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 Program
www.spe.org/dl
1
1
Effects Of Complex Reservoir Geometries And
Completion Practices On Production Analysis In Tight
Gas Reservoirs
Stuart A. Cox
Marathon Oil Company
Society of Petroleum Engineers
Distinguished Lecturer Program
www.spe.org/dl
®
Outline
¾Introduce production analysis
¾Reservoir geometries
- Synthetic cases
- Field examples
¾Completion parameters
- Synthetic cases
- Field examples
3
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?
4
Conditions
¾Reservoir geometries
– Stress dependent permeability
– Radial composite
– Two-layer system
¾Completion parameters
– Hydraulic fracture clean-up and damage
– Liquid loading
5
Production Analysis
¾ Rate, time, pressure analysis
¾ Long term pressure drawdown test
¾ Type-curve matching technique
¾ Major Assumptions
-
6
Single-phase fluid
Constant reservoir / completion properties
Volumetric production
Bottomhole pressure known
Applications of
Production Analysis
¾Determine effective drainage volume
¾Estimate drainage area
¾Estimate reserves / productive life
¾Identify infill drilling potential
¾Estimate reservoir flow capacity
¾Completion performance evaluation
7
Example of Reservoir Flow Geometry
on Diagnostic Type Curve
Infinite Conductivity Fracture
100
Boundary Dominated
Uniform Flow
Kh = 3.27 md-ft
Xf = 200 ft
Infinite Acting Flow
10
PwD
Boundary Dominated
Linear Flow
1
0.1
0.0001
0.001
0.01
0.1
tDA
8
1
10
100
Field Example of Flow Characteristics
East Texas
FirstBoundary
90 Days at 1,017 Days
East Texas Example
2 to 1Example
Rectangular
100
Kh = 3.3 md-ft
Boundary Dominated
Flow
Inifite Acting
Pseudo Radial Flow
PwD or PwD'
10
Uniform Flux Fracture
Xf = 380'
1
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.1
0.0001
0.001
0.01
0.1
tDA
9
1
10
100
Field Example of Flow Characteristics
East Texas Example 2 to 1 Rectangular Boundary at 1,017 Days
100
Kh = 3.3 md-ft
Boundary Dominated
Flow
Inifite Acting
Pseudo Radial Flow
PwD or PwD'
10
Uniform Flux Fracture
Xf = 380'
1
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.1
0.0001
0.001
0.01
0.1
tDA
10
1
10
100
Base Simulation Cases
Model Parameters
Uniform – 40 acres
Formation top, ft
10,000
Initial reservoir pressure, psi
Net pay, ft
40
Gas specific gravity
0.65
Effective Gas Perm. md
0.05
Fracture half -length, ft
200
Fracture Conductivity, md-ft
Simulation Controls
• Flowing tubing pressure 350 psia
• Production time 2 years
• Single layer model
11
5,000
75
Base Case Radial Flow
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
100
Match
Kh = 2.0 md-ft,
Xf = 165 ft,
Area= 40 Acres,
Simulation
2.0 md-ft
165 ft
40 Acres
PwD or PwD'
10
1
0.1
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
1
10
Pressure, Psia
100
tDA
400
¾ Results match simulation
¾ Average pressure 2,630 psi after 2 years
12
5000
Stress Dependent Permeability
Matrix
13
Natural Fracture
Stress Dependent Permeability
¾ Reduced flow capacity
Permeability Multiplier
¾ Reduction in reservoir and
completion flow capacity
10
1
0.1
0.01
0.001
¾ Flowing pressure 450 psi
14
0
1000
2000
3000
4000
Change inNet
NetStress,
Stress,Psi
psi
5000
6000
7000
Stress Dependent Permeability
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
100
Match
Kh = 1.0 md-ft
Xf = 180 ft
Area= 32 Acres
PwD or PwD'
10
1
0.1
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
1
10
Pressure, Psia
100
tDA
400
¾ Flow capacity is reduced, radial flow
¾ False depletion stem
¾ Thirty year effective drainage area of 34 acres
15
5000
Natural Fracture
Parameters
¾ Fracture spacing 30 ft
¾ Flow capacity 2.0 md-ft
- Matrix = 0.005 md
- Natural fracture = 0.045 md
16
Type Curve Match
Infinite Conductivity Fracture in 3 to 1 Rectangular Boundary at 2 years
100
Match
Kh = 0.68 md-ft
Xf = 165 ft
Area= 23 Acres
PwD or PwD'
10
1
0.1
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
tDA
¾Reduced flow capacity
¾Linear flow
¾Limited drainage area
17
1
10
100
Pressure Profile After Two Years
No Stress dependent Permeability
Stress dependent Permeability
Pressure, Psia
400
Pressure, Psia
5000
400
¾ False depletion stem draining ~ 20 Acres
¾ Linear flow
¾ PA after 16 years resulted in a 40 acre
18
5000
Field Example
Stress Dependant Permeability
