(2) gas reservoir engineering application toolkit

7/15/2019 (2) Gas Reservoir Engineering Application Toolkit

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GREAT

Gas Reservoir Engineering Application Toolkit

Version 1.2

SiteLark LLC

Technical Support: [email protected]

Website: www.sitelark.com


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GREAT


1. Introduction............................................................................................ 3

2. Descriptions ............................................................................................ 4

2.1 Decline Curves................................................................................. 4

2.2 Flash Calculation............................................................................. 4

2.3 Fluid Property Calculation ............................................................ 5

2.4 Isochronal Testing........................................................................... 5

2.5 Bottomhole Pressure Calculation.................................................. 6

2.6 Gas Pressure Transient Analysis................................................... 62.7 P/Z Analysis ..................................................................................... 7

2.8 Volumetric Calculation................................................................... 7

2.9 Zfactor Calculation ......................................................................... 8

2.10 Gas Reservoir Simulator ................................................................ 8

3. Examples ................................................................................................. 9

4. General Instructions ............................................................................ 17

4.1 Installation ..................................................................................... 17

4.2 Execution........................................................................................ 18

5. Troubleshooting ................................................................................... 18

6. Contact .................................................................................................. 20

7. Book References................................................................................... 20


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GREAT


1. Introduction

GREAT is an Excel-based complete set of gas reservoir engineering modules e.g. p/z,Hurst-Everdingen water

influx calculation, gas

sampling and phase

behavior, gas injection,gas deliverability

prediction, single-phase

gas simulator and muchmore. The motivation

behind this tool is toempower engineers witha host of cost-effective

yet useful petroleum

engineering tools that are

typically used to solveday-to-day reservoir

engineering problems.

Although very useful inits own right, there is

always a need to custom-

built modules for veryspecific problems at hand. GREATs Excel framework and modular structure

undeniably provide a foundation for further modifications.

GREAT has two versions, namely, Standard and Premium. The various tools in the

Standard toolkit are:1. Decline Curves2. Flash Calculation3. Fluid Property Calculation4. Isochronal Testing5. Bottom hole Pressure Calculation6. Gas Pressure Transient Analysis7. P/Z Analysis8. Volumetric Calculation9. Zfactor Calculation

In addition, the Premium version contains the following toolkit

1. Gas Reservoir Simulator


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2. Descriptions

The following sections describe each of the tools in some detail. Thesedescriptions explain the purpose and the algorithms used in the models. By

reading this section users should have a better understanding of the models.

Additionally, they can further read about them in the literature1-3 should they beinterested to lean more about their intricacies.

2.1 Decline Curves

Three distinct decline curve methods (Exponential, Harmonic and Hyperbolic)are presented for estimating gas recoveries and performance from long term gas

production data either from individual wells or entire fields. Along with the

reservoir and fluid properties, the program calculates initial rate, decline rate,ultimate recovery and time to abandon. The charts depict the match of actual

data points with their corresponding model values.

The decline curve tool has the following features:

o Decline curve analysis is based on Exponential, Harmonic and Hyperbolicdeclines.

o Hyperbolic decline can be matched by adjusting model parameters (b andD).

o Model is fit with linear regression.

2.2 Flash Calculation

In mutlicomponent flow it is often desired to check whether the mixturecomposition is stable i.e. whether it would persist as a single phase or split into

two-phases in equilibrium. If latter is true then the compositions of each of these

phases are important to know. Such situation can arise in reservoir flow or at thesurface when separator output is needed as a design parameter or to meet

contractual agreement (quantity of NGL and dry gas). This module calculates the

fraction composition of each components in liquid and gaseous phase given the

feed compositions, pressure and temperature. An Equation of State (PengRobinson and Soave Redlich Kwong) based approach is utilized to perform the

computation. In particular, under the assumption of equilibrium, fugacity of each

component is equated in both phases and solution is achieved using SuccessiveSubstitution iterative method. Additionally, this module can be further used to

create phase envelops (P-T diagrams).

The flash calculation tool has the following features:


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o Multicomponent flash routineo Stability analysis to catch stable, single-phase mixtureso EOS-based (SRK and PR) fugacity computationo Successive substitution method of convergenceo Easy to use framework

2.3 Fluid Property Calculation

As is well known, laboratory experiments are the most reliable way to calculate

fluid properties. In their absence however, correlations presented in thisworkbook could be very useful. For dry gas, based on component properties the

specific gravity is first computed. This is then used to calculate the Zfactor and

Bg (Note: This module reports Zfactor for a given temperature and pressurecondition. Use other module (Sec. 2.9) to calculate Zfactor for a range of

pressure values ). On the other hand, the surface separator production data is also

incorporated in the case of wet gas, to compute the fraction of dry gas and itscorresponding mixture properties.

