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A Parametric Study on the Benefits of Drilling Multilateral and Horizontal Wells in
Coalbed Methane Reservoirs
Nikola Maricic, Chevron Shahab D. Mohaghegh, WVU
Emre Artun, WVU
October 12, 2005
ATCE 2005Dallas, TexasSPE Paper # 96018
- Coal is an unusual reservoir rock, with highly complex reservoir characteristics.
- A significant amount of gas is stored in the coal rock, rather than the pore space.
- CBM production is a complex phenomenon, which has a unique behavior that needs careful consideration.
motivation > CBM Background
motivation > CBM Background
Butt cleat
Face
clea
t
Fluid production from natural fractures…
Gas desorption from coal surface…
Molecular diffusion through the coal matrix…
1
2
3
Fluid production from natural fractures…
Gas desorption from coal surface…
Molecular diffusion through the coal matrix…
1
2
3
motivation > CBM Background
TimePr
oduc
tion
rate
, MSC
F/d
or b
bl/d
Phase 1
Phase 1:- Constant water production- BHP declines and reaches
its minimum.- Gas rate starts to incline.
motivation > CBM Background
TimePr
oduc
tion
rate
, MSC
F/d
or b
bl/d
Phase 1 Well dewatered
Phase 2:- Significant decline in
water rate- Negative decline in gas
rate
Peak gas rate
motivation > CBM Background
TimePr
oduc
tion
rate
, MSC
F/d
or b
bl/d
Phase 1 Well dewatered
Peak gas rate
Phase 3:- Gas rate starts to decline- Water production
becomes negligible
motivation > Unconventional Wells for CBM
- Very large areas of CBM reservoirs can be drained with horizontal well configurations.
- In a very short time, coal system can be dewatered and peak gas rate can be reached.
- Most optimum well direction, shape, and position must be determined after considering economical limits.
motivation > Parametric Studies
- Changing one variable at a time to see its influence.
- With the current computer power, an important optimization tool to analyze behaviors of different types models.
- Helps us to understand and optimize complex processes such as CBM recovery.
1. Investigate different horizontal and multilateral well configurations for the most optimum CBM recovery.
2. Investigate influences of different types reservoir characteristics on CBM recovery.
Objectives
Conducting parametric studies in order to:
Flow chart
CBM reservoir model
Study on well configurations
Well shapes
Total horizontal length
Spacing between laterals
Study on reservoir
characteristics
Identify the most optimum well configuration
Permeability
Time constant
Gas content
Study on thickness
The Model
- A dual-porosity, homogeneous reservoir model with Cartesian grid system.
- Model properties:
- 31*31 grid block system, each block: 120*120 ft2
- Constant vertical depth = 1,000 ft- Constant drainage area = 320 acres- Production period = 15 years.- Initial default thickness = 4 ft
Study 1
Well Configurations
- Five horizontal well shapes have been identified to investigate:
Horizontal Well Shapes
1. Total Horizontal Length (THL)2. Spacing Between Laterals (SBL)
Investigated Parameters
A lateral well
Vertical depth: constant
Spacing between laterals (SBL)
Spacing between laterals (SBL)
Total Horizontal Length (THL)
Methodology
Determine the Spacing between Laterals (SBL)
Determine the well shape (single, dual, tri, quad, pinnate)
Change the Total Horizontal Length from min. to max.
LOOP 1
LOOP 2
0
5
10
15
20
25
340 840 1340 1840 2340 2840
Horizontal Length, ft
Gas
Rec
over
y, %
Single-lateral
Dual-lateral
0
0.05
0.1
0.15
0.2
0.25
0.3
0 1000 2000 3000 4000 5000 6000
Horizontal Length, ft
Gas
Rec
over
y, %
SBL = 170 ft
SBL = 340 ft
SBL = 680 ftSBL = 1,360 ft
SBL = 2,720 ft
SBL = 2,720 ft
SBL = 1360 ft
SBL = 680 ft
SBL = 340 ft
SBL = 170 ft
Tri-lateral
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 1000 2000 3000 4000 5000 6000 7000 8000
Horizontal Length, ft
Gas
Rec
over
y, %
SBL = 170 ft
SBL = 340 ft
SBL = 680 ft
SBL = 1,360 ft
SBL = 1,360 ft SBL = 680 ft
SBL = 340 ft
SBL = 170 ft
Quad-lateral
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Horizontal Length, ft
Gas
Rec
over
y, %
SBL = 170 ft
SBL = 340 ft
SBL = 680 ft
SBL = 170 ft
SBL = 340 ft
SBL = 680 ft
Pinnate (Fish-bone)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 5000 10000 15000 20000 25000 30000
Horizontal Length, ft
Gas
Rec
over
y, %
SBL = 120 ftSBL = 240 ftSBL = 360 ftSBL = 480 ftSBL = 600 ftSBL = 720 ft SBL = 120 ft
SBL = 240 ftSBL = 360 ft
SBL = 480 ftSBL = 600 ft
SBL = 720 ft
Best Producers Comparison
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
Horizontal Length, ft
Gas
Rec
over
y, %
Duallateral (SBL = 2,720 ft)
Trilateral (SBL = 1,360 ft)
Quadlateral (SBL = 680 ft)
Pinnate (SBL = 240 ft)
Duallateral(SBL = 2,720 ft)
Quadlateral(SBL = 680 ft)
Trilateral(SBL = 1,360 ft)
Pinnate(SBL = 240 ft)
Suggested configuration for gas recovery:Quadlateral, with SBL = 680 ft, THL = 8,000 ft.
Now, let’s look at economics.
