15. Gas Well Testing Field Case Studies

Chapter 15

Gas Well Testing Field Case Studies

15.1 Introduction This chapter presents various field case studies in low, high permeability, and fractured carbonate gas wells including summary, conclusions, and recommendations. It also includes a gas well test evaluation sheet, state report forms, and various cross plotting techniques before and after workovers.

15.2 Gas Well Test Evaluation Sheet Well Data and Basic Parameters ~

Filed name Well name Zone number Interval Reservoir datum Estimated reservoir pressure Reservoir temperature Net hydrocarbon thickness Gas saturation Porosity Fluid viscosity Compressibility Hydrocarbon porosity Fluid gradient Z-factor Well radius Drainage radius Cumulative production prior to test

aaaabbbb ccccfeet feet ss psia 0 R feet fraction fraction cP psi" 1 fraction psi/ft — feet feet mmcf

Gas Composition "Gas IH 2 SI CO 2 IN 2 I Ci I C 2 I C 3 11C4 I nC 4 I iC 5 I nC 5 I C 6 I C 7 + composition

MoI % I I 1 1 1 1 1 1

1

1

1 1

Well Test Data Choke size I Rate I Duration I Cumulative I Final BHP I Final THP (-/64") (mmcfd) (min) time(hr) (psia) (psia)

Amerada Data Amerada no.

Serial no.

Last calibrated data

Depth of Amerada

Interpreted data: MBH correction: tp (hr) tpDA (dimensionless time) Buildup slope m(mmpsia 2 /cP) if(Pwfo) (mmpsia2/cP) f (Pwf) At=i (mmpsia2/cP) Calculated data: kh = k=

1.632 x 10%cT , md m kh/h

ft

,'=i.i5i r » ( * > i * - * Q ^ > _ L rn m(Ap)j' = 0.867 m / Ap =

k log

hix ctrl

+3.231

J

Pressure data: HPi) = F = 4ntDA Af

is(Pwf) = f(pR) = eF

15.3 Shallow Low-Pressure and Highly Productive Gas Reservoirs The following example illustrates how to determine the stabilized deliverability curve and AOF. Example 15-1 Determining Stabilized Deliverability Curve and AOF from the Test Data A gas well produces from a shallow low-pressure, highly productive reservoir. The well has been tested by a multirate test and the results are plotted in Figure 15-1. One and one-half durations of each flow period was enough to reach stabilization of flowing wellbore pressure. In fact, it was observed that pressures stabilized almost instantaneously after each rate change. Solution The log-log backpressure plot gives a straight line which defines a backpressure exponent n = 1 /slope = 0.56. The backpressure coefficient is

Slope, B = 0.001607 Indicates pressure loss due to high velocity flow Dqsc

Intercepts = 0.00145 Indicates pressure loss due to steady state skin s

Gas flow rate, mmscfd

Figure 15-1. Linear plot for determining high-velocity effect on gas well performance.

calculated from the curve as C = 8.0 x 10 6 (ll,200) a56 = 43,204 scf/day/psia2 The backpressure equation then is qsc = 43,204(p| — / ^ ) 0 ' 5 6 and the absolute open flow is 45.538 mmscfd. A Cartesian plot of Ap2/qsc versus qsc (Figure 15-1) gives a straight line (except for a small deviation and the low rate point). The intercept of the line is A = 0.00145 psia2/scfd/D and the slope is B = 1.607 x 10-3

Pdia2/SCM

mmscfd or, when expressed in scf/d, * = 1.607xl0-9 p S i a 2 / S C f d / d scfd/d The low n value and the high B value indicate large rate-dependent skin. The slope B in Figure 15-1 indicates the significance of the high-velocity effect on the productivity of the well. A large slope implies large rate-dependent skin. The intercept A is related to steady-state skin factor. If the rate needs to be written in terms of flowing pressure, the quadratic equation can be solved as follows: ^A2+ qsc =

4B(pl-p2wf)-A

YB ^(0.00145)2 +4(1.607 x I O " 9 ) ^ - plf) - 0.00145

=

2(1.607 x 10"9)

This equation can be used to calculate the AOF for this example.

15.4 Recommended Form of Rules of Procedure for Backpressure Tests Required by State Regulatory Bodies All backpressure tests required by a state regulatory body shall be conducted in according with the procedures set out by the state regulatory body except for those wells in pools where special testing procedures are applicable.1"3 The calculations shall be made in the manner prescribed in the appropriate test examples. The observed data and calculations shall be reported on the prescribed forms. Gas produced from wells connected to a gas transportation facility should not be vented to the atmosphere during testing. When an accurate test can be obtained only under conditions requiring venting, the volume vented shall be the minimum required to obtain an accurate test. All surface

pressure readings shall be taken with a dead weight gauge. Under special conditions where the use of a dead weight gauge is not practical, a properly calibrated spring gauge may be used when authorized by the state regulatory body. Subsurface pressures determined by the use of a properly calibrated pressure bomb are acceptable. The temperature of the gas column must be accurately known to obtain correct test results; therefore a thermometer well should be installed in the wellhead. Under shut-in or low-flow-rate conditions, the external temperature may distort the observed wellhead temperatures. Whenever this situation exists the mean annual temperature should be used.

