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Primary funding is provided by
The SPE Foundation through member donations and a contribution from Offshore Europe
The Society is grateful to those companies that allow their professionals to serve as lecturers
Additional support provided by AIME
Society of Petroleum Engineers Distinguished Lecturer Programwww.spe.org/dl 1
Effects Of Complex Reservoir Geometries And Completion Practices On Production Analysis In Tight
Gas Reservoirs
Stuart A. CoxMarathon Oil Company
Society of Petroleum Engineers Distinguished Lecturer Programwww.spe.org/dl
®
3
Outline
Introduce production analysisReservoir geometries- Synthetic cases - Field examples
Completion parameters- Synthetic cases - Field examples
4
Purpose
Address the following question:When performing production analysis, can complex reservoir geometries and completion practices cause linear flow, limited fracture half lengths and limited drainage areas to be predicted?
5
Conditions
Reservoir geometries–Stress dependent permeability –Radial composite–Two-layer system
Completion parameters–Hydraulic fracture clean-up and damage–Liquid loading
6
Production Analysis
Rate, time, pressure analysis
Long term pressure drawdown test
Type-curve matching technique
Major Assumptions- Single-phase fluid- Constant reservoir / completion properties- Volumetric production- Bottomhole pressure known
7
Applications of Production Analysis
Determine effective drainage volumeEstimate drainage areaEstimate reserves / productive lifeIdentify infill drilling potential Estimate reservoir flow capacity Completion performance evaluation
8
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
Infinite Conductivity Fracture
Kh = 3.27 md-ftXf = 200 ft
Boundary DominatedUniform Flow
Boundary DominatedLinear Flow
Infinite Acting Flow
Example of Reservoir Flow Geometry on Diagnostic Type Curve
9
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Boundary Dominated Flow
East Texas Example 2 to 1 Rectangular Boundary at 1,017 Days
Kh = 3.3 md-ft
Inifite Acting Pseudo Radial Flow
Uniform Flux Fracture Xf = 380'
Field Example of Flow Characteristics
East Texas Example First 90 Days
10
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Boundary Dominated Flow
East Texas Example 2 to 1 Rectangular Boundary at 1,017 Days
Kh = 3.3 md-ft
Inifite Acting Pseudo Radial Flow
Uniform Flux Fracture Xf = 380'
Field Example of Flow Characteristics
11
Base Simulation Cases
Uniform – 40 acres Model ParametersFormation top, ft 10,000
Initial reservoir pressure, psi 5,000
Net pay, ft 40
Gas specific gravity 0.65
Effective Gas Perm. md 0.05
Fracture half -length, ft 200
Fracture Conductivity, md-ft 75
Simulation Controls• Flowing tubing pressure 350 psia
• Production time 2 years
• Single layer model
12
Base Case Radial Flow
Results match simulationAverage pressure 2,630 psi after 2 years
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
Match Simulation Kh = 2.0 md-ft, 2.0 md-ftXf = 165 ft, 165 ftArea= 40 Acres, 40 Acres
Pressure, Psia
400 5000
13
Stress Dependent Permeability
Matrix Natural Fracture
14
Stress Dependent Permeability
Reduced flow capacity
Reduction in reservoir and completion flow capacity
Flowing pressure 450 psi0.001
0.01
0.1
1
10
0 1000 2000 3000 4000 5000 6000 7000Net Stress, Psi
Perm
eabi
lity
Mul
tiplie
rChange in Net Stress, psi
15
Stress Dependent Permeability
Flow capacity is reduced, radial flowFalse depletion stemThirty year effective drainage area of 34 acres
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
Match Kh = 1.0 md-ftXf = 180 ftArea= 32 Acres
400 5000
Pressure, Psia
16
Natural FractureParameters
Fracture spacing 30 ftFlow capacity 2.0 md-ft - Matrix = 0.005 md- Natural fracture = 0.045 md
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Type Curve Match
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 3 to 1 Rectangular Boundary at 2 years
Match Kh = 0.68 md-ftXf = 165 ft Area= 23 Acres
Reduced flow capacity Linear flowLimited drainage area
18
Pressure Profile After Two Years
No Stress dependent Permeability Stress dependent Permeability
False depletion stem draining ~ 20 AcresLinear flowPA after 16 years resulted in a 40 acre
400 5000
Pressure, Psia
400 5000
Pressure, Psia
19
Field Example Stress Dependant Permeability
Well Location North Dakota Carbonate ~ 10,000 ft
Flow capacity Natural fractured
Completion Horizontal
20