Suggested configuration for gas recovery:Quadlateral, with SBL = 680 ft, THL = 8,000 ft.
Now, let’s look at economics.
Quad-lateral Economics
$(500,000.00)
$(400,000.00)
$(300,000.00)
$(200,000.00)
$(100,000.00)
$-
$100,000.00
$200,000.00
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Horizontal Length, ft
Net
Pre
sent
Val
ue (N
PV)
SBL = 170 ft
SBL = 340 ft
SBL = 680 ft
SBL = 170 ft
SBL = 340 ft
SBL = 680 ft
After considering economics:Quadlateral, with SBL = 680 ft, THL = 3,100 ft.
After considering economics:Quadlateral, with SBL = 680 ft, THL = 3,100 ft.
Study 2
Reservoir Characteristics
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
1.000
Single vertical well Four-spot vertical Horizontal
Well Configuration
Gas
reco
very
4 ft
12 ft
3 x 4 ft -Permeability
-Time constant
-Gas content
Permeability
0.160
0.180
0.200
0.220
0.240
0.260
0.280
0.300
0.320
0.340
0.360
0 2 4 6 8 10 12 14 16
Permeability, md
Gas
Rec
over
y
Time Constant
- Time required for gas to be desorbed off the coal surface and diffuse through the coal into the cleat system.
- Also known as sorption time, and an important parameter for time-to peak-gas.
- May vary from less than a day, to 300 days based on the coal composition, rank, and cleat spacing.
Time Constant
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
10 days 50 days 150 days 200 days
Time Constant, days
Gas
Rec
over
y
Time constants ranging between 10 days to 200 days brought a recovery range of
0.26-0.29.
Time constants ranging between 10 days to 200 days brought a recovery range of
0.26-0.29.
Time Constant
0
50,000,000
100,000,000
150,000,000
200,000,000
250,000,000
300,000,000
350,000,000
0 2 4 6 8 10 12 14 16
Years
Cum
. gas
pro
duce
d, ft
3
10 days50 days150 days200 days
Different gas recoveries were caused by the difference in the amount of cumulative gas
production occurred in the 1st year production.
Different gas recoveries were caused by the difference in the amount of cumulative gas
production occurred in the 1st year production.
Time Constant
$(100,000.00)
$(50,000.00)
$-
$50,000.00
$100,000.00
$150,000.00
$200,000.00
25 45 65 85 105 125 145 165
Time Constant, days
Net
Pre
sent
Val
ue (N
PV)
Difference in 1st year cumulative production yielded a ≈ $200,000 difference in the Net
Present Value.
Difference in 1st year cumulative production yielded a ≈ $200,000 difference in the Net
Present Value.
Gas Content
- Very critical parameter that influences the gas recovery from coal.
- Gas content values: 350 scf/ton, 395 scf/ton, 450 scf/ton
- Five different isotherms were generated by changing Langmuir pressure and volume constants.
Gas Content
VL, scf/ton
PL, psi
Gas content, scf/ton
Modification 1 480 167.5 350
Northern App. 542.03 167.5 395Modification 2 480 96.84 395Modification 3 617.5 167.5 450Modification 4 480 30 450
Langmuir volume constant
Langmuir pressure constant
Gas Content
0.00
100.00
200.00
300.00
400.00
500.00
600.00
0 100 200 300 400 500 600
Pressure, psia
Gas
Con
tent
, scf
/ton
Modification 1Northern AppalachiaModification 2Modification 3Modification 4
Gas Content
0.160
0.180
0.200
0.220
0.240
0.260
0.280
0.300
0.320
0.340
0.360
320 340 360 380 400 420 440 460 480 500
Gas Content, scf/ton
Gas
Rec
over
yChanging VlChanging Pl
Mod. 1
N. App.
Mod. 3
Mod. 2
Mod. 5
Study 3
Thickness
h1
h2
Thickness
Case 1
Case 2
Case 3
Single layer, h = 4 ft
Single layer, h = 12 ft
Three layers, each h = 4 ft
hTOTAL = 12 ft
Thickness
Well Configuration 1 Single well
Four-spot
Horizontal
Well Configuration 2
Well Configuration 3
Thickness
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
1.000
Single vertical well Four-spot vertical Horizontal
Well Configuration
Gas
reco
very
4 ft
12 ft
3 x 4 ft
Thickness
$-
$500,000.00
$1,000,000.00
$1,500,000.00
$2,000,000.00
$2,500,000.00
Single vertical well Four-spot vertical Horizontal
Well Configuration
Net
Pre
sent
Val
ue (N
PV)
Summary
- A parametric study to determine benefits of horizontal and multilateral wells is presented.
- Different well configurations such as single-, dual-, tri-, quad-laterals, and pinnate were investigated by changing:- Spacing between laterals (SBL)- Total horizontal length (THL)
- Influences of some reservoir characteristics were investigated by changing:- Permeability
- Time constant
- Gas content
- Thickness
Conclusions
- CBM recovery can significantly benefit from unconventional well configurations.
- Considering a 320 acres of drainage area, the optimum configuration that the results of this study suggests is:- Quadlateral well configuration:
SBL = 680 ft, and THL = 3,100 ft.
- Time constant has a significant effect on the first year production, although it does not have a considerable effect on overall recovery.
Acknowledgements
- Authors would like to thank Computer Modeling Group, Inc. for providing the software that was used in this study.
Acknowledgements
- Authors would like to thank Computer Modeling Group, Inc. for providing the software that was used in this study.
October 12, 2005
ATCE 2005Dallas, TexasSPE Paper # 96018
A Parametric Study on the Benefits of Drilling Multilateral and Horizontal Wells in
Coalbed Methane Reservoirs