The fluid property tool has the following features:

o Mixture critical temperature and pressureo Molecular weight and specific gravity of gaso Zfactor, Bg and viscosity of gaso Wet gas property calculation based on surface separator production

information

2.4 Isochronal Testing

Three of the most common deliverability testings are presented, namely, flow-

after-flow, isochronal and modified isochronal testing. Since single point testing

is a special case of one of these, this is not implemented separately. Based on theconstant terminal rate solution of pseudopressure based homogeneous, isotropic,

radial flow diffusivity equation, various methods fit models to observed

production and pressure data. The methods differ in the representation ofpressure drop and rate relationship via various model parameters. Linear

regression is used to compute these model parameters which are then used to

predict AOF (for various estimation of static reservoir pressure). This AOF canbe used to compute IPRs and also be reported to regulatory agencies to scheduleprorating. Estimation of stabilization time, permeability and skin factor are also

reported.

The isochronal testing tool has the following features:

o Isochronal, modified isochronal and flow-after-flow tests with charts


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o Rawlins-Schellhardt and Houpeurt analysis techniqueso Brar-Aziz and Stabilized-C methods where stabilization point is hard to

obtain

o Pseudopressure based computationo Reports AOF under multiple static pressure conditions

2.5 Bottomhole Pressure Calculation

This tool computes the bottomhole static/flowing pressure given the surface

pressure, temperature and gas properties. Each segments of the well (vertical orinclined) is discretized into compartments. The number of compartments is

denoted by the "number of increments" input data. Starting from the surface

static/flowing wellhead pressure and temperature, the algorithm iterativelycalculates the pressure in each compartment. It is iterative because Zfactor and

friction factor which are needed to calculate the next pressure are functions of

pressure. Therefore, both these parameters and the pressure need to bedetermined simultaneously such that the pressure gradient equation is satisfied.Each compartment pressure is sequentially computed as described above. The

final pressure is the static/flowing bottomhole pressure for this well.

The bottomhole pressure calculation tool has the following features:

o Incremental pressure (static and flowing) calculation based on surface P andT

o IPR based on the reservoir propertieso VLP computation for varying tubing sizes

o Wells can have multiple segments of varying lengths and inclinations

2.6 Gas Pressure Transient Analysis

Over the years, starting from the Ei-function solution of radial, single-phase

diffusivity equation for homogeneous reservoirs, analysis techniques have been

developed to analyze drawdown and buildup test results. This module implementsthree such techniques with special consideration to gas well testing. First,

drawdown testing methodology is implemented where based on rate and pressure

response both skin and permeability can be estimated. Second, methodology toestimate non-darcy flow coefficient based on pressure response of a single yethigh rate is incorporated. This iterative technique calculates the composite skin

and then reports the individual components, non-darcy skin and darcy skin.

Finally, a buildup test analysis procedure is also implemented. Here, the actualpressure vs time data is regressed to obtain a straight line which is subsequently

used to compute permeability and skin. All calculations are done based on

adjusted pressure i.e. modified pseudopressures.


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The bottomhole pressure calculation tool has the following features:

o Multirate drawdown test, single high constant rate drawdown for non-darcyflow and buildup test with prior constant rate production

o

Linear regression to fit straight lines to measured datao Estimate skin, permeability and non-darcy coefficiento User control to ignore points for linear regression

2.7 P/Z Analysis

In a gas reservoir with water influx the pressure behavior is affected by both gas

and water production and water influx. A typical P/Z straight lineapproximation is no longer valid. Eye-balling raw data and fitting a line to

account for the water influx may also lead to erroneous in-place numbers, in

some cases significantly. This module solves the problem numerically. Theproblem is framed such that for every time-step the gas material balanceequation is iteratively solved with water influx equation. A Newton's method is

employed for better convergence. P/Z are calculated and automatically plotted

against observed values. In practice, however, the first approximation of inputsmight not give a good match of P/Z. Appropriate inputs (e.g. in-place gas,

aquifer size and shape) can be manually changed to obtain a good match and

thereby converge to a meaningful and accurate value of OGIP and aquifergeometry.

For geopressured reservoirs normal OGIP computation is supplemented by two

other methods (geopress I and geopress II). I is where formationcompressibility is known and II is where formation compressibility is calculated

as part of the analysis.