Depletion stem
Field Example Production Analysis Results
Rate & Pressure HistoryLog-Log Plot
Limited ReservoirFlow capacity 13.6 md-ftEffective length 520 ft
21
Pressure Build Modeling
1E-3 0.01 0.1 1 10 1001
10
100
Log-Log plot: p-p@dt=0 and derivative [psi] vs dt [hr]
Log-Log Plot
22
Actual Pressure Build UpLog-Log Plot
100 Hour TestStimulated well performanceNo Boundaries
23
Actual Pressure Build UpPressure Match
24
Radial Composite
25
Radial Composite
Two regions considered - Inner region – 5 acres, 2 md-ft- Outer region – 35 acres, 0.02 md-ft
Results- Reduced effective drainage area - PA match shows linear flow- Long-term complex transient behavior- PA after 25 years results show 40 acres
26
Type Curve Match
Linear flow with limited drainage area
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 3 to 1 Rectangular Boundary at 2 years
Case 3 Kh = 2.0 md-ftXf = 165 ft Area= 7 Acres
27
Pressure Profile After Two Years
Blue area is the 5 acre higher flow capacity areaRadial shape reflected in the model
400 5000
Pressure, Psia
28
Two Non-Communicating Layers
Description – Top layer – 466 ft x 1320 ft, ~ 14 acres
– Length to width aspect ratio of 2.8– Bottom layer – 40 acre– Flow capacity for each layer
- 2 md-ft- 20 md-ft
29
High permeability layer dominates geometryGas in place matches the actual volumeSingle layer model can not capture the complex geometry
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 2.8 to 1 Rectangular Boundary at 2 years
Case 4 Kh = 16 md-ftXf = 125 ft Area= 28 Acres
High Flow Capacity Channel
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
Case 4 Kh = 20 md-ftXf = 125 ft Area= 28 Acres
Low Flow Capacity Channel
30
Pressure Profile After Two YearsHigh Permeability Channel
Both layers drainedLinear flow geometry from PA
400 5000
Pressure, Psia
31
Hydraulic Fracture Clean-up
Initial fracture conductivity set at 2 md-ft60 day clean-up to a final fracture conductivity of 75 md-ftResult show short effective length
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
Match Simulation Kh = 2.0 md-ft, 2.0 md-ftXf = 37 ft, 165 ftArea= 40 Acres, 40 Acres
32
0.01
0.1
1
10
100
0.0001 0.001 0.01 0.1 1 10 100tDA
P wD
or P
wD'
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Infinite Conductivity Fracture in 2.2 to 1 Rectangular Boundary at 2 years
Match Simulation Kh = 1.8 md-ft, 2.0 md-ftXf = 181 ft, 165 ftArea= 17 Acres, 40 Acres
Fracture Conductivity Reducing
Fracture conductivity reduced by 1% each day for the two years False depletion stem and linear flow
400 5000
Pressure, Psia
33
Liquid Loading –What’s The Problem?
– Additional back pressure on formation?– Poor estimate of actual bottomhole
pressure from surface data?– Imbibition of water into the formation while
the well is flowing and static?– Will the well improve if unloaded?– Do loaded wells result in a false depletion
stem and reservoir shape?
34
Wellbore Dynamics - Loading
35
Wyoming Field Example
36
Field Example Production Analysis
0.1
1
10
100
1000
0.0001 0.001 0.01 0.1 1 10 100 1000 10000 100000tDA
P wD
or P
wD'x
0.1
Actual PwDActual PwD'Analytical PwDAnalytical PwD'
Radial Flow in 1 to 1 Rectangular Boundary Pre-loading
Match Kh = 20 md-ftSkin = -1.1 Area = 190 Acres
37
Imbibition Under Flowing Conditions
Laboratory work by Stim-Lab
38
Liquid Loading
Standing liquid promotes near-well damage through spontaneous imbibition.In field applications it is common to see both linear flow and false depletion stemsPressure profile from the model confirms the false depletion
Two Year Pressure ProfileLate Time SLC
550 5000
Pressure, Psia
39
Sims Gas Unit No. 1
0
250
500
750
1000
8/1/2004 8/1/2005 8/1/2006
Gas
Rat
e &
Tub
ing
Pres
sure
, (m
scf/D
, psi
)
0
250
500
Wat
er R
ate,
bbl
/D
MCFDFTPBWPD
Installed Pumping Unit
East Texas Well
East Texas Field Example
40
West Texas Well
West Texas Field Example
41
Observations
Complex flow conditions can cause PA to incorrectly predict flow geometry and drainage area.Actual reservoir properties can be reproduced through PA when the reservoir and the fracture are producing at a pseudo steady state conditions. When these conditions are not achieved, PA can not be expected to provide unique solutions.The cases presented highlight the need to incorporate all available data into the analysis of the well’s performance and recognize the limitations of the technique being used to analyze well performance.
42
Questions?
43