Reservoir Stress-Sensitivity

BGD Smart

JM Somerville

M Jin

Reservoir Stress-Sensitivity

• Reservoir properties and therefore behaviour influenced by changes in stress

• Caused by either changes in pore pressure or temperature, or combination

• Properties = permeability, dimensions, integrity

Stress-Sensitivity Scales

• Near wellbore– permeability – (stress skin cf skin caused by

invasion)– failure

• Increasingly distant from the wellbore– permeability

• Whole reservoir– permeability, directional floods

• Field– compaction, subsidence, seal alteration

Stress-Sensitivity Scales

• Near wellbore – Influenced by UBD– permeability – (stress skin, no skin

caused by invasion)– failure

• Increasingly distant from the wellbore– permeability

Reservoir Stress-Sensitivity: a multi-disciplinary challenge

More Realistic Reservoir

Model

Better Decisions

Reservoir Stress-Sensitivity: a multi-disciplinary challenge

More Realistic Reservoir

Model

Better Decisions

Stress Sensitivity

Better Decisions Re:-• Reserves• Well design• PI• Well locations• Production strategy• Reservoir management (inc 4D seismic)• Seal integrity• Compartmentalisation• Facilities• Efficacy of UBD technology and methodology

All impacting recovery factor and costs

HWYH-399

Breakout from CBIL(A)

Key:

Drilling-inducedtension from STAR

HWYH-394

Key:

Drilling-inducedtension cracks

All from STAR

Bed boundary

Fracture

Unclassified, possiblestylolite

The Conceptual Model

• The reservoir consists of blocks or layers of intact rock bounded by discontinuities

• The reservoir is stressed in an anisotropic manner

• The whole system exhibits hysteresis

Thinly-bedded interval in the Annot Sandstone.This intervalis underlain and overlain by more ‘massive’ sandstones.

The Reservoir

“Intact” Rock

Discontinuities

Boundary and Local Stresses within the Reservoir

Reservoir

Boundary or Regional Stresses

hh H

v

hHh

Local Stresses

v

v

v

Intact Rock Properties(stress-sensitive values where appropriate)

Ambient porosity and permeability Elastic constants E and v Biot’s coefficient Failure (Fracture) Criteria Vp and Vs velocities Vp anisotropy at ambient conditions Permeability at reservoir stress

conditions Palaeomagnetic trial

Tests with Specimen in Triaxial Cell

Stress-Sensitive Values of:-

• Elastic Moduli• Biot’s Coefficient• Permeability• Vp,Vs• Failure Criterion

1

1

22

1

2

P and S waves

Fluid flowing at pressure

Single State Triaxial Testing

1

1

2 = constant

Failure

1

1

2 2

1

1

2

x1

1

2’

2’’

2’’’

2’’’’

x

x

x

xx

x

x

Tan = Triaxial Factor

Failure Criterion - Triaxial Factor

Multi-Failure State Triaxial Testing

1

2

x1

1

2’

2’’

2’’’

2’’’’

x

x

x

x

x

x

x

Tan = Triaxial Factor

P Wave Velocity at 27.5MPa versus Porosity

y = -111.63x + 6753

R2 = 0.7776

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

Porosity (%)

P W

ave

Vel

ocity

(m

/s)

Series1

UTMN 1307

HRDH 704

HWYH 325

HWYH 394

HWYH 399

Vp at 27MPa vs Porosity

Modulus of Elasticity at 27.6MPa versus Porosity

y = -1.7701x + 68.839

R2 = 0.5076

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

Porosity (%)

Mo

du

lus

of

Ela

stic

ity

(G

Pa)

Series1UTMN 1307

HRDH 704HWYH 325

HWYH 394HWYH 399

Young’s Modulus at 27 MPa vs Porosity

Angle of Internal Friction versus Porosity

y = -1.3045x + 49.54R2 = 0.7722

0

10

20

30

40

50

60

70

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

Porosity (%)

Ang

le o

f In

tern

al F

rict

ion

Series1

UTMN 1307

HRDH 704

HWYH 325

HWYH 394

HWYH 399

Angle of Internal Friction vs Porosity

Sampling Rationale - Intact Rock

Petrophysical Property

Rock Mechanics Property

Sample Core,

then Test

Wireline Log

Correlation

Populating Model - Intact Rock

Correlation

Synthetic Rock Mechanics Log

Convert Reservoir Characterisation Model into a Geomechanical Model

The Process• Populate the Conceptual Model with

properties and data

• So create a Geomechanical Model of the reservoir (plus surrounding rock)