The P/Z analysis tool has the following features:

o P/Z calculationo Water influx calculationo Comparison to actual production datao Iteration between gas MB and water influxo Geopressured reservoirso

Newton's Method for Faster Convergenceo Output Numerical and Graphical Format

2.8 Volumetric Calculation

For dry gas, volumetric algorithm calculates the volume at initial conditions.

For wet gas and gas condensate cases, the initial productions at separator


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conditions are also required. At most, three stage separators can be used. Based

on the data, dry gas fraction at reservoir conditions can be estimated.Subsequently, the algorithm computes the recoveries at various abandonment

pressure conditions. Similar approach is taken for water drive reservoirs where

vertical sweep efficiency and residual gas saturation are additionally required.

Output section presents the OGIP of dry gas or additionally condensatevolumes.

The volumetric calculation tool has the following features:

o In-place volumes of dry gas and condensateo Depletion and water drive reservoirs can be modeled

2.9 Zfactor Calculation

Based on the chosen EOS ("Z-Calc-EOS"), gas Zfactor of the mixture iscalculated by solving the appropriate cubic equation. This is done for each ofthe pressure points. Both Zfactor and Bg are reported as a function of pressure.

On the other hand, "Z-Calc-Grav" calculates the same properties using

correlations.

The Zfactor calculation tool has the following features:

o Bg calculationo Zfactor calculationo Viscosity calculation

2.10 Gas Reservoir Simulator

Bulk of the reservoir software available in the market caters broadly to two

groups of users. On one hand, well testing software helps analysts to determinethe type of the reservoir (single or double porosity), type of boundaries (faults,

aquifers etc.), reservoir and wellbore properties etc. However, this is typically

achieved in a constant rate or bottomhole pressure situation and within areservoir with homogeneous properties. On the other hand, there are full-blown

reservoir simulators that can handle any level of sophistication in terms of type

of fluids, reservoir heterogeneity, wellbore and surface constraints, drive andphase behavior mechanisms. Unfortunately, not only these products areexpensive, but they also demand a high level of expertise to run them.

Moreover, in many instances the simulator seems to be an overkill. The gas

reservoir simulator is strategically placed to bridge this gap. It provides ameans to solve practical problems, especially when near wellbore flow potential

has to be resolved.


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Single phase gas diffusivity PDE is solved in two dimensions. Control Volume

Finite Element method is utilized to discretize the partial Differential Equation(PDE). The difference equations are linearized using Newtons method.

Reservoir properties (permeability, porosity and initial pressure) can be

prescribed to be spatially varying. Well boundary conditions can be imposed as

either constant rate or pressure. However, constant rate wells will beautomatically switched to constant pressure when it hits the pressure constraint

(maximum or minimum). Moreover, well recurrent data can be input to

account for varying production history and attributes of wells. The pressure andrate histories of each well in the simulation model are reported.

The gas reservoir simulator tool has the following features:

o Single phase gas reservoir simulatoro Spatially variable properties on a fixed grido Fully implicit and CVFE formulationo

Impose variable rate conditions using "Flush Well Recurrent Data" buttono Output well rates and pressureso Easy to use framework

3. Examples

The value of a gas asset will typically be gauged by the following reservoir relatedquestions:

a. What are the properties of my gas?

b. What is the quality of my produced stream?c. How big is my reservoir?d. Do I have water influx?e. What are the properties of my reservoir?f. How much can I produce from a typical well?g. How long can I produce from my asset?h. Can I integrate all available knowledge of fluid and reservoir properties and

determine the longer term productivity potential of my wells in the presence of

offset wells?

Following exercises show how the different toolkits within GREAT can be used to

systematically answer the above questions.

What are the properties of my gas?

Following figure shows a typical gas composition measured from a producedstream. The respective critical temperature, pressure and molecular weights are

also listed. Common pure component properties can easily be found from

literature.


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Component

Name

Liq Mole

Fraction

Vap Mole

Fraction

Molecular

Weight Crit Temp R Crit Press psia

N2 00.0236

28.013 227.16 493.1

CO2 0 0.0164 44.01 547.58 1071

H2S 00.1841

34.08 672.35 1306

CH4 0 0.77 16.043 343 666.4

C2H6 0 0.0042 30.07 549.59 706.5

C3H8 0 0.0005 44.097 665.73 616

i-C4H10 0 0.0003 58.123 734.13 527.9

n-C4H10 0 0.0003 58.123 765.29 550.6

i-C5H12 0 0.0001 72.15 828.77 490.4

n-C5H12 0 0.0001 72.15 845.47 488.6

C6H14 0 0.0001 86.177 913.27 436.9

C7+ 0 0.0003 114.231

Figure 1. Component mole fractions and properties

The fluid property module (FluidProp.xls) was then utilized to compute the mixture(gas) properties, Zfactor, Bg and viscosity at 2000 psia and 200 F. The results are shownin Figure 2.

