GAS LIFT STABILITY ISSUES

8/10/2019 GAS LIFT STABILITY ISSUES

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To tackle this issue of unstable flow in gas lift wells, an R&D project has been taken up to design amodified profile in the gas lift valve orifice to achieve critical flow at a much lesser pressure differential

compared to conventional square edged orifice.

Gaslift valve seats for normally used port sizes of 3/16 inch, inch and 5/16 inch were selected for

experimentation. The modifies seat profiles of the above port sizes were fabricated with convergent

divergent profile with a total angle of divergent section limited to 10 and 12 degree respectively. Theexit diameter of the relevant port sizes in each case was theoretically calculated to achieve a critical

flow rate at approximately 90 % of the downstream pressure to upstream pressure ratio. Theschematic diagram of a standard square edged seat and a modified convergent divergent seatprofile is shown in Figure-1

Conventional Square edged profile

Modified divergent profile

Figure 1: Existing square edge and modified convergent divergent gaslift valve seat profile

Dynamic tests using Constant injection pressure testing ( CIPT ) method were carried out on each ofthe modified seat to generate gaslift valve flow performance curves. The results indicate that the

critical flow rates were achieved at downstream pressure to upstream pressure ratio of approx 87 %,85 % and 82 % for port sizes of 3/16 , 1/4 and 5/16 port sizes respectively. Details ofexperimentation and analysis of results can be obtained from Reference-1.

The seat profile modification will allow the valves to be operated in the critical region even withlesser pressure differential of about 100 200 psi, as presently being practiced in the field and yet will

pass the maximum flow rate at that low pressure differential. Any pressure fluctuations in tubing willnot affect the gas throughput and thus eliminate instability in the tubing, helping to improve oilproduction.

Field Trial ImplementationOnce the experimentation in the laboratory is completed and results are verified through dynamic

testing of modified profile seat gas lift valves, pilot field implementation has been taken up to validatethe results and benefits thereof by installing these valves in two gas lift wells of Onshore Ankleshwar

Asset.

One well each from two producing fields of Ankleswar Asset, namely, Ankleswar and Gandhar fields

have been chosen for implementation of Improved gas lift valve technology. Parameters of Gandharand Ankleswar field differ significantly from each other in many ways, which is described below,


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Parameter Gandhar field Ankleswar field

Gas Injection Pressure, Kg/cm2 100-105 35-40

Average well depth, mts 2500-3000 1000-1200

API of Crude oil, Deg 38 45

Formation GOR, v/v 100-200 50-100

Casing size, inches 7 and 9 5/8 5

Average Water cut, % 5 to 50 85 to 90

These two sets of field conditions cover almost entire range of operating conditions existing indifferent fields of ONGC as well as other Indian E&P companies.

Candidate wells for installation of modified gas lift valves were selected based on Having good potential (expected rate ~ 100 m3/d), so that tubing pressure at the operating

valve depth will be higher and close to Gas injection pressure.

Availability of independent gas injection line, Gas measurement facilities in place

Results Of Field Implementation Study

Ankleswar # 293

Two numbers of GLVs with modified profile seats were installed on the upper part of the tubing

string in addition to two normal valves in the lower portion of the string. In the Gas lift wells ofAnkleswar field, due to good reservoir pressure support, generally first or second valve from topwill be the operating valve. For this reason, modified profile valves are installed in top two valves

in this well. After installation and completion, well was activated with gas lift and following studieswere carried out.

Gradient survey Two pen recorder survey Well testing under varying operating conditions

Well details and Gas Lift design of Ank-293 are given in Table-1 and Table-2 below.