• Impose process-induced changes on the Geomechanical Model using analytical or numerical solutions

• Numerical offers more realism than analytical – hence coupled modelling

Coupled Modelling

Fluid Flow Simulator

Stress-Analysis Simulator

Change in Pore Pressure, Temperature,

Saturations

Change in Effective Stresses

Rock Movements, Change in Stress and Strain

Change in Permeability

More realistic simulation results

Reservoir and o/b stresses, strains and

displacements

Differentiating Filter (Synthetic)

Fluid Flow Simulator

Stress-Analysis Simulator

Change in Pore Pressure, Temperature,

Saturations

Change in Effective Stresses

Rock Movements, Change in Stress and Strain

Change in Permeability

Enhanced 4D Seismic Interpretation/Reservoir

Management

Saturation-Related changes in Impedance

Stress-Related changes in Impedance

Changes in Velocity and Density

Differentiating Filter (Synthetic)

Fluid Flow Simulator

Stress-Analysis Simulator

Change in Pore Pressure, Temperature,

Saturations

Change in Effective Stresses

Rock Movements, Change in Stress and Strain

Change in Permeability

Enhanced 4D Seismic Interpretation/Reservoir

Management

Saturation-Related changes in Impedance

Stress-Related changes in Impedance

Changes in Velocity and Density

More realistic simulation results

Reservoir and o/b stresses, strains and

displacements

Example 1

UKNS, Perm Stress Sensitivity

(ECLIPSE coupled with VISAGE)

Production Prediction: permeability reduction

The diagram shows the absolute reduction (k1-k18). The maximum reduction in permeability is in the central part of the field

Perm sensitivity modelled with hysteresis

0.37000

0.37500

0.38000

0.38500

0.39000

0.39500

42000 44000 46000 48000 50000

mean stress (kPa)

k/k0

Series1

(ECLIPSE Output)

Stress Sensitive Permeability with hysteresis

0.37000

0.37500

0.38000

0.38500

0.39000

0.39500

42000 44000 46000 48000 50000

mean stress (kPa)

k/k0

Series1

Injection in Miller induced unloading

Injection in South Brae induced unloading in Miller Field

Depressurisation in Miller

Oil Production Rate is sharply reduced because the permeability reduction in the area causes a reduction in BHP and leads to a increase in gas production

Comparison of GOPR Predictions

(ECLIPSE Output)

Horizontal Ground Displacements - 1

Horizontal Ground Displacements - 2

Horizontal Ground Displacements - 3

Stress Ratio vs. time

Close to well

Far from well

Between wells

kq p

q p

3

1

3

3 2

/

/

Stress Status in p-q terms (anisotropy)

close to wells

far from wells

Stress Path Distribution

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61

S1

S4

S7

S10

S13

S16

S19

S22

S25

S28

S31

S34

S37

S40

S43

S46

S49

S52

S55

S58

k/k0

%

q

p

K stress path sensitive for Unconsolid Sand

90-100

80-90

70-80

60-70

50-60

40-50

30-40

20-30

10-20

0-10

Permeability Stress Path Sensitivity

Normalised Permeability Contoursk(%):

MOBIL "U"- Field: Unconsolidated Sand

Mean Effective Stress, p' (MPa)

#C4C2P2

#C4C2P4

#C4C2P6

#C4C4P1A

#C4C5P1

0

10

20

30

40

0 10 20 30 40 50

<= 30.0

<= 50.0

<= 70.0

<= 90.0

<= 35.0

<= 55.0

<= 75.0

<= 95.0

<= 40.0

<= 60.0

<= 80.0

<= 100.0

<= 45.0

<= 65.0

<= 85.0

> 100.0

Diff

eren

tial

Str

ess,

q

(M

Pa)

N/A(UCMS)

MATLAB

Excel

1

11

21

31

41

51

61

S1

S4

S7

S10

S13

S16

S19

S22

S25

S28

S31

S34

S37

S40

S43

S46

S49

S52

S55

S58

0

100

k

q

P'

p-q-k 3D

100-200

0-100

Compaction and subsidence

XY

Z

Model: MODL01L005: TIME/MONTHS *******Nodal DISPLACE YMax/Min on model set:Max = .504E-3Min = -.461E-1

