n Spring2005

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NEWS

1

Engineering and Business News from Fekete Associates Inc. Spring 2005

Fekete Introduces Numerical Models Into F.A.S.T. - | Tech Talk: Perforation Inflow Test Analysis - | What’s News at Fekete -p1 p2-3 p4

INSIDE THIS ISSUE

Fekete Introduces Numerical Models into

F.A.S.T. Software

Analytical solutions have been theunderpinnings of Well Test Analysis and Rate

Transient Analysis. They have served us well,

and continue to do so. Modifications such as

pseudopressures, material-balance-time and

p s e u d o t i m e h a v e e x t e n d e d t h e i r

applicability. However, these traditional

methods are not always suitable for solving

complex systems such as heterogeneous

reservoirs and multiphase flow. In such

circumstances, numerical solutions are more

appropriate.

A U T H O R

Louis Mattar is presidentof Fekete Associates Inc.

Examples where Fekete has found numerical

solutions to be more suitable than

traditional analytical solutions include:

Production data analysis of oil reservoirsbelow bubble point (F.A.S.T. RTA™).

Well test analysis of oil wells below bubblepoint (F.A.S.T. WellTest™).

Transient flow of gas and water in coal bedmethane systems (F.A.S.T. CBM ™).

Injection of acid gas in depleted gas

reservoirs or in aquifers (F.A.S.T. VirtuWell™).

Multi-well heterogeneous reservoirs(F.A.S.T. PoolSim ™).

The next release of F.A.S.T. RTA™ will include

a numerical model that has been tailored for

pressure transient and rate transient

analysis. The first set of numerical models to

be released will be the multiphase radial

model, composite radial, edge-water drive

and hydraulically-fractured well. Features

include wellbore storage, skin, and pressure-

dependent permeability. Non-Darcy flow in

the fracture and the reservoir, local grid

refinement, horizontal well models, and

wellbore hydraulics will be in future releases.

Numerical models are being constructed so

that they can be easily customized for

Fekete’s various software programs. For

example, when implemented into F.A.S.T.

CBM™, the numerical simulator will be

capable of handling transient, multi-phase

flow as well as gas desorption and matrix

shrinkage. Another example is in F.A.S.T.

VirtuWell™, where the acid gas injectionmodel will incorporate equations of state.

Numerical models can be quite complex and

it is no trivial task to make them easy to use.

Because Fekete software is written for use by

our engineers on consulting projects, we will

ensure that the implementation is user-

friendly and practical. We will generate grids

automatical ly , incorporate de faul t

correlations for fluid PVT data and simplify

the relative-permeability data manipulation.

Wherever possible, results and trends will be

presented visually to help the engineer arrive

at a meaningful interpretation.

Fekete Introduces Numerical Models

DON'T KNOW WHO TO CALL?

DO YOU NEED INFORMATION OR SUPPORT?

HELP IS HERE FOR YOU!!

Hello! I'm Melissa Gillard, a 9 year oil patch sales veteran. A few months ago I had the privilege of joining Fekete Associates Inc. as their customer service specialist.

On behalf of Fekete I want you to know that YOU, our clients, are important to us. It is my goal tohelp you get the service you both need and deserve. If you are unsure of who to call, PLEASECALL ME. I will help you reach the person who can answer your questions, walk you through atechnical issue, arrange for training or setup a demonstration for one of our software programs. Iam here to help you with any of your needs, so please feel free to contact me by either phone oremail at: (403) 213-4229 | 1-800-645-2488 (toll free N.A.)| [email protected]

Unparalleled Customer Service

Melissa Gillard Is Fekete’sNew Customer Service Specialist

8/20/2019 n Spring2005


Flow Regimes

It is obvious that the data for PITA is significantly influenced bwellbore storage. It is also evident that the data is directlinfluenced by the flow capacity (k h) and skin. The critical par*of any analysis is distinguishing between the data that idominated by wellbore storage (afterflow), and the data that idominated by the reservoir response (reservoir flow).

Just as in traditional well testing, the best way to differentiatbetween these flow regimes is to plot the derivative. However, thderivative for PITA is different from the traditional derivative o

well testing. The PITA derivative, also known as the “impuls2derivative,” is defined as t dp/dt, the product of the square of timand the well known primary pressure derivative (PPD). A typicaderivative is shown in Figure 2. It illustrates that the early-timdata (wellbore storage) has a slope of 2 (well test derivative has slope of 1), and the late-time data (reservoir flow) has a slope of (flat line– same as well test derivative).

Once the derivative has been plotted, it is easy to recognize reservoir flow exists. If it does, reservoir pressure anpermeability can be determined. At least some of the late-timdata should fall on the flat part of the derivative, as shown in

Introduction

Well testing has served the industry faithfully for decades as

the primary and most reliable means of:

Quantifying deliverability

Characterizing the reservoir

Evaluating the condition of the well

However, for the last few years, oil and gas producers have been

searching for alternatives that would yield the desired

information in less time, in a more environmentally friendly

manner, and at a cheaper cost than conventional well tests.

