used toconfirm fluid contacts and fluid density gradients. Onthat basis, the different hydraulic compartments of thereservoir will be determined and tied into the geologicalmodel. Often, field production has already started whileadditional development wells are being drilled. In thenew wells, pressure gradients already may reflect theinfluence of the production on the reservoir pressure.On those wells, the reservoir simulator, if in place, willbe used to predict vertical pressure profiles to be con-firmed by the wireline tester measurements. Any differ-ences would be used to refine the geological model andintroduce suitable compartments in the dynamic model.This stage is crucial in reservoirs with a large number ofstacked layers such as deltaic deposits (which are themost prolific hydrocarbon-producing reservoirs). Forthese, wireline pressure measurements are an invalu-able aid to reservoir dynamics characterization, becausethere is no other practical way of assessing vertical andlateral communications and the volumetrics of thesesmall individual accumulations. This application con-tributed to the immediate acceptance of the first wire-line testing tool, the RFT* Repeat Formation Tester.Until this tool’s introduction in the mid-1970s, distrib-uted pressure measurements had been unavailable toreservoir engineers.

The main objective of conventional testing, if per-formed on the new development wells, is to measure anyskin resulting from formation damage. If skin is absent,the wells can produce at their full potential. If a highskin is detected, it must be corrected before putting thewells on line.

Another reason for testing development wells is toprepare them for stimulation operations, which may benecessary to produce them economically. This iscommon with many reservoirs in low-producibility areas,where the operator needs an early return on invest-ments. When unstimulated production rates would beuneconomical, the operator often spends as much as $1 million or more on extensive stimulation operations,such as hydraulic fracturing. Recovering this investmentcould take months. Conventional well testing is espe-cially important to assess the productivity gainsachieved through measuring skin, determining fracturelength and its hydraulic conductivity, and assessing thefinancial risks.

Fundamentals of Formation Testing n Overview 5

Production and injection well tests In the production phase, the objectives of testing shiftfrom flow evaluation to reservoir monitoring, data col-lection for history matching of reservoir simulators, andproductivity tests to assess the need for stimulation. Welltests are performed to check for skin resulting frommigration of fines to the near-wellbore region, and toassess the need for acidizing to remove those fines.Partial completion effects are diagnosed and remediedby reperforating or by extending perforation intervals.Gravel-packed wells are tested to evaluate the gravel-pack skin (which is often very large) and decide on anysubsequent treatment.

Most of these tests are performed using a pressuregauge suspended from an electric cable or a slickline, orusing a production logging tool that will provide comple-mentary measurements such as downhole flow rate. Thecomplexity of tests will depend on the well and may varyfrom a simple buildup test to a series of step-rate testsintended to analyze the dynamic performance of com-plete multilayer systems.

Cased hole wireline testing also may be performed toobtain formation pressures in layered sequences, collectsamples of bypassed oil detected by saturation measure-ments and, in some cases, measure permeability.

Productivity well tests versus descriptive reservoir testsThe interpretation of well tests is one of the most wide-spread sources of dynamic reservoir data. Tests on oiland gas wells are performed at various stages of drilling,completion and production. The test objectives rangefrom simple measurements of reservoir pressure to char-acterization of complex reservoir features. Well tests canbe classified either as single-well productivity tests ordescriptive reservoir tests.n Single-well productivity tests are conducted to

– determine well deliverability– characterize formation damage and other sources

of skin– identify produced fluids and determine their

respective volume ratios– measure reservoir pressure and temperature– obtain representative fluid samples suitable for PVT

analysis– evaluate completion efficiency– evaluate workover or stimulation treatments.

n Descriptive reservoir tests are conducted to– assess reservoir extent and geometry– determine hydraulic communication between wells– characterize reservoir heterogeneities– evaluate reservoir parameters.

Goals of well test interpretationsTable 1 lists the interpretation objectives of typical welltests. Five types of tests have been considered:n openhole wireline test on an exploration or appraisal

welln conventional DST on an exploration or appraisal welln openhole wireline test on a development welln conventional DST on a development welln cased hole test on a production or injection well,

using a pressure gauge or a production logging tool.