Figure 2. Output from GREAT (FluidProp.xls)

Often times it is desired to get the properties (Zfactor, Bg, viscosity etc.) as a function of

pressure. This is typically true to create input PVT data sets for simulation. In such an

occasion it is beneficial to utilize the Zfactor.xls toolkit of GREAT. This toolkit cancalculate these properties both from correlations and gas specific gravity (whencompositions are not available) or from EOS based computations. Following table

exhibits a typical output of this toolkit.


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Figure 3. GREAT output showing gas properties as a function of pressure

What is the quality of my produced stream?

Related to gas properties it is often desirable to know if the produced stream would split

into liquid and gas at separator pressure and temperature conditions. Given the feed

compositions and the properties of the components, Flash.xls can be used to compute theliquid and vapor mole fractions, if the feed stream splits into two phases. Also, the

fraction of each component in liquid and vapor can also be computed. Alternatively, thistool can also be used to generate P-T diagrams to understand the reservoir fluid

conditions during a recovery path (pressure decline).

How big is my reservoir and do I have water influx?

These two questions are better answered in conjunction with each other. The pzCalc.xlstoolkit can be used to determine the in-place gas based on production data and fluid

properties. The algorithm incorporates effects of water influx as well. The size of theaquifer can be adjusted to obtain a desirable match. Figure 4 shows an example match of

model to observed pressure and production data.


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4700

4750

4800

4850

4900

4950

5000

5050

5100

5150

0 5 10 15 20 25

Gp (Bcf)

P/Z

P/Z (Obs)

P/Z (Calc)

Figure 4. P/Z match of observed data points to model as a function of cumulative production

Furthermore, two additional algorithms are included for geopressured reservoirs. Thesealgorithms provide means to control number of points to choose to fit a straight line.

This manual adjustment of selection of points improves matching model to data.

Volumetreic.xls can be used to compute the gas in place based on volumetricassumptions. In case of wet-gas, based on additional information of initial production at

the surface, fraction of dry gas in the reservoir can also be calculated. Additionally, if

volumetric sweep efficiency is known, then recovery factor in the presence of water

influx can also be obtained. Following is a typical output of Volumetreic.xls:

13Original Gas in Place

(Bcf)

10.488Original Gas

in Place (Bcf)

10.488Original Gas in

Place (Bcf)

10.354

Original

Condensate

in Place

(MMSTB)

0.173

Abandonment Dry GasDry Gas with

Water InfluxWet Gas

Pressure (psia) Reserves Recovery Factor Reserves Recovery Factor Gas ReservesRecovery

Factor

1500 4.339 0.414 7.619 0.726 7.485 0.726

1200 5.661 0.540 8.236 0.785 8.102 0.785

1100 6.096 0.581 8.439 0.805 8.305 0.805

1000 6.526 0.622 8.639 0.824 8.506 0.824

900 6.952 0.663 8.838 0.843 8.704 0.843

800 7.371 0.703 9.034 0.861 8.900 0.861

750 7.579 0.723 9.130 0.871 8.997 0.871

600 8.192 0.781 9.417 0.898 9.283 0.898

500 8.592 0.819 9.604 0.916 9.470 0.916

400 8.986 0.857 9.787 0.933 9.653 0.933

300 9.372 0.894 9.968 0.950 9.834 0.950

200 9.752 0.930 10.144 0.967 10.011 0.967

100 10.124 0.965 10.318 0.984 10.184 0.984 Figure 5. GREAT output showing recovery factors as a function of abandonment pressures and

OGIP

What are the properties of my reservoir?

PTransient.xls toolkit provides three different ways to estimate properties of reservoir(permeability, skin and non-Darcy flow coefficients), namely, multi-rate drawdown,


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single, high constant rate drawdown for non-Darcy coefficient and constant rate buildup

test. Depending upon the well management decisions (whether shut-in is feasible), oneor the other type of information is usually available. The algorithm calculates the

composite skin and reports the true skin as well.

