Gas Field Engineering - Deliverability Tests

8/9/2019 Gas Field Engineering - Deliverability Tests

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GAS FIELD ENGINEERING

Deliverability Tests

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CONTENTS

5.1 Radius of investigation5.2 Time to stabilization5.3 Introduction to Deliverability Testing

5.4 Flow-after-flow tests5.5 Isochronal tests5.6 Modified Isochronal tests5.7 Classifications, limitations, & use of Deliverability tests

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LESSON LEARNING OUTCOMES

At the end of the session, students should be able to:

Understand radius of investigation & stabilization time

Calculate deliverability of gas wells by using various flowtests

Understand classifications, limitations , and the use ofdeliverability tests

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Radius of Investigation

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r e = radius of reservoir boundary

r i = radius of investigation

r w = radius of wellbore


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Why do we care about radius ofinvestigation?

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To tell if the flow has been stabilized.Flow is transient before r inv reaches r e

Flow is stable i.e. pseudo-steady state if r inv = r e

Result Analysis is different for transient vs pseudo-steady state


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Radius of Investigation

for r inv < r e .

As long as the radius of investigation is less than r e,stabilization has not been reached and the flow is said to be

transient . Gas well tests often involve interpretation of dataobtained in the transient flow regime.

If r inv = r e the flow is pseudo-steady-state.

When the radius of investigation reaches the exterior boundary,r e , of a closed reservoir, the effective drainage radius is given by

r d = 0.472 r e

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(4.65)

(4.66)


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Time to Stabilization

Time to stabilization can be determined by

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(4.63)


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ExampleCalculating Radius of Investigation

Solu t ion Using Eq, time to stabilization is

Using Eq, the radius of investigation is

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Gas Well Deliverability-Before

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Early estimates of gas well performance were conducted byopening the well to the atmosphere and then measuring theflow rate.

Such open flow practices were:-- wasteful of gas,-- dangerous to personnel and equipment, and-- damaging to the reservoir.-- provided limited information to estimate productive

capacity under varying flow conditions


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Gas Well Deliverability-Now

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The productivity of a gas well is now determined withdeliverability testing. It provides:

-- Rate-pressure behavior for the well in form of aninflow performance curve (IPR) or gas backpressure curve.

-- Finds theoreticalmaximum flow ratepossible for the wellcalled AbsoluteOpen Flow (AOF).


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Gas Well Deliverability Testing

The stabilized flow capacity or deliverability of a gas wellis required for planning the operation of any gas field.

The Absolute Open Flow (AOF ) potential of a well is definedas the rate at which the well will produce against a zerobackpressure . It cannot be measured directly but may beobtained from deliverability tests .

Most common types of deliverability tests:

Flow-after-flow test

Isochronal test

Modified isochronal test

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Test Schematic

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Analyzing Deliverability Test

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There are two basic relations currently in used to analyzedeliverability test data:

-- Empirical relationship of Rawlins and Schellhardt based on500 wells data.

C = flow coefficientn = deliverability exponent

-- Theoretical relationship by Houpeurt derived from radialdiffusivity equation accounting for non-Darcy flow effects.

a = laminar flow coefficientb = turbulence coefficient
http://petrowiki.org/images/6/61/Vol4_page_0007_eq_001.pnghttp://petrowiki.org/File:Vol4_page_0005_eq_001.png


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Flow-after-Flow Test

Figure (5.1) Conventional flow rate and pressure diagrams.

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Flow-after-flow tests

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The test is often referred to as a four-point test , because manytests are composed of four flow rates, as required by variousregulatory bodies.

This test is performed by producing the well at a series ofstabilized flow rates and obtaining the corresponding stabilizedflowing bottom-hole pressures .

In addition, a stabilized shut-in bottom-hole pressure isrequired for the analysis.

A major limitation of this test method is the length of timerequired to obtain stabilized data for low-permeability gasreservoirs.


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Analysis of Flow-after-flow tests

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log = log + log ( 2 2 )

Dividing both sides by n and re-arranging to bring pressureterms on left hand side because pressure is dependent variable

(the one that is not controlled) whereas qsc is independentbecause it is changed at will).

log ( 2 2 ) =

log

log

Compare this to the equation of straight line:y = mx +C

m = 1/n

Rawlins and SchellhardtsEmpirical Equation:


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Empirical Method

Eqn. reveals that a plot of ( P 2) = P 2 R P 2w f versus q s c onlog-log scales should result in a straight line having a slope of 1/n.

