Guo, Boyun / Computer Assited Petroleum Production Engg 0750682701_chap13 Final Proof page 184
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3.1.2007 9:07pm Compositor Name: SJoearun
ARTIFICIAL LIFT METHODS
Tubing pressure (Pt)
Casing pressure (Pso)
Gfa
Dv
Depth
0
D
Point of gas injection
Point of balance
D-Dv
∆Pv
G
fb
PR
Flowing bottom hole pressure Pressure
0
Figure 13.4 Pressure relationship in a continuous gas lift.
In a field-scale evaluation, if an unlimited amount of lift gas is available for a given gas lift project, the injection rate of gas to individual wells should be optimized to maximize oil production of each well. If only a limited amount of gas is available for the gas lift, the gas should be distributed to individual wells based on predicted well lifting performance, that is, the wells that will produce oil at higher rates at a given amount of lift gas are preferably chosen to receive more lift gas. If an unlimited amount of gas lift gas is available for a well, the well should receive a lift gas injection rate that yields the optimum GLR in the tubing so that the flowing bottom-hole pressure is minimized, and thus, oil production is maximized. The optimum GLR is liquid flow rate dependent and can be found from traditional gradient curves such as those generated by Gilbert (Gilbert, 1954). Similar curves can be generated with modern computer programs using various multiphase correlations. The computer program OptimumGLR.xls in the CD attached to this book was developed based on modified Hagedorn and Brown method (Brown, 1977) for multiphase flow calculations and the Chen method (1979) for friction factor determination. It can be used for predicting the optimum GLR in tubing at a given tubing head pressure and liquid flow rate. After the system analysis is completed with the optimum GLRs in the tubing above the injection point, the expected liquid production rate (well potential) is known. The required injection GLR to the well can be calculated by
qg,inj ¼ GLRinj qo ,
(13:2)
where qo is the expected operating liquid flow rate. If a limited amount of gas lift gas is available for a well, the well potential should be estimated based on GLR expressed as qg,inj , (13:3) GLR ¼ GLRfm þ q where qg is the lift gas injection rate (scf/day) available to the well. Example Problem 13.1 An oil well has a pay zone around the mid-perf depth of 5,200 ft. The formation oil has a gravity of 26 8API and GLR of 300 scf/stb. Water cut remains 0%. The IPR of the well is expressed as " 2 # pwf pwf 0:8 , q ¼ qmax 1 0:2 p p where qmax ¼ 1500 stb=day p ¼ 2,000 psia. A 21⁄2 -in. tubing (2.259 in. inside diameter [ID]) can be set with a packer at 200 ft above the mid-perf. What is the maximum expected oil production rate from the well with continuous gas lift at a wellhead pressure of 200 psia if a. an unlimited amount of lift gas is available for the well? b. only 1 MMscf/day of lift gas is available for the well?
(13:1)
Solution The maximum oil production rate is expected when the gas injection point is set right above the packer. Assuming that the pressure losses due to friction below the injection point are negligible, the inflow-performance curve for the gas injection point (inside tubing) can be expressed as pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pvf ¼ 0:125 p½ 81 80ðq=qmax Þ 1 GR ðD Dv Þ,
Then the required gas injection rate to the well can be calculated by
where pvf is the pressure at the gas injection point, GR is the pressure gradient of the reservoir fluid, D is the pay
GLRinj ¼ GLRopt,o GLRfm , where GLRinj ¼ injection GLR, scf/stb GLRopt,o ¼ optimum GLR at operating flow rate, scf/stb GLRfm ¼ formation oil GLR, scf/stb.