Reservoir pressureThe primary data obtained from reservoir testing arecontinuous measurements of reservoir pressure versustime; flow rate is typically controlled to follow a plannedschedule. In effect, formation pressure is probably thesingle most important measurement in the productionhistory, dynamics and economics of the field. Pressuremeasurements are used in volumetric calculations(reserves), dynamic reservoir property determinations(permeability), reservoir characterization (compart-mentalization), fluid characterization (phase behavior,fluid properties) and well completion design (lifting sys-tems). Pressure also provides information on the evolu-tion of reservoir energy and fluid contacts with time, andit is an essential input to reservoir simulation models.Further, repeated and comprehensive use of pressuremeasurements is fundamental to the success of produc-tion optimization programs. Such programs address notonly the well itself but the whole reservoir and even thesurface facilities.

In addition to well testing applications, reservoirpressure testing is used inn determining rock stresses and hydraulic fracturing

characteristicsn optimizing well flow characteristics and control of

hydraulic lossesn optimizing surface production systemsn determining material balancen characterizing reservoir energy.

6

Fundamentals of Formation Testing n Overview 7

Table 1. Target Objectives of the Interpretation of Various Types of Well Tests

Exploration or Exploration or Development Development Production or Appraisal Well Appraisal Well Well Well Injection WellOpenhole Wireline DST Openhole Wireline DST Cased Hole Wireline

(Gauges or Production Logging tool)

Pressure-depth profile Yes Yes

Vertical permeability barriers Yes Yes

Reservoir fluid densities Yes Not if differential from gradients depletion

Reservoir fluid contacts Yes Not if differential depletion

Vertical flow patterns Yes Yesfrom pressure profile

Drawdown and buildup Yes Yesmobilities

Anisotropic permeability Yes Yesdetermination

Fluid samples Yes Yes Yes Yes Yes

Representative fluid samples Yes Little control Needs initial Needs initial Needs initial on quality reservoir fluid reservoir fluid reservoir fluid

Reservoir deliverability kh/µ Yes Yes

Skin factor Yes Yes Yes

Reservoir heterogeneities Yes Yes Yesand flow model

Reservoir volume Yes

Reservoir boundaries Yes Yes Yes

Horizontal wells Usually no Yes Yesexplorationhorizontalwells

Horizontal interference tests Yes Yes Yes

Vertical interference tests Yes Yes

Average reservoir pressure Yes Yes Yes

Treatment efficiency Yes

Layered reservoirs testing Yes

Determination of PI and AOF Yes

Multipoint gas well tests Yes

Injection well tests Yes

Flow measurements Possible but Yes Recommendedinfrequent

The measurement commonly known as “reservoirpressure” is a measurement of the pore fluid pressure ppin a porous reservoir. The reservoir pore fluid pressure isthe fraction of the overburden pressure that is supportedby the fluid system. The other fraction, the effectivestress σv′ is supported by the rock. The overburdenstress (σv) is caused by the weight of the fluid and rocksin the lithostatic column above the measured point. Porepressure is linked to rock stresses with the relation

(1)

The static pressure measurement always results fromsome form of transient test, where a specific volume offluid is withdrawn from the well before the pressures areallowed to stabilize. The efficiency of wireline testing,where static pressures can be acquired at the rate ofpossibly one measurement every few minutes, resultsfrom the small volume of the fluid sample. Conversely, inconventional well testing, static pressures take muchlonger to stabilize only because of the larger amount offluid withdrawn, which creates pressure disturbancesobservable at much greater distances into the reservoir.The term “sandface pressure” refers to the value of thepressure existing at the boundary between the reservoirand the wellbore, whether the reservoir flows (draw-down or flow tests) or not (shut-in or buildup tests).Ideally the sandface pressure would be the pressuremeasured by a wireline tester (considering the probepenetration to be nil) or the well pressure—static orflowing—measured at depth in the well by a hangingpressure gauge.

To illustrate the relationship between the reservoirpressure and the reservoir dynamic properties, this bookwill review the essentials of steady-state (or stabilized)flow in reservoirs and the propagation of pressure inreservoirs under the effect of transient flow conditions.

Steady-state radial flow in reservoirsFlow through a homogeneous reservoir into a wellbore isconsidered radial when the flowlines are horizontal, par-allel and converge toward the wellbore axis. Infinite-acting radial flow (IARF) is a special case of transientflow regime illustrated in Fig. 4. In IARF, an idealizedcylindrical model can be used to calculate flow rates anddescribe the pressure distribution away from the well-bore.

If h is the thickness of the flowing interval, the cross-sectional area of flow at distance r from the axis of thewellbore is 2πrh, and the flow velocity v through thisarea for a production rate Q is

(2)

Darcy’s steady-state equation can be written toexpress the flow velocity

(3)

where k is the reservoir permeability and µ is the reser-voir fluid viscosity.