Once a value of slope m has been determined from the plot, ithas to be inversed to find n .

C can then be calculated byusing data from one of the tests

that falls on the line.Then AOF is calculated. AOF is used for these tests. AOF ismaximum rate at which the well could flow against thetheoretical back pressure at the sand face

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(4.30)

log ( 2 2 ) =1

log 1

log


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Figure (5.2) Deliverability test plot

A plot of typical flow-after-flow data is shown in Figure

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Theoretical Methods

Deliverability equations based on theoretical methods are :

1. Pressure solution technique

2. Pressure-squared technique

3. Pseudo pressure technique

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Example 5-1Stabi l ized Flow Test An alysis

A flow-after-flow test was performed on a gas well located in alow-pressure reservoir. Using the given test data, determine thevalues of n and C for the deliverability equation, AOF , and flowrate for P wf =175 psia .

Solu t ion

Flow-after-flow Test Data are shown in Table 5-1.

A plot of q s c versus ( P 2R P 2w f ) is shown in Figure(5.2)

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Table (5-1) Flow-after-flow Test Data

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p R = 201 psia(p R)2 40401 psia 2

Test q sc p wf (p wf )2 (p R)2 - (p wf )2(mscfd) (psia) (psia 2) (psia 2)

1 2,730 196 38,416 1,9852 3,970 195 38,025 2,376

3 4,440 193 37,249 3,1524 5,550 190 36,100 4,301

For AOF, P wf = 0, thus (p R)2 - (p wf )2 = 40401 - 0 = 40401 psia 2


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1,000 10,000 100,000AOFFlow Rate, qsc (mscfd)

p w

f =

0

Note that the fraction hasbeen inversed tocompute n directly.

Also note that the twopoints selected forcalculating n are bothfalling on the line. YouCan also read pointsfrom the line itself whichsometimes gives betteraccuracy.

The origin should bethe biggest corner box

and can not be zeroEvery log cycle increasesby a multiple of 10


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From test4 which falls on the line, calculate C:

Therefore, the deliverability equation is

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Isochronal Test

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The isochronal test is a series of single-point

tests developed to estimate stabilized

deliverability characteristics without actually

flowing the well for the time required to achieve

stabilized conditions at each different rate .

- The flow-after-flow tests are very accurate but

take a long time to run specially for tight reservoir.

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Isochronal Test

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- The flow-after-flow tests are very accurate but

take a long time to run specially for tight reservoir.

-To speed up the testing process, the isochronaltests were developed.

-Isochronal tests faster yet fairly accurate

-Isochronous means time dependent, i.e.processes where pressure data must be taken atfixed time intervals.

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Isochronal Test

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Why Isochronal Tests are Quicker?

Because less time is required to build to

essentially initial pressure after short flow

periods than to reach stabilized flow at each

rate in a flow-after-flow test.

Th b hi d I h l T


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Theory behind Isochronal Tests

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- The isochronal test is based on the principle that the radiusof drainage during a flow period does not depend upon rate.

- The r d depends only on the length of time for which thewell is flowed.

- Therefore, the pwf measured at the same time periodsduring each rate cycle correspond to the same radius ofdrainage.

- Thus, isochronal test data can be analyzed using the sametheory as a flow-after-flow test , even though stabilized flowis not attained.

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Isochronal Test

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Shut in periodsare different

Pressure buildsUp to Pavg

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Isochronal Test

A deliverability test designed as a series of drawdown

and buildup sequences such that:

each drawdown is at different qsc,

each drawdown is of the same duration and the flowrate does not necessary reach rate stabilization

each buildup reaches pressure stabilization (i.e. P wf

builds up to the same pressure as it was at the start of

the test) where data must be delivered at fixed times.

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Isochronal Test

Flow the well at a fixed rates for a set period of time t ,noting the P wf at several fixed time intervals t 1, t 2, t 3, t 4 etc.

Shut-in the well and wait until pressure is almost stabilize.