Time (hrs) Pw f (psia ) Q (MSCF/D) PlotY Function PlotX Function

0 6287.1 4,855.980.01 6296.6 4,855.98 200,001.00

0.0149 6301.1 4,865.77 134,229.19

0.0221 6307.8 4,870.40 90,498.74

0.0329 6317.7 4,877.31 60,791.27

0.0489 6332.1 4,887.51 40,900.80

0.0728 6353.1 4,902.35 27,473.53

0.108 6383.5 4,923.98 18,519.52

0.161 6427.1 4,955.30 12,423.36

0.24 6488.6 5,000.21 8,334.33

0.356 6573.6 5,063.54 5,618.98

0.53 6687.9 5,151.02 3,774.58

0.788 6834.7 5,268.59 2,539.07

1.17 7011.8 5,419.43 1,710.40

1.74 7208.3 5,601.16 1,150.43

2.59 7405.9 5,802.44 773.20

3.86 7586 6,004.42 519.13

5.74 7738.7 6,188.12 349.43

8.53 7864.9 6,343.56 235.47

12.7 7971.4 6,471.80 158.48

18.9 8065.6 6,579.86 106.82

28.1 8153.2 6,675.31 72.1741.8 8234.4 6,763.96 48.85

62.1 8313.4 6,846.05 33.21

92.4 8389.6 6,925.82 22.65

137 8463.7 7,002.67 15.60

204 8534.9 7,077.34 10.80

304 8602.9 7,149.00 7.58

452 8666.6 7,217.37 5.42

672 8725.3 7,281.36 3.98

1000 8777.6 7,340.28 3.00

Properties

Permeability

(mD)0.030

Composite Skin -0.468

Non-Darcy Coeff

(D/MSCF)0.000E+00

True Skin

Figure 6. GREAT input and output for the pressure transient toolkit, reporting skin and non-darcy

flow information

How long can I produce from my asset?

Regulatory obligations mandate AOF reporting by operators. Isochronal.xls toolkitprovides three distinct algorithms, namely, Isochronal, modified Isochronal and flowafter flow tests. These methods, based on the test information (time, pressure and rate),

compute AOF as a function of static drainage pressures. Algorithms like Brar and Aziz

and Stabilized C methods are more useful when either stabilized point is not available or

it is not practical to get stabilized data because of very high stabilization time. Followingis a typical output from this toolkit.


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R a w l i n s-S c h e l l h a r d t H o u p e u r t

n (a v e r a g e ) 0 . 6 4 a 8 . 9 8 E + 0 5C 4 . 3 3 E -0 4 b (a v e r a g e ) 1 . 8 7 E + 0 4

A O F (M M S C F / D ) 2 3 4 . 1 9 A O F (M M S C F / D ) 2 1 2 .0 6

6 6

S t a t i c D r a i n a g e

P r e s su r e ( p s ia ) A O F (M M S C F / D )

S t a t i c D r a i n a g e

P r e s su r e ( p s ia ) A O F (M M S C F / D )

3 5 0 0 . 0 0 1 8 5 . 6 1 4 3 5 0 0 . 0 0 1 7 3 . 0 8 4

3 0 0 0 . 0 0 1 5 6 . 7 2 1 3 0 0 0 . 0 0 1 4 8 . 9 2 8

2 5 0 0 . 0 0 1 2 7 . 3 4 6 2 5 0 0 . 0 0 1 2 3 . 4 4 1

2 0 0 0 . 0 0 9 7 . 9 1 1 2 0 0 0 . 0 0 9 6 . 7 2 9

1 5 0 0 . 0 0 6 9 . 0 5 6 1 5 0 0 . 0 0 6 9 . 0 7 1

1 0 0 0 . 0 0 4 1 . 7 2 1 1 0 0 0 . 0 0 4 1 . 1 0 0

Figure 7. GREAT output showing AOF as a function of pressure for two different methods

PressureInWellbore.xls is another module in the toolkit that can be used to computestatic and flowing bottom hole pressures given the surface conditions. The wellbore can

be approximated by several segments of any lengths and deviations. In addition, this tool

can also be used to obtain Inflow Performance Curves and Vertical Lift Profiles. Figure8 shows such an example

0

1000

2000

3000

4000

5000

6000

7000

0 2 4 6 8 10 12 14 16

Rate (MMSCF/D)

BottomholePressure(psia)

IPR-1

IPR-2

IPR-3

IPR-4

IPR-5

VLP-1

VLP-2

VLP-3

VLP-4

VLP-5

Figure 8. GREAT output of IPR and VLP for an example well


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Finally, to answer the question of this section, DeclineCurves.xls can be used to fitobserved production data using Arps (Hyperbolic, Exponential and Harmonic declines)

methods.