Table-1

Well No Ank # 293

Casing Size, in 5.5

Tubing Size, in 2.875

Perforation depth,Mts

1090

SBHP, Kg/cm2 94

GLV Depths, Mts GLV Port size, inches

GLV1, mts 252 12/64 (Modified profile)

GLV2, mts 443 12/64 (Modified profile)

GLV3.mts 589 12/64 (Conventional)

GLV4, mts 726 16/64 (Conventional)

PI, m3/d/kg/cm2 2

W/C, % 78


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Table-2a GL Design of Ank # 293

Valve No. Depth TVD Tv(degrees F) Port Size R Pt Psc Pd&Pvc

Feet 1/64 inch Psi psi psi

1 827.5 117.8 12 0.038 157.5 422.7 430.8

2 1456.8 131.8 12 0.038 219.2 415.7 429.8

3 1934.2 142.5 12 0.038 268.7 408.1 426.5

4 2384.2 152.6 16 0.066 317.5 397.7 419.7

Table-2b GL Design of Ank # 293 (contd)

Valve No. OP Pso Pd@91F TRO Set to

psi psi psi Psi psi

1 441.6 433.1 409.1 425.3 425

2 438.1 423.6 397.5 413.2 413

3 432.7 413.9 386.8 402.1 400

4 426.9 404.6 373.9 400.3 400

Gradient survey

Results of the two gradient surveys conducted in Ankleshwar-293 are placed in Table-3.

Table-3 Gradient Survey results of Ankleshwar-293

Gradient surveyDate: 20-9-05

Gradient survey Date: 21-10-05

Depth, Pressure Depth, Pressure

mts Kg/cm2 mts Kg/cm2

0 14.6 0 10.6

100 18.3 100 13.8

254 24.2 254 20.2

468 36.9 468 31.6

652 49.8 652 45.2

803 61.1 803 57.3

900 68.4 900 65.2

Two pen recorder survey

This is a useful method, where in Casing and Tubing head pressures are continuously

recorded over a period of 24 hours to monitor the stability of pressure profiles in casing and

tubing. A smooth and steady curve on the recording chart clearly indicates the stableoperation of Gas lift in the well. Twp pen chart recorded in Ankleshwar#293 is placed in Fig-2.

Inner recording (Red) is THP having range of 0-500 psi and the outer recording (Green) isCHP having range of 0-1000 psi. This chart indicates a fairly stable flow in tubing, which

confirms the advantage of using modified Gas lift valves.

Well testing under varying operating conditions

Well testing for liquid and gas rates has been done with three different bean sizes on tubing.Results are placed in Table-4 below.


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Table-4 Well test data of Ank # 293

Test date Bean size Qliquid W/C Qg,total Qg,inj

MM M3/d % M3/d M3/d

20-9-05 8 51.84 78.55 4028 289510-10-05 10 86.4 66.8 4257 2858

21-10-05 12 115 NA 4610 2923

Figure-2 Two-pen chart of Ankleshwar # 293 (29-9-05)

Another main benefit of Improved gas lift valve technology is the ability of achieving critical flowacross GLV with a much less pressure differential compared to Conventional square edged seat

GLV. This was verified and validated by measuring the injection gas flow rates in to the well withdifferent tubing head pressures. Tubing head pressures were varied by varying the size of chokeinstalled on the wellhead. Varying THP will obviously result in varying pressure differentials

across the GLV as well as sand face. By looking at data in Table-5 below (Compiled fromGradient survey and well testing Data above), it is seen that Injection gas rate is fairly stable even

with varying pressure differentials across the valve.

Table-5

Test date Qg,total Qg,inj THP Pt@valve Pinj@valve Pd/Pup

M3/d M3/d Kg/cm2 Kg/cm2 Kg/cm2 %

20-9-05 4028 2895 14.6 24.2 32 75.625

10-10-05 4257 2858 10.6 21.6 32 67.5

21-10-05 4610 2923 9.4 20.2 32 63.125

Hence it can be concluded that critical flow has been achieved at a pressure differential of 75 %

compared to 53% in conventional square edged gas lift valve seats.

CHP

THP


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Liquid flow rates from this well with conventional gas lift valve seats (Before work over job ofWater shut off) and with Modified profile seats (After work over job) are indicated in table-6 below.

Table-6

Test date Bean size Qliquid W/C

MM M3/d % Remarks

Before WOJ July'05 10 65.66 93.1 With Conventional GLV

After WOJ 10-10-05 10 86.4 66.8 With modified GLV

Gandhar # 310

Three numbers of GLVs with modified profile seats were installed on the bottom part of the tubing

string in addition to two unloading valves in the upper portion of the string. Well details and Gaslift design are placed in Table-7 and Table-8 respectively below.

