Reducing Well Test Deferments from Clusters of Gas-Lifted Sub-Sea Wells

1. Reducing Well Test Deferments from Clusters of Gas-Lifted Sub-Sea Wells Philip HolwegWim der Kinderen (Shell Expro) API Gaslift WorkshopWoodbank, Aberdeen12 November 2001

2. Reducing Well Test Deferments fromClusters of Gas-Lifted Sub-Sea Wells Outline: • Why sub-sea clusters are difficult to test • Well testing by difference - methodology • Example from Gannet • Benefits of testing by difference • Conclusions

3. Issues in Well Testing Subsea Clusters #1 Significant deferment when well testing subsea • flow line usually is a production constraint • test line often used to maximise production • multi-rate tests result in sub-optimal production • choke wells to change THP • change gaslift to assess gaslift performance • deferring production for duration of the test • long stabilisation times for long tie-backs • fluids must travel from near well bore to facilities to accurately measure BSW, GOR • stabilisation time even longer for gaslifted wells • also need to stabilise annulus pressure

4. Issues in Well Testing Subsea Clusters #2 Often no dedicated test line available • single well on large flow line causes more deferment • even longer stabilisation period due to low velocities • single well on large flow line can induce slugging • need to choke well back to stabilise - further deferment • long averaging period required • risk of wax and hydrate deposition • reduced flow rates cause lower pressures and temperatures

5. Testing Wells Together - by Difference Approach: • Test wells together • usually in normal configuration • Induce controlled changes • change THP through choke or gaslift change • Measure production change by difference • Feed data into linear PQ curve model • calculate individual well performance (PQ curves) for all wells

6. 100 Well Model 90 80 Linearised 70 60 Gaslift(103 m3/d) THP (bar) 50 40 40 20 0 30 20 10 0 0 100 200 300 400 500 600 700 800 900 Gross Flow Rate (m3/d) Linearised PQ Curves

7. Linearised PQ curves rewrite: where: the constant C includes reference lift gas rate

8. Calculation for Dual Well Test

9. Generalised Methodology • More test points than unknowns improves accuracy • Measurement uncertainty can be included • THP data points can be replaced by PDG measurements • improves accuracy • Require one single well test to avoid singular data set • unless PDG is available (measure SIBHP using PDG) • Methodology works also with more than 2 wells

10. Examples from Gannet • 7 fields tied back to facilities at single platform • 6 subsea fields • Long tie backs • 3 to 15 km • most fields are gaslifted • limited test lines Field Distance (km) Gaslift Test Line D 15   E 14   F 12   G 5  

12. Gannet D - Schematic Overview Andrew Tay Test Sep GD-01 Gannet A 15.5 km to Gannet A MPM Bulk Sep GD-02 R31 6” 6” GD-03 4” (blocked) R32 Bulk Sep GD-04 4” (gas lift) Gannet G GD-06 Gannet D

13. Gannet D - Dual Well Test Programme Well 1 Well 2 Total THP(bar) THP(bar) Gaslift(103 m3/d) Gaslift(103 m3/d) Q gross(t/d) Test 1 0 56.6 0 602.6 0 Test 2 0 67.8 41.2 518.3 0 Test 3 71.1 38.5 69.6 41.1 583.6 Test 4 58.1 57.6 38.8 905.3 36.8 Test 5 53.1 52.2 0 1001.4 44.7 Test 6 49.5 50.6 0 862.5 20.8

14. 80 70 60 Single Test 1 50 Single Test 2 Combined Test 3 40 THP (bar) Combined Test 4 Combined Test 5 Combined Test 6 30 20 10 0 0 200 400 600 800 1000 1200 Gross Flow Rate (t/d) Gannet D - Derived PQ Curves

15. 80 70 60 Single Test 1 50 Single Test 2 Combined Test 3 THP (bar) 40 Combined Test 4 Gaslift: 20 38 45 0 40 .103 m3/d Combined Test 5 Combined Test 6 30 GD-01 GD-04 20 10 0 0 200 400 600 800 1000 1200 Gross Flow Rate (t/d) Gannet D - Derived PQ Curves

16. 80 70 60 Single Test 1 50 Single Test 2 Combined Test 3 THP (bar) 40 Combined Test 4 Gaslift: 20 38 45 0 40 .103 m3/d Combined Test 5 Combined Test 6 30 GD-01 GD-04 20 10 0 0 200 400 600 800 1000 1200 Gross Flow Rate (t/d) Gannet D - Derived PQ Curves

17. Conventional 1000 1 2 3 4 5 6 800 600 Rate (m3/d) 400 200 0 normal operation stabilisation measurement By Difference 1000 800 600 Rate (m3/d) 400 200 0 0 24 48 72 96 120 144 168 Time (hours) Benefits - Example for 2 Well Cluster Assumptions: • Individual rate: 500 m3/d • Combined rate: 800 m3/d Time(hours) Deferment(m3) Conventional 105 1540 By difference 76 320 Benefit 25% 80%

18. Testing by Difference - Provisos • Need capability to measure cluster production • difficult when multiple fields are commingled • Wells must be stable over test period • gas coning wells may be less suitable • Still need one test point with single well flowing • unless a PDG is available • Assumption of linearised PQ curves not always valid • especially for gaslifted wells at higher THPs • apply multiple slopes for one well • gather more test points

19. Conclusions - Testing by Difference • Significant reduction in well test deferment • especially for gaslifted subsea wells • typically saves 80% hydrocarbon deferment & 20 % test time • avoids adverse flowline conditions • slugging • low p, T • PDGs allow further deferment reduction • no single well test point required • Quality of well tests not affected • approximation of linearised PQ curves usually proves not to be a limitation • extend linear model per well if necessary • PDGs make this method more accurate & simpler