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

450.00

0 1000 2000 3000 4000 5000 6000

Time (Days)

Rate(MSCF/D)

Actual

Model

Figure 9. Hyperbolic decline curve example from GREAT. Both D and b can be adjusted to fit the

observed rate data

Can I simulate my reservoir?

Standard version of GREAT provides all the above mentioned toolkits that among otherthings help determine fluid and reservoir properties, extent of the reservoir, volumes and

well deliverability. Sometimes it is necessary to witness the subject well performance in

the presence of other neighboring wells (interference effect) with spatially varying

reservoir properties. Moreover, this model typically needs to be calibrated to historicalproduction of the wells included in the study (history matching). This calibrated model is

then utilized to forecast the performance of the gas reservoir. To provide a seamless

platform to model this coupled, complex scenario of reservoir heterogeneity in the midst

of multiple well productions, GREAT-Premium provides Simulator xxx.xls, a gasreservoir simulator. The simulation model can be posed by introducing all the necessary

geologic, fluid and well attributes. Figure 10 shows the well input section of Gas

Reservoir Simulator (GRS).


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Max Number

of Wells4

Max

Number of

Recurrent2 Current

Recurrent2

Starting

Recurrent

Time150

Variable

Name

Well No.

(1)

Well No.

(2)

Well No.

(3)Well No. (4)

Well No.

(5)Well No. (6)

Well No.

(7)

Well No.

(8)

Well No.

(9)

Well No.

(10)

WellIndex1 2 3 4 0 0 0 0 0 0

Well Name

PROD-1 PROD-2 PROD-3 INJ-1

WellType

0 0 0 1 0 0 0 0 0 0

Rate (SCF/D)

1.00E+05 6.00E+04 6.00E+04 8.00E+04 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00

X-Direction

Location (ft)

333.33 933.33 1666.67 600 0 0 0 0 0 0

Y-Direction

Location (ft)

250 250 250 450 0 0

Radius (ft)

0.25 0.25 0.25 0.25 0 0 0 0 0 0

BHPMax (psi)

1060 1060 1060 8000 0 0 0 0 0 0

BHPMin (psi) 100 100 100 100 0 0 0 0 0 0

Skin0 0 0 0 0 0 0 0 0 0

X-Direction

Ending

Location (ft)

333.33 933.33 1666.67 1200 0 0 0 0 0 0

Y-Direction

Ending

Location (ft)

250 250 250 450 0 0 0 0 0 0

PI Factor 1 1 1 1 1 1 1 1 1 1

Num of

Completions1 1 1 1 1 1 1 1 1 1

Figure 10. Well input section of GRS. Notice that upto 10 wells can be simultaneously simulated

The simulator then prescribes the rates, bottomhole pressure and the well block pressure,

for each of the wells (see Figure 11).


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Time (Days)

Rate

(MSCF/D)

Pressure

(psia)

Block

Pressure

(psia)

Rate

(MSCF/D)

Pressure

(psia)

Block

Pressure

(psia)

Rate

(MSCF/D)

Pressure

(psia)

Block

Pressure

(psia)

Rate

(MSCF/D)

Pressure

(psia)

Block

Pressure

(psia)

0.01 100 5647.22 5992.13 60 5788.32 5995.28 0.00 6000.00 6000.00 0.00 6000.00 6000.00

0.02 100 5647.22 5992.13 60 5788.32 5995.28 0.00 6000.00 6000.00 0.00 6000.00 6000.00

0.03 100 5641.21 5986.07 60 5784.70 5991.64 0.00 6000.00 6000.00 0.00 6000.00 6000.00

0.69 100 5569.50 5913.77 60 5741.55 5948.28 0.00 6000.00 6000.00 0.00 5999.73 5999.93

4.34 100 5494.52 5838.13 60 5697.52 5904.03 0.00 6000.00 6000.00 0.00 5994.75 5997.27

23.64 100 5367.88 5710.44 60 5629.78 5835.93 0.00 5999.51 5999.51 0.00 5959.89 5963.59

43.64 100 5343.45 5685.80 60 5616.79 5822.88 0.00 5999.36 5999.36 0.00 5951.84 5955.5163.64 100 5266.87 5608.56 60 5574.97 5780.84 0.00 5997.82 5997.82 0.00 5915.43 5915.57

83.64 100 5250.57 5592.13 60 5565.82 5771.64 0.00 5997.40 5997.40 0.00 5907.15 5906.45

103.64 100 5188.13 5529.29 60 5528.45 5734.09 0.00 5994.47 5994.47 0.00 5870.44 5865.49