Table-7

Well No G # 310Casing Size, in 7

Tubing Size, in 2.875

Perforation depth, Mts 2812

SBHP,Kg/cm2 195

GLV Depths GLV Port size,inches

GLV1,mts 942 12/64 (Conventional)

GLV2,mts 1737 12/64 (Modified)

GLV3.mts 2386 16/64 (Modified)

GLV4,mts 2700 16/64 (Modified)

PI,m3/d/kg/cm2 0.4

W/C, % 90

Table-8a GL Design of G # 310

Valve No. Depth TVD Tv(degrees F) Port Size R D Pc Pt Psc Pd&Pvc

Feet 1/64 inch psi psi psi

1 3093.2 152.4 12 0.038 85.5 449.7 1452.9 1551.6

2 5701.8 200.8 12 0.038 162.9 708.5 1447.5 1623

3 7829.5 240.2 16 0.066 215.3 931.7 1394.9 1621.3

4 8858.3 259.3 16 0.066 232.7 1043.1 1347 1591.3

Table-8b GL Design of G # 310 (contd)

Valve No. OP Pso Pd@91F TRO Set to

psi psi psi psi Psi

1 1595.1 1493.4 1367.9 1421.9 1420

2 1659.1 1479.5 1310 1361.7 1360

3 1670 1436.5 1226.6 1313.3 1315

4 1630 1379.5 1169.4 1252 1250

After installation and completion, well was activated with gas lift and all the above-mentionedstudies (as in Ank-293) were carried out in this well also.


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Gradient survey results, Two pen recorder chart and well test results are placed in Table-9,Figure-3 and Table-10 respectively.

Table-9 Gradient Survey results of Gandhar # 310

Gradient survey Date: 10-9-05 Gradient survey Date: 22-9-05

Depth, Pressure Depth, Pressure

mts Kg/cm2 mts Kg/cm2

0 1.8 0 1.2

951 7.6 941 20.9

1300 9.2 1300 11.1

1746 10.9 1746 5.3

2000 12.2 2000 6.6

2397 14.4 2397 7.8

2750 19.0 2705 12.1

Table-10 Well test data of G # 310

Test date Bean size Qliquid W/C Qg,totalQg,inj

(Estimated)

MM M3/d % M3/d M3/d

22-9-05 No bean 17.28 8 8303 5,000

How ever, results in this well are erratic and inconclusive. Possible reasons for this behavior are,

a) Potential of this well was much less than expected,

b) Due to the low potential in this well, this well was behaving like an intermittent gas lift wellwith choke control (Which can be seen from the Two pen chart).

c) Due to low potential nature of this well, injection gas rate from surface is restricted through asmall size choke.


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Figure-3 Two-pen chart of Gandhar # 310 (22-9-05)

Inner recording (Red) is THP having range of 0-1000 psi and the outer recording

(Green) is CHP having range of 0-2000 psi.

Conclusions

1. To achieve critical flow across gas lift valve seat at a lesser pressure differential compared tothe conventional square edged seat, profile of the seat has been modified.

2. Laboratory experimental results indicated that with the modified profile seats, critical flow hasbeen achieved at a Down stream to Up stream pressure ratio of 87 %, 85 % and 82 % for portsizes of 3/16 , 1/4 and 5/16 port sizes respectively.

3. Modified gas lift valves have been installed in two wells of Ankleswar Asset (Ank # 293 &G #310).

4. In Ank # 293, Gas injection rates are constant even with varying pressure differential across

the GLV. This is because, with modified GLV, the critical rates could be achieved at a muchhigher-pressure differential (75 %).

5. From Two Pen recording chart of Ank # 293, it can be seen that flow in the tubing is quite

stable.6. Results from the studies in Ank-293 validated the expected benefits of improved flow stability

in tubing and achieving stable gas rates even with varying pressure differential across GLV.

7. No useful conclusions could be drawn in G # 310, basically due to low potential nature of thecandidate well.

8. Field application of Modified gas lift valve technology has validated the benefits seen during

experimentation in the laboratory.

ReferencePetrotech-2005 Paper ID No.49 titled Improved Gaslift Valve Technology presented in New Delhi,

January, 2005

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GAS LIFT STABILITY ISSUES