15.5 Appropriate State Report Forms The appropriate state report forms are as follows.

Texas Gas Well12 • • • •

Uses tubing pressures Square root chart entries for gas measurement GE system dialogue Answers transferred to G-1 form

New Mexico Gas Well1'2 • • • •

Uses tubing pressures Deviated well UCS system dialogue Answers transferred to preprinted state form C-122

Oklahoma Gas Well3 • • • • •

Uses casing pressures Single-point test Case No. 1 assigned to input data GE system dialogue Answers presented in report form

Offshore Gas Well Using IOCC Procedure3 • Uses bottom-hole pressures • UCS system dialogue • Answers presented in report form for natural gas Oklahoma

15.6 Stimulation Efforts Evaluation, Summary, and Recommendations This section presents theoretical and practical aspects of methods used to determine absolute open flow potential (AOF), formation permeability, overall skin factors, average reservoir pressure, and gas in place in low- and highpermeability gas reservoirs. Test analysis methods examined include deliverability, Horner, type curves, and reservoir limit test analysis. It also includes a brief summary, conclusions, and recommendations of two field case studies. One case is for a low-permeability gas reservoir; the other is for a highpermeability gas reservoir. These two cases demonstrate well test analysis applications in low- as well as high-permeability gas reservoirs.

Low-Permeability Gas Well, Nilam Gas Field, Indonesia Case Studies: Nilam Gas Field, Well # N-38/gas, Zone GSOA Nilam gas field is in Kalimantan, Indonesia, and is "offshore." The reservoir is 12,950 ft deep and consists of layers of clay and sandstone. The overall thickness is about 52 ft with average porosity of about 14 to 20%. The empirical deliverability equations are qsc = 1.3152 x 10" 6 (~p2R - plh) (wellhead conditions) qsc = 0.5997 x 10~6 (~p2R - plf) (bottom-hole conditions) Stabilized flow equations are also developed using the LIT(\J/) approach to estimate deliverability potential of this gas well against any sandface pressure. The values of exponent n = 1 and formation permeability = 8.274 mD indicate, that it is a low-permeability gas reservoir (see Table 15-1 for a summary of results). The laminar-inertial-turbulent (LIT) flow equations are ~ is(Pwh) = 45.5574 sc q + 2.1429q 2 c (wellhead conditions)

^(PR)

is(pR) - if(pwf) = 9l.S213qsc

+ 0.1785 q)c (bottom-hole conditions)

Returning again to the Forscheimer equation, ~p\ — p^ = Aqsc + Bq2c, kh is small (339.23 mD), A qsc becomes large, and the B q2c term can become negligible (not necessarily zero) when compared to the laminar pressure drop term. We could then write qsc = j(~p2R — P^f)1'0Calculate the following quantities n

J2S> = 76.145 i n

Y^q = 27.087

Table 15-1 Summary of Results

Shut-in Ratel Rate 2 Rate 3 Rate 4 Extended rate Final shut-in n C AOF

Wellhead pressure (psia)

Bottom-hole pressure (psia)

Flow rate (mmscf/d)

Choke size (inch)

2388 2015 1640 1365 1015 1015 2388 1.0 1.3152 x 10" 6 7.50

3700 3144 2566 2158 1836 1721 3700 1.0 0.5997 x 10~6 8.21

— 2.397 5.214 6.144 7.186 6.148 —

16 24 32 48 32

mmscfd/psia mmscfd

Table 15-2 Specific Results of Pressure Buildup Analysis Using Four Rate Tests Parameters

Estimated values

qsc VKPvv/i) xjf(Vwfo) m kh k s' s D \lr(AP)skin xjf(Pi) xJr(PR) Static gradient

6.148 mmscfd 690xl0 6 psia 2 /cP 669xl06psia2/cP 21.0x106 psia2/cP 339.23 mD-ft 8.274 mD +16.869 +3.649 2.137511 64.44 mmpsia2/cP 861.12 mmpsia2/cP 772.0 mmpsia2/ cP 0.110 psi/ft

Remarks

Apparent skin True skin Turbulent factor 995 psia 3955 psia 3702 psia

See Table 15-3 See Table 15-3 True skin From Horner plot

J2