123.64 100 5174.39 5515.46 60 5520.07 5725.67 0.00 5993.73 5993.73 0.00 5862.14 5856.25

143.64 100 5118.40 5459.07 60 5483.97 5689.40 0.00 5989.25 5989.25 0.00 5825.69 5816.02

163.64 100 5105.99 5446.57 60 5475.93 5681.32 0.00 5988.15 5988.15 0.00 5817.53 5807.02

183.64 100 5058.52 5398.75 60 5485.51 5690.94 60.00 5605.83 5811.85 80.00 5878.77 5830.88

203.64 100 5047.13 5387.27 60 5477.53 5682.93 60.00 5604.39 5810.40 80.00 5870.71 5822.14

223.64 100 5006.80 5346.61 60 5484.35 5689.78 60.00 5538.33 5744.03 80.00 5876.52 5821.80

243.64 100 4995.86 5335.58 60 5476.48 5681.87 60.00 5536.56 5742.25 80.00 5868.57 5813.26

263.64 100 4959.91 5299.33 60 5477.85 5683.25 60.00 5486.45 5691.89 80.00 5866.79 5807.26

283.64 100 4949.40 5288.74 60 5470.11 5675.47 60.00 5484.39 5689.82 80.00 5858.98 5798.94

303.64 100 4916.90 5255.95 60 5466.98 5672.32 60.00 5441.34 5646.55 80.00 5853.00 5789.99

323.64 100 4906.72 5245.68 60 5459.36 5664.66 60.00 5439.03 5644.23 80.00 5845.31 5781.84

343.64 100 4876.39 5215.08 60 5452.67 5657.94 60.00 5399.76 5604.75 80.00 5836.25 5770.52

363.64 100 4866.45 5205.04 60 5445.17 5650.40 60.00 5397.21 5602.19 80.00 5828.69 5762.49

383.64 100 4837.44 5175.88 60 5435.69 5640.87 60.00 5360.54 5565.38 80.00 5817.30 5749.28

403.64 100 4827.69 5166.08 60 5428.31 5633.45 60.00 5357.79 5562.62 80.00 5809.88 5741.42

423.64 100 4799.94 5138.20 60 5416.69 5621.77 60.00 5323.16 5527.85 80.00 5796.77 5726.87

443.64 100 4790.41 5128.61 60 5409.42 5614.46 60.00 5320.22 5524.90 80.00 5789.47 5719.18463.64 100 4763.58 5101.64 60 5395.96 5600.93 60.00 5287.17 5491.71 80.00 5774.71 5703.26

Well No. 1 Well No. 2 Well No. 3 Well No. 4

Figure 11. GRS output of rates, well block pressure and bottomhole pressures for an example

simulation study

4. General Instructions

This section first describes the installation related instructions and then how to executethe different tools in the software.

4.1 Installation

There are no specific steps for installation for this package. The package comes as a

Windows zip file (WinZip is readily available as a freeware from sites like

www.zdnet.com). Once unzipped there are two top level directories; 1. GREAT-Standard and 2. GREAT-Premium, the second will appear only if the premium version of

the software is purchased. Inside GREAT-Standard there are nine directories, one each

for the modules as described in Section 2. Inside each of these directories are the

Excel files that are pertinent to that module. For instance, inside DeclineCurves

directory, DeclineCurves.xls is the driver workbook which when opened presents themodule. In some directories, e.g. Flash and Zfactor, there are other sub-directories and

batch files as well which for the purposes of execution (as described in the next Section)can be safely ignored. These directories are used to store intermediate files and the batch

files are used to run the executables residing in the same directory. Once again, the file

of interest is the Excel file. In particular, for the Flash directory it is Flash.xls and forthe Zfactor directory it is the Zfactor.xls file.


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4.2 Execution

Execution of the modules is equally simple and intuitive. Each module has an

introduction page (first page). This page is designed to provide three helpful sections i.e.

1. Features 2. Instructions and 3. Algorithm. Features briefly list the crucial

characteristics and deliverables of the module. Instructions guide the user through thetypical steps to input, execute and observe outputs. Finally, algorithm section briefly

describes the numerical/analytical techniques utilized to obtain the solution. Interested

users are requested to review the References (listed at the bottom of this document) forfurther information on algorithms and methods.

In general, the WHITE cells are for input, GREY cells are for outputs from the module(do NOT enter any values here since they will be overwritten) and finally, cells of any

other color are either headers or variable description. Typical steps are to enter

admissible values in the WHITE cells, click on the execution button specific to themethod of choice and observe the outputs, both tabular and graphical. For some

algorithms, iterations are needed to converge to the solution. For instance, in pzCalc.xls,one has to perturb aquifer geometry, gas reservoir extent etc. to obtain a good match

between observed and model parameters.

