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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

®

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Outline

Introduce production analysisReservoir geometries- Synthetic cases - Field examples

Completion parameters- Synthetic cases - Field examples

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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?

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Conditions

Reservoir geometries–Stress dependent permeability –Radial composite–Two-layer system

Completion parameters–Hydraulic fracture clean-up and damage–Liquid loading

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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

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Applications of Production Analysis

Determine effective drainage volumeEstimate drainage areaEstimate reserves / productive lifeIdentify infill drilling potential Estimate reservoir flow capacity Completion performance evaluation

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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

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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

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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

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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

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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

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Stress Dependent Permeability

Matrix Natural Fracture

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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

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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

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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

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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

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Field Example Stress Dependant Permeability

Well Location North Dakota Carbonate ~ 10,000 ft

Flow capacity Natural fractured

Completion Horizontal

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Depletion stem

Field Example Production Analysis Results

Rate & Pressure HistoryLog-Log Plot

Limited ReservoirFlow capacity 13.6 md-ftEffective length 520 ft

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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

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Actual Pressure Build UpLog-Log Plot

100 Hour TestStimulated well performanceNo Boundaries

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Actual Pressure Build UpPressure Match

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Radial Composite

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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

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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

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Pressure Profile After Two Years

Blue area is the 5 acre higher flow capacity areaRadial shape reflected in the model

400 5000

Pressure, Psia

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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

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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

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Pressure Profile After Two YearsHigh Permeability Channel

Both layers drainedLinear flow geometry from PA

400 5000

Pressure, Psia

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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

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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

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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?

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Wellbore Dynamics - Loading

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Wyoming Field Example

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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

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Imbibition Under Flowing Conditions

Laboratory work by Stim-Lab

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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

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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

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West Texas Well

West Texas Field Example

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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.

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Questions?

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