The trend has inevitably been towards tests of shorter

duration. Although it is accepted that results from short tests

may not be as reliable as those from conventional well tests, it

is reasonable to accept that the results are valuable in assisting

with decisions about field development, when an increased

margin of error can be tolerated.

Offshore, in addition to the potentially exorbitant cost of

testing (several millions of dollars), the drive towards “green”

(shorter) tests is fuelled by environmental considerations such

as flaring of hydrocarbons. In North America, the marginal

economics of low deliverability wells requires the use of

“cheap” tests. Either way, there is an increasing trend towards

green tests to replace conventional well tests.

One such green test consists of simply allowing the well to flow

into the closed wellbore after perforating (closed chamber

test). The buildup pressure data is collected for a period of

hours or days, depending on the reservoir’s flow potential.

These tests have been variously called: Slug test, Surge Test,

Perforation Inflow Diagnostic (PID), or Closed Chamber Test.

flow rate is not measured but can be estimated using closedchamber calculations, provided the assumption of single-phaseflow can be justified. Figure 1 shows the typical profiles omeasured pressure and calculated gas influx rate for a

“perforation test” of a gas well. As shown here, the influx ratedeclines rapidly.

Theory

Following a critical review of the literature, and in collaboration

with Dr. Mehran Pooladi-Darvish of the University of Calgary, we

have developed a complete and systematic analysis that yields an

estimate of initial reservoir pressure, permeability and skin. We

have called this procedure “Perforation Inflow Test Analysis”(PITA). Two publications containing full technical details have

been submitted for presentation at the CIPC in Calgary (June

2005) and at the SPE ATCE, in Dallas (October, 2005).

Starting with the fundamental equations in Laplace space, wederived early-time and late-time solutions of the transientpressure behaviour. Early-time data is used to estimate skin, andlate-time data is used to estimate initial pressure andpermeability. In gas wells, the pressure data is usually measuredat the wellhead and converted to bottomhole. This conversion isprimarily due to hydrostatic head because the influx rate into thewellbore diminishes rapidly (and friction is negligible). The influx

A

U T H O R

2

Marty Santo is a senior technical advisor at Fekete andleads the F.A.S.T. WellTest™ development team.

Minimum Test Duration: Start of Reservoir Dominated Flow

TECH TALK: Perforation Inflow Test Analysis (PITA)

8/20/2019 n Spring2005


Figure 2, in order to get a reliable analysis. If this data exists, thenskin can be calculated from the early-time data. If there is noreservoir flow, then an unique interpretation is not possible.

Analysis

In traditional well test interpretation, we start analyzing the datafrom early-time to late-time. In PITA, we start with late-timedata to obtain reservoir pressure and permeability. We thenanalyze the early-time data (where the derivative slope is 2) toobtain skin. Even though a complete analysis can be obtainedfrom the derivative plot alone, it is useful to generate the inversepseudo-time plot to confirm the analysis.

The fundamental equations were derived in terms of liquid flow(pressure and time). However they are easily converted to gasflow by replacing pressure with pseudo-pressure and time withpseudo-time.

A summary of the procedure for analyzing a Perforation InflowTest for a gas well is presented below:

1. Convert measured wellhead pressure to sandfacepressure (in some tests, the sandface pressure ismeasured directly).

2. Convert sandface pressure to pseudo-pressure(Ψ).

3. Convert time to pseudo-time (t ).a2

4. Calculate impulse derivative (t d Ψ /dt) and plot versus ta

on log-log scales (Figure 2).5. Determine start of late-time reservoir dominated flow

(Figure 2).6. Analyze late-time data by plottingΨ vs 1/t on reversea

Cartesian coordinates (Figure 3). A straight line throughthe valid late-time data gives permeability and reservoirpressure from:

3

7. Analyze the early-time data by plotting Ψ vs t ona

Cartesian coordinates (Figure 4). If permeability has firstbeen determined from step 6, skin can be obtained fromthe slope of a straight line, anchored on Ψ , the minimumw0

wellbore pressure (cushion pressure), and passingthrough the valid early-time data (corresponding toderivative slope = 2)

sV

t kh

w

awi

ww

)2.141)(24(

)( 00

ψ ψ ψ ψ

−+=

a

wiw

iw

t kh

V

2

)()2.141)(24(0

ψ ψ ψ ψ

−+=

The analysis procedure is well grounded in theory. We now have amuch better sense of the interpretation and the validity of theseshort tests because of our clear understanding of the flowregimes. In practice, we know that the longer we flow, the better

the results. Therefore, it is important to validate the PITA resultsby comparing them with those from other tests (e.g. permeabilityobtained from longer flow/buildup or reservoir pressure obtainedfrom static gradient). Because of well cleanup and other suchconsiderations, the value of permeability and skin can bedifferent between various tests. Until we have enough experienceto determine to what extent PITA can be relied on, Feketerecommends comparing results as often as possible, and weencourage analysts to publish their results.

Conclusions

1.We have developed a systematic and comprehensive analysisof data obtained from Perforation Inflow Tests.

2.We have developed clear diagnostics to identify when theanalysis is applicable and when the results are reliable.