Perhaps the most challenging of all inputs is the Gas Simulator which is part of GREAT-

Premium. The input, execution and output parts of this module are explained in details in

the next section. However, even here, first, values are entered in the WHITE cells, inputsare created by appending time dependent data and finally the program is executed to

generate output.

5. Troubleshooting

I have downloaded the zip file but where are the modules?

You have to unzip the file to get either GREAT-Standard alone or with GREAT-

Premium directories.

I have unzipped the file, now what do I do?

You are ready to use the tools. Lets say, you want to know the properties of the gas

using correlations. You should go into the FluidProp directory and double-click on

FluidProp.xls to open the Excel workbook. (Note: The assumption is you haveExcel 2000 or higher on your machine.) Enter values in the WHITE cells and click on

the button to obtain the results. It is that simple!

I have entered values, clicked the button yet I get an Excel error message?

Check the values for admissible entries. Each method expects certain types of inputs (of

which some are mandatory). Perhaps either some data is missing or you have entered


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values that are not allowed. In any case, please contact technical support at

[email protected].

I have entered the values, clicked on the execution button but cant find my output?

Sometimes based on your screen resolution, you could be limited to how much you cansee on the monitor screen. If both input and output do not fit into one viewing area,

please scroll right and/or down to get to the output section. The tabular outputs are, in

general, on the same page as the inputs. The graphical outputs, however, are presentedon separate worksheets.

Could you explain how to run the simulator?

From the Gas-Simulator directory open the Simulator xxx.xls file (where xxx is theversion number). First step is to enter the reservoir properties and then click the Write

Constant Grid Prop button to propagate properties to the entire grid. It can be visualized

in the GridProp worksheet. Interestingly, during the history match phase, one cancome back to this page to perturb grid properties at appropriate locations. Next, the fluid

and simulation option properties are entered at the respective WHITE cells.

Next is to create the well data. The well data (especially during history matching)

typically occurs in recurrent data sets i.e. in several intervals of time. From one intervalto the next many events can happen to a well. For instance, the completions can change,

rates can change, productivity can improve because of workover operations or a producer

can be changed into an injector or vice versa. Additionally, wells can be put onproduction at a certain time

1. To incorporate these changes, first click the Reset

Recurrent Data button to clean all previous recurrent information and start afresh. Then

enter the well information for the first recurrent time interval. After finishing data entry,

click Flush Well Recurrent Data to write this section into memory. Thereafter, moveon to the next recurrent interval and incrementally alter/modify the information pertinent

to that particular time interval. Once again, click Flush Well Recurrent Data to write

this section into memory. Repeat this until maximum number of recurrent intervals isreached.

Scroll down to the section marked OUTPUTS. In Step 1, click Generate Input buttonto collect and parse the input information built so far into Simulator input format. In Step

2, click Simulate button to run the simulator. An MS-DOS window will surface and

show several intermediate outputs of the simulation run2. At the successful completion of

this simulation run, click Generate Output button. This will write the output (wellrates, bottom hole flowing pressures and well block pressures) for each well. This can

then be plotted for history match. Training is available at request.

1Important: Remember to enter the maximum numbers of wells and recurrent time intervals at the

beginning of the simulation2 Do NOT disturb this MS-DOS window while the simulation is in progress


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6. Contact

Mailing Address: Phone: EmailDeepankar Biswas 972-818-7450 (o) [email protected]

SiteLark LLC 469-222-5436 (cell) [email protected]

5939 Smoke Glass www.sitelark.comDallas TX 75252

7. Book References1. Lee, J. and R. Wattenbarger: Gas Reservoir Engineering, SPE Textbook Series

Vol. 5, Second Printing, 2002.

2. Poston, S.W. and R.B. Robert: Overpressured Gas Reservoirs, Society ofPetroleum Engineers, Second Printing, 2002.

3. Ertekin, T., J.H. Abou-Kassem and G.R.. King: Basic Applied ReservoirSimulation, SPE Textbook Series Vol. 7, 2001.

Note: Only some book references are listed here which could be a good starting

point. There are countless other SPE paper references that can be obtained either

from the books or from the SPE library and therefore, are not enumerated here.

(2) gas reservoir engineering application toolkit

Documents

gas reservoir simulator

gas deliverabilityprediction

gas injection

volumetric calculation

zfactor calculation

thestandard toolkit

pz analysis

fluid property calculation4