Perform the above cycle four times but each time at adifferent rate .

The behaviour of the flow rate and pressure with time wasillustrated in the earlier Figure for increasing flow rates.

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E l 5 2


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Example 5-2Iso ch ron al Test

An isochronal test was conducted on a well located in areservoir that had an average pressure of 1952 psia. The wellwas flowed on four choke sizes , and the flow rate and flowingbottom-hole pressure were measured at 3 hr and 6 hr for eachchoke size. An extended test was conducted for a period of 72 hr

at a rate of 6.0 MMscfd, at which time p w f was measured at 1151psia . Using the data in Table 5-2.The slopes of both the 3-hr and 6-hr lines are apparently equal.(see Fig. 5-4). Use the first and last points on the 6-hr test tocalculate n from Eqn. gives.

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R P


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Table (5-2) Isochronal Test Data

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Average Reservoir pressure = 1952 psia


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Figure 5-4 Deliverability data plot Example 5-42. 33


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Using the extended flow test to calculate C using Eqn:

Solu t ion

1. Given the data in Table 4-2, the deliverability equation for q sc inmscfd is

2. To calculate AOF , set P wf = 0 :

3. In order to generate an inflow performance curve , pickseveral values of P wf and calculate the corresponding q sc

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Well inflow performance responses are shown in Table 5-3.The inflow performance curve is plotted in Figure 5-6.

Table 5-3 Well Inflow Performance Responses

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Figure 5-6. Well inflow performance response Example 5-2. 36


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Modified isochronal tests

In normal isochronal test, time for pressureto build up to during sh ut -in may beimpractically long.

Modified isochronal test shortens test timesbecause well does not build up to averagereservoir pressure after each flow period

Why Needed?
http://petrowiki.org/File:Vol5_page_0781_inline_001.pnghttp://petrowiki.org/File:Vol5_page_0781_inline_001.png


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The modified isochronal test is lessaccurate than the isochronal test becausethe well does not build up to

What is the downside?

p & q history of a typical modified isochronal test
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p w f & q history of a typical modified isochronal test

Equal Shut in times thatare = or > flow periods

Shut in pressure is notReaching P avg


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Conducted like an isochronal test, except theshut-in periods are shorter and of equal

duration.

The shut-in periods should equal or exceed thelength of the flow periods.

A final stabilized flow point usually is obtainedat the end of the test but is not required foranalyzing the test data.

Test Procedure


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Shut-in sandface pressure p s is used instead of P avg


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Rawlins-Schellhardt analysis technique


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Rawlins-Schellhardt analysis technique


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Wellhead Deliverability

In practice it is sometimes more convenient to measure thepressures at the wellhead .

These pressures may be converted to bottom-hole

conditions by the calculation procedure suggested byCullender and Smith.

However, in some instances, the wellhead pressures mightbe plotted versus flow rate in a manner similar to bottom-hole curves.

Classifications Limitations


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This may be known directly from previous tests, such as drill-stem or deliverability tests, conducted on the well or from theproduction characteristics of the well.

If such information is not available , it may be assumed thatthe well will behave in a manner similar to neighbouringwells in the same pool, for which data are available.

Time to stabilization may be estimated using Equation (4.63) .

Radius of investigation can be found from Equation (4.65) .

Classifications, Limitations,and Use of Deliverability Tests

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Classifications Limitations


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If time to stabilization is of the order of few hours, flow-after-flow test may be conducted.

Otherwise, one of the isochronal tests is preferable.

Classifications, Limitations,and Use of Deliverability Tests

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Classifications Limitations Use of Deliverability Tests


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Classifications, Limitations, Use of Deliverability TestsNext Figure shows types, limitations, and uses of deliverabilitytests. In designing a deliverability test, collect and utilize all

information which includes:logsdrill-stem testsprevious deliverability tests conducted on that wellproduction historygas and liquid compositionstemperaturecore samplesgeological studies

Knowledge of the time required for stabilization is a veryimportant factor in deciding the type of test to be used for determining the deliverability of a gas well.

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Figure. Types, limitations, and uses of deliverability tests. 49


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Thank You

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Q & A

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Gas Field Engineering - Deliverability Tests