¾Well Location
North Dakota
¾Carbonate
~ 10,000 ft
¾Flow capacity
Natural fractured
¾Completion
Horizontal
19
Field Example
Production Analysis Results
Log-Log Plot
Rate & Pressure History
Depletion stem
¾ Limited Reservoir
¾ Flow capacity
¾ Effective length
20
13.6 md-ft
520 ft
Pressure Build Modeling
Log-Log Plot
100
10
1
1E-3
0.01
0.1
1
10
Log-Log plot: p-p@dt=0 and derivative [psi] vs dt [hr]
21
100
Actual Pressure Build Up
Log-Log Plot
¾ 100 Hour Test
¾ Stimulated well performance
¾ No Boundaries
22
Actual Pressure Build Up
Pressure Match
23
Radial Composite
24
Radial Composite
¾ Two regions considered
- Inner region – 5 acres, 2 md-ft
- Outer region – 35 acres, 0.02 md-ft
¾ Results
-
25
Reduced effective drainage area
PA match shows linear flow
Long-term complex transient behavior
PA after 25 years results show 40 acres
Type Curve Match
Infinite Conductivity Fracture in 3 to 1 Rectangular Boundary at 2 years
100
Case 3
Kh = 2.0 md-ft
Xf = 165 ft
Area=
7 Acres
PwD or PwD'
10
1
0.1
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
1
10
100
tDA
¾ Linear flow with limited drainage area
26
Pressure Profile After Two Years
Pressure, Psia
400
5000
¾ Blue area is the 5 acre higher flow capacity area
¾ Radial shape reflected in the model
27
Two Non-Communicating Layers
¾Description
– 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
28
High Flow Capacity Channel
Low Flow Capacity Channel
Infinite Conductivity Fracture in 2.8 to 1 Rectangular Boundary at 2 years
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
100
100
Case 4
Kh = 16 md-ft
Xf = 125 ft
Area= 28 Acres
Case 4
Kh = 20 md-ft
Xf = 125 ft
Area= 28 Acres
10
PwD or PwD'
PwD or PwD'
10
1
0.1
0.01
0.0001
0.1
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
tDA
1
10
1
100
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
1
tDA
¾ High permeability layer dominates geometry
¾ Gas in place matches the actual volume
¾ Single layer model can not capture the
complex geometry
29
10
100
Pressure Profile After Two Years
High Permeability Channel
Pressure, Psia
400
¾Both layers drained
¾Linear flow geometry from PA
30
5000
Hydraulic Fracture Clean-up
Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years
100
Match
Simulation
Kh = 2.0 md-ft,
2.0 md-ft
165 ft
Xf = 37 ft,
Area= 40 Acres,
40 Acres
PwD or PwD'
10
1
0.1
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
1
10
100
tDA
¾ Initial fracture conductivity set at 2 md-ft
¾ 60 day clean-up to a final fracture conductivity of 75 md-ft
¾ Result show short effective length
31
Fracture Conductivity Reducing
Infinite Conductivity Fracture in 2.2 to 1 Rectangular Boundary at 2 years
100
Match
Simulation
Kh = 1.8 md-ft,
2.0 md-ft
165 ft
Xf = 181 ft,
Area= 17 Acres,
40 Acres
PwD or PwD'
10
1
0.1
0.01
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
tDA
1
10
Pressure, Psia
100
400
¾Fracture conductivity reduced by 1%
each day for the two years
¾False depletion stem and linear flow
32
5000
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?
33
Wellbore Dynamics - Loading
34
Wyoming Field Example
35
Field Example
Production Analysis
Radial Flow in 1 to 1 Rectangular Boundary Pre-loading
1000
Match
Kh = 20 md-ft
Skin = -1.1
Area = 190 Acres
PwD or PwD'x 0.1
100
10
1
0.1
0.0001
Actual PwD
Actual PwD'
Analytical PwD
Analytical PwD'
0.001
0.01
0.1
1
10
tDA
36
100
1000
10000
100000
Imbibition Under Flowing Conditions
Laboratory work by Stim-Lab
37
Liquid Loading
Late Time SLC
Two Year Pressure Profile
Pressure, Psia
550
¾ Standing liquid promotes near-well damage through
spontaneous imbibition.
¾ In field applications it is common to see both linear flow and
false depletion stems
¾ Pressure profile from the model confirms the false depletion
38
5000
East Texas Field Example
Sims Gas Unit No. 1
East Texas Well
Installed
Pumping Unit
750
250
500
250
0
8/1/2004
39
500
MCFD
FTP
BWPD
0
8/1/2005
8/1/2006
Water Rate, bbl/D
Gas Rate & Tubing Pressure, (mscf/D, psi)
1000
West Texas Field Example
West Texas Well
40
Observations
¾ Complex flow conditions can cause PA to incorrectly predict
flow geometry and drainage area.
¾ 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.
¾ 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.
41
Questions?
42
43