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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