1. Reducing Well Test Deferments from Clusters of Gas-Lifted Sub-Sea Wells
Philip Holweg Wim der Kinderen
(Shell Expro)
API Gaslift Workshop Woodbank, Aberdeen 12 November 2001

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

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

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

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

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

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