Combining Eq. 2 and Eq. 3, we get

(4)

Eq. 4 can be integrated in two different ways:n From r = rw (rw is the wellbore radius, where the

pressure is the sandface pressure, pwf) to r = re (re isthe external or drainage radius, where the pressure isthe static pressure, pi), giving the steady-state flowrate:

(5)

8

Figure 4. Infinite-acting radial flow.

Well

pwf

rw

pi

re

r

h

vQrh

=2π

.

Qkh p p

r

r

i wf

e

w

=−( )

2π

µln

.

σ σv v pp= ′ + .

vk dp

dr=

µ,

vQ

khdrr

= µπ2

.

In oilfield units,

(6)

n From r to re, giving the pressure response at distancer:

(7)

The distribution of pressure versus distance from thewellbore is shown on Fig. 5. The greatest pressure lossoccurs within a short distance (in this example, about100 ft). The pressure distribution is independent ofreservoir permeability. As pressure varies with the loga-rithm of radius, the pressure measured at the sandfaceis little affected by the drainage radius in normal pro-duction conditions. On the other hand, because rw(which in practice is taken as the casing outside diame-ter) can vary considerably, this parameter has a largerinfluence on both the pressure distribution and the flowrates.

This pressure profile applies only to laminar flow andslightly compressible (e.g., liquid) flows. At very highrates, and for compressible fluids (e.g., gas), flow nearthe wellbore may become turbulent and pressure gradi-ents may become steeper than predicted by Fig. 5.

In practice, the presence of skin often affects thepressure profile away from the wellbore. Skin is a condi-tion of flow impairment or enhancement close to the

wellbore (the “skin”) created by various conditions suchas formation damage (or stimulation). Skin effect is theequivalent of a pressure drop added to (or subtractedfrom) the sandface pressure that would exist in theabsence of skin. The pressure profile away from the well-bore will be affected by the existence or absence of skin,as illustrated in Fig. 6.

Transient flowA step change in the production rate of a well causes apressure disturbance that propagates radially outwardinto the reservoir at a velocity determined by thehydraulic diffusivity of the total reservoir and fluidsystem:

(8)

where φ is the reservoir porosity and Ct is the compress-ibility of the total reservoir and fluid system. The veloc-ity of propagation is independent of the magnitude of thechange causing the disturbance. This is analogous to theobservation that the velocity of ripples caused by throw-ing a pebble into a pond is independent of the size of thepebble.

There are two basic types of pressure transient tests:n Pressure drawdown tests are performed after the well

has been shut in for a period sufficient to establishstatic pressure conditions. The well is opened and pro-duced at a steady flow while the pressure (and option-ally the rate) change is observed at the sandface.

n Pressure buildup tests are performed after the wellhas flowed for a period sufficient to establish a radialflow regime. The well is closed while the pressure(and optionally the rate) change is observed at thesandface.

Fundamentals of Formation Testing n Overview 9

Figure 5. Pressure profile in radial flow regime.

Pressure

0 100 200 300 400 500

Distance (ft)

pwf

pi

Pressure distribution due to radial flowthrough a homogeneous formation

Half total pressure drop

15 ft from wellbore

Figure 6. Comparison of pressure profiles with and withoutskin damage.

Qkh p p

r

r

i wf

e

w

=−( )

7 08..

µln

p p p p

r

r

r

r

r i i wf

e

e

w

( ) .= − −( )

ln

ln

kCtφµ

,

pi

k1<k0Pressure distributionwith skin damagePressure distributionwithout damage

pwf

Pressurepskin

The “open” (or “flow”) and “close” (or “shut-in”)cycles can be generated at the surface by changing thechoke or closing the master valve, or downhole by usinga downhole shut-in valve or a wireline tester flow controlschedule.

Classic transient test analysis is based on solutions tothe partial differential equations describing fluid flowthrough porous media in the period during which theflow around the wellbore is radial and has not encoun-tered any distant reservoir heterogeneities or bound-aries.

The radial diffusivity equation attempts to model theradial propagation of pressure through a reservoir ofspecified characteristics (permeability, porosity, com-pressibility), where a fluid of specified properties (den-sity, viscosity, compressibility) is flowing. In the diffusiv-ity equation, time is also a variable, which allowspressure modeling to be made both as a function of timeand of distance from an observation point (typically thewell).