3.When the influx is very small, is it because of lowpermeability or is it formation damage? If sufficient data isavailable, it is possible to determine the difference betweenlow permeability or damage. If the data quality is poor, or thetest duration is inadequate, one must rely on “gut feeling” or experience, or consider more elaborate testing procedures,such as downhole shut-in.

4. Validate PITA by comparing results to those obtained fromother tests.

Fekete’s Well Testing ServicesFekete recognizes that well test interpretation means more thanpressure transient analysis. We analyze approximately 1000well tests annually and develop leading edge software toperform comprehensive analysis of the pressure/flow datacaptured in the field. Please call Reza Ali, manager of WellTesting, or Marty Santo at (403) 213-4200 to discuss youranalysis requirements.

TECH TALK: Perforation Inflow Test Analysis (PITA)

8/20/2019 n Spring2005


What’s News at Fekete

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Video #5 Release in April

Ralph McNeil, senior technical advisor at Fekete, presents “GasGathering System Modeling: A 5 Step Approach to ProductionOptimization.” This highly practical approach is based on more than20 years of modeling experience. It takes the modeling process andbreaks it down into the:

Pipeline Pressure Loss Match: Identify and categorize problempressure losses.

Well Deliverability Match: Analyze well deliverability potentialand understand well producing dynamics.

Compressor Capacity Curve Match: Reconcile capacity versusfield deliverability issues.

Base Case: Brings steps 1 through 3 together and confirms themodel works at the match point.

Production History Match: Compare base case forecast toproduction history for each well to validate the original gas-in-place input.

Breaking the modeling process into manageable sub-topics makesidentification and evaluation of optimization opportunities such asline looping, well unloading and installation of booster compressioneasier. Since results can be generated before project completion,recommendations can be acted upon more quickly than usingprevious modeling practices.

Video 5 will be released in April, 2005. Past video releases are availableon our website at www.fekete.com.

Fekete Associates Inc. | Phone: 403-213-4200 | Toll Free: 1-800-625-2488 | Fax: 403-213-4298 | Email: [email protected] | Website: www.fekete.com

FieldNotes & Electronic Data Acquisition

F.A.S.T. FieldNotes™ has been used to electronically monitor

production tests in remote locations such as the Arctic.

Configured to work with electronic McDaniel and turbine meters,

readings are directly transmitted to F.A.S.T. FieldNotes™, in real

time, and the rates automatically calculated and trended on plots.

Fekete has developed the F.A.S.T. ModBus SnapIn™ interface to

communicate with any electronic data instrumentation. It doesn’t

matter whether you have industry standard RTU instrumentation

or specialized proprietary well site electronics. Imagine

downloading subsurface recorders or monitoring wellheadinstrumentation in real time, all from the same piece of software.

The F.A.S.T. ModBus SnapIn™’s versatile configuration allows for:

Real time tubing & casing pressure/temperaturemeasurements.

Electronic reading as input for any F.A.S.T. FieldNotes™ metertype (eliminating the need for time consuming and costly metercalculations within the RTU).

Multiple-sensor production test monitoring.

For proprietary well site electronics, ask your instrumentation

vendor if they have a SnapIn for their electronic equipment

(SnapIn developer's tool kits are available). SnapIns not only work

in F.A.S.T. FieldNotes™, they are also supported in F.A.S.T.

ValiData™ and F.A.S.T. WellTest™.

F.A.S.T. FieldNotes™ is currently being rewritten to take

advantage of our new data table format, our engineering plotting

package and Fekete's fully customizable reporting package.

Features include: Flow-Shut-in Events (eliminates the need to

zero meters), Variable Gas Properties to account for frac f lowback,

Injection Meters and Improved Multi-threaded real time data

acquisition (SnapIns).

Need Custom Programming for Your Fekete Software?

Fekete uses custom programming to connect our software, that uses

production data (such as F.A.S.T. RTA™ or F.A.S.T. CBM™), to an

internal database. This creates a direct connection to the data and

minimizes the work required to analyze it. With database integration

our software becomes more efficient for our customers to use. Custom

programmed software can also be used to handle/manipulate outpu

data.

For more information, contact Chris Janostin at (403) 213-4200.

PAS File Submission

Fekete software is continually upgraded to be compliant with

current AEUB PAS file formats.

F.A.S.T. FieldNotes™ takes raw measurements in the field,calculates rates, and certifies and generates a PAS-PRD file.

F.A.S.T. ValiData™ imports pressures and rates, performs staticgradient analysis and prepares data for well test analysis,generating PAS-GRD, PAS-TRG or PAS-DST files.

F.A.S.T. WellTest™ imports PAS files, analyzes the data, andgenerates the final PAS-TRG file.

Need Custom Programming for Your Fekete Software?

Fekete uses custom programming to connect our software, that uses

production data (such as F.A.S.T. RTA™ or F.A.S.T. CBM™), to an

internal database. This creates a direct connection to the data and

minimizes the work required to analyze it. With database integration

our software becomes more efficient for our customers to use. Custom

programmed software can also be used to handle/manipulate outpu

data.

For more information, contact Chris Janostin at (403) 213-4200.

n Spring2005

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