-.4E-1-.35E-1-.3E-1-.25E-1-.2E-1-.15E-1-.1E-1-.5E-20.5E-2.1E-1.15E-1.2E-1

21-JAN-2000 10:53 compac05.cgmFEMGV 6.1-02 : HERIOT-WATT UNIVERSITY

XY

Z

Model: MODL01L018: TIME/MONTHS *******Nodal DISPLACE YMax/Min on model set:Max = .194E-1Min = -.339E-1

-.4E-1-.35E-1-.3E-1-.25E-1-.2E-1-.15E-1-.1E-1-.5E-20.5E-2.1E-1.15E-1.2E-1

21-JAN-2000 11:10 compac18.cgmFEMGV 6.1-02 : HERIOT-WATT UNIVERSITY

Compaction IN 1995 in which the result of injection is shown

Compaction in 1987 1

2

Example 2

UKNS, Seismic Stress-Sensitivity

(ECLIPSE, VISAGE, H-WU software)

Features of a 2D flow model grid embedded

for coupled geomechanical simulation

Overburden

Sideburden

Faults

Well

Gas , Water in the flow model grid

Caprock

Displaced shape of the geomechanical model

Surface subsidence

Differential compactionacross faults in reservoir

Typical location of shearstrain on faults

(VISAGE Output)

Mean effective stress distribution at the end of the simulation

Localized effectsat faults

Perturbed stress fieldabove and below reservoir

Unperturbed stress field(constant gradient) Apparent deepening of reservoir

due to decreasing pore pressure

(VISAGE Output)

Time-lapsed compressional acoustic impedance

Initial gas-water contact Changes in reservoirdue to fluid movement

Changes in reservoirdue to pore pressure decline

Changes in overburden/caprockdue to stress redistribution

Top of caprock

(VISAGE Output)

Initial Modelling: Before Production Begins

Time Lapse Model: Saturation Changes Only

Time Lapse Model: Saturation + Stress

Reflector at top of caprock

Reservoir base

Reservoir top

Time-lapsed seismic trace model

Pull-up in reflector eventdue to stress change effects

Perturbations at reflector event due to fluid change effects

Where are we now?

• Extreme examples of reservoir stress-sensitivity accepted: Ekofisk, HP/HT, Gulf of Mexico, Angola?

• The processes required exist in usable form

• Non-uniform levels of commitment• What about the more subtle

reservoirs?

Technical Challenges

• Discontinuity distributions• Discontinuity properties• Rel perm stress-sensitivity• In situ stress state• Coping with anisotropy• Seamless software

Organisational Challenges

• Realising the full value of the data we already have

• Cost vs value of the process

• Coping with multi-disciplinarity

Is this too much to ask for?

Shared analysis

Shared belief

Fully owned

decisions

Better performance

Decision Making• Straight from the geomechanical model,

aided possibly by some calcs, e.g.– fracture density = well locations for max PI– subsidence = yes or no

• With the aid of coupled modeling, e.g– improvement of appraisal– impact of perm sensitivity = recovery, GOR etc– Ground movements and subsidence = threat

to wells and facilities– 4D seismic enhancement = better management

Thank You

What do we want to achieve today?

• Overview of the main tasks of the project

• Select candidate reservoirs for study

• Set up communications

• Agree next meeting date 17th August?

Hysteresis

K

Increasing Stress

Hysteresis

K

Increasing Stress

X

Hysteresis

K

Increasing Stress

UBD site history very important

Effective Stress around the wellbore

Time

Drilling Completion Production

Failure Level

Multi-Disciplinary

Tasks assembling

data for Model

Deliverables

feedback toimprove

characterisation

feedback to improve

characterisation

Basin process simulations

*Genetic Units expertise Analogue studies

*Geomechanics of fracture genesis

*Published and proprietary studies

Stress-Sensitive Reservoir Modelling

and Coupled Simulations (Ground

movements, Fluid Flow and 4D seismic)

Characterise Reservoir Rocks Characterise Reservoir Faults & Fractures

Characterise Structural Setting of the Reservoir

*Log analysis

Reservoir Geomechanical Model

Better Decisions Reservoir Management

*Geomechanical Core Analysis*Structure and anisotropy analysis from Seismic

Creation of the Geomechanical Model

Stress-Sensitive Coupled Modelling

Building the Geomechanical Model