The radial diffusivity equation results from a combi-nation of three formulas:n Darcy’s law, presented in Eq. 3.n The mass conservation equation:

(9)

where ρ is the density of the flowing fluid and t is therunning time.n The equation-of-state of a slightly compressible fluid:

(10)

By making certain assumptions, the system shown inEqs. 3, 9 and 10 simplifies, and the diffusivity equationcan then be expressed in its most common form andapplied to transient pressure testing.

(11)

The assumptions governing the validity of the radialdiffusivity equation are as follows:n Isothermal conditions exist.n There are negligible gravitational effects.n The flowing fluid is single phase.n The reservoir is homogeneous, isotropic, incompress-

ible and of constant porosity.n The permeability is independent of pressure.

n The fluid viscosity is constant and independent ofpressure.

n The pressure gradients are small.n The fluid is slightly compressible.n The flow is laminar (the velocity has no component

normal to the flow).Whether the final assumption is satisfied depends on

the Reynolds number for reservoir flow. The Reynoldsnumber, NRe, (Eq. 12) is a dimensionless function of fluidvelocity v, fluid density ρ, fluid viscosity µ and cross-sectional diameter of flow d stated in oilfield units as

(12)

At values of the Reynolds number less than 2000, theflow is laminar. This is the most common situation inpractice. In some very high-rate oil wells, however, andin many gas wells (where fluid viscosity is low), theReynolds number surpasses the 2000 to 3000 transitionalrange, and the flow becomes turbulent. The diffusivityequation can still be used in those situations, providedpseudofunctions of pressure and time are considered toaccount for the turbulence of the flow.

The radial diffusivity equation can be solved in manyways. One of the most useful results applies to transientradial flow, where flowlines are horizontal, perpendicu-lar to the wellbore and convergent to it. In this case, theradial diffusivity equation is approximated to

(13)

where rD, tD and pD are dimensionless variables relatedto the corresponding physical parameters r, t and p by

(14)

(15)

and

(16)

∆ t and ∆p are the elapsed time and pressure change,respectively, referenced to the end of the previous shut-in or flowing period.

10

d

dt r

d rv

dr

φρ ρ( )=

( )1,

Cddpt = 1

ρρ

.

d p

dr rdpdr

C

kdpdt

t2

2

1+ =φµ

.

Ndv

Re = 7742ρ

µ.

pt

rD

D

D

=

+

12

0 809072

ln . ,

rrrDw

= ,

t tk

C rD

t w

= ∆φµ 2

,

p pkh

QD = ∆ 2πµ

.

The formulation of pD enables us to calculate thepressure value following an initial disturbance (thedrawdown) at any point in space and time near theorigin of the disturbance (i.e., the well).

An interesting result is that, at any specific time, thepressure disturbance is inversely proportional to ln(rD

2).Thus, the magnitude of the disturbance is at a maximumnear its origin (the wellbore) and rapidly diminishesaway from the wellbore. Lower-permeability reservoirswould generate lower diffusivity and transmit pressuredisturbances more slowly. Also, because the expressionof pD includes the reservoir deliverability kh/µ, thehigher the reservoir deliverability, the smaller the pres-sure differentials and vice versa. This explains why it isdifficult to measure significant pressure differentials inhigh-deliverability reservoirs, where high-resolutionpressure gauges must be used.

Another meaningful result concerns the radius ofinfluence of a pressure disturbance as a function of time.It is intuitive that the longer the well test, the deeper isthe region investigated. The derivation of the dimen-sionless pressure pD shows that the radius of influence ofa pressure disturbance is proportional to the square rootof time (in oilfield units):

(17)

This theory is shown schematically in Fig. 7, whichexplains why testing wells to observe distant reservoirboundaries rapidly becomes prohibitively expensivebecause of the time involved.

The following chapters describe the measurementtechnologies and the interpretation techniques of for-mation testing.

Fundamentals of Formation Testing n Overview 11

Figure 7. Propagation of a pressure disturbance as a functionof time.

Ps

Pwf1

Pwf2

Pwf 3

Pwf 4

Pwf 5

Pwf 6

rd1 rd2 rd 3 rd4 rd 5 rd 6 rd…

log t

t = 1 ft

t = 100 dayst = 10 days

t = 1 dayt = 1 hrt = 1 in.

rkt

Cit

=

948

0 5

φµ

.

.