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 Stress Sensitivity Better Decisions

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 Key: Breakout from CBIL(A) Drilling-induced tension from STAR

HWYH-394 Key: Drilling-induced tension cracks Bed boundary Fracture Unclassified, possible stylolite All from STAR

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 interval is underlain and overlain by more ‘massive’ sandstones.

The Reservoir

“Intact” Rock Discontinuities

Boundary and Local Stresses within the Reservoir

s

v Boundary or Regional Stresses

s

h

s

h Reservoir

s

v

s

H

s

v

s

h Local Stresses

s

H

s

v

s

h

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

s

1 P and S waves Fluid flowing at pressure

Stress-Sensitive Values of:-

s

2

e

1

e

2

s

2

• • • • •

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

s

1 Tests with Specimen in Triaxial Cell

s

2

s

1

s

1

e

1 Failure

s

2 = constant

s

2

s

1 Single State Triaxial Testing

e

1

s

1 x x x x

s

2 ’’’’

s

2 ’’’

s

2 ’’

s

2 ’

e

1

s

1 x x x x Tan = Triaxial Factor Failure Criterion - Triaxial Factor

s

2

s

1 x

s

2 ’ x

s

2 ’’ x

s

2 ’’’ x

s

2 ’’’’

e

1

s

1 x x x x Tan = Triaxial Factor

s

2 Multi-Failure State Triaxial Testing

P Wave Velocity at 27.5MPa versus Porosity

9000 8000 7000 6000 5000 4000 3000 2000 1000 0 0.0

Series1 UT MN 1307 HRDH 704 HWYH 325 HWYH 394 HWYH 399 5.0

10.0

15.0

20.0

Porosity (%) y = -111.63x + 6753 R 2 = 0.7776

25.0

30.0

35.0

Vp at 27MPa vs Porosity

Modulus of Elasticity at 27.6MPa versus Porosity

120.00

100.00

80.00

60.00

40.00

20.00

0.00

0.0

Series1 UT MN 1307 HRDH 704 HWYH 325 HWYH 394 HWYH 399 y = -1.7701x + 68.839

R 2 = 0.5076

5.0

10.0

15.0

20.0

Porosity (%) 25.0

30.0

35.0

Young’s Modulus at 27 MPa vs Porosity

Angle of Internal Friction versus Porosity

70 60 50 40 30 20 10 0 0.0

Series1 UT MN 1307 HRDH 704 HWYH 325 HWYH 394 HWYH 399 5.0

10.0

15.0

20.0

Porosity (%) 25.0

30.0

y = -1.3045x + 49.54

R 2 = 0.7722

35.0

Angle of Internal Friction vs Porosity

Sampling Rationale - Intact Rock Wireline Log Rock Mechanics Property Sample Core, then Test Correlation Petrophysical Property

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

More realistic simulation results Fluid Flow Simulator Change in Pore Pressure, Temperature, Saturations Change in Permeability Change in Effective Stresses Rock Movements, Change in Stress and Strain Stress-Analysis Simulator Reservoir and o/b stresses, strains and displacements

Fluid Flow Simulator Change in Pore Pressure, Temperature, Saturations Change in Permeability Change in Effective Stresses Rock Movements, Change in Stress and Strain Enhanced 4D Seismic Interpretation/Reservoir Management Differentiating Filter (Synthetic) Saturation-Related changes in Impedance Stress-Related changes in Impedance Changes in Velocity and Density Stress-Analysis Simulator

More realistic simulation results Fluid Flow Simulator Change in Pore Pressure, Temperature, Saturations Change in Permeability Change in Effective Stresses Rock Movements, Change in Stress and Strain Stress-Analysis Simulator Enhanced 4D Seismic Interpretation/Reservoir Management Differentiating Filter (Synthetic) Saturation-Related changes in Impedance Stress-Related changes in Impedance Changes in Velocity and Density 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

0.39500

0.39000

0.38500

0.38000

0.37500

0.37000

42000 44000 46000 48000 50000

mean stress (kPa)

Series1

(ECLIPSE Output)

Stress Sensitive Permeability with hysteresis

Injection in Miller induced unloading

0.39500

0.39000

0.38500

0.38000

0.37500

0.37000

42000 44000 46000 48000 50000

mean stress (kPa) Injection in South Brae induced unloading in Miller Field

Series1

Depressurisation in Miller

Comparison of GOPR Predictions 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 (ECLIPSE Output)

Horizontal Ground Displacements - 1

Horizontal Ground Displacements - 2

Horizontal Ground Displacements - 3

Stress Ratio vs. time

Between wells Close to well

k

= D D s s 3 1 ў = 3 -

p p

Stress Status in p-q terms (anisotropy)

close to wells far from wells

Stress Path Distribution

k(%):

Permeability Stress Path Sensitivity p-q-k 3D MOBIL "U"- Field: Unconsolidated Sand

40 #C4C2P6 #C4C4P1A #C4C2P4 30 #C4C2P2 20 10 N/A(UCMS) #C4C5P1 MATLA B 0 0 10 20 30 Mean Effective Stress, 40

p'

(MPa) 50 Normalised Permeability Contours <= 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

100 0

K stress path sensitive for Unconsolid Sand P'

S58 S55 S52 S49 S46 S43 S40 S37 S19 S16 S13 S10 S7 S4 S1 S34 S31

p

S28 S25 S22

q

90-100 80-90 70-80 60-70 50-60 40-50 30-40 20-30 10-20 0-10 61 51 21 41 31

q

11 1 Excel 100-200 0-100

Compaction and subsidence

FEMGV 6.1-02 : HERIOT-WATT UNIVERSITY Model: MODL01 L005: TIME/MONTHS ******* Nodal DISPLACE Y Max/Min on model set: Max = .504E-3 Min = -.461E-1

Compaction in 1987

1 Y X FEMGV 6.1-02 : HERIOT-WATT UNIVERSITY Model: MODL01 L018: TIME/MONTHS ******* Nodal DISPLACE Y Max/Min on model set: Min = -.339E-1 2 Y Z X 21-JAN-2000 10:53 compac05.cgm

.2E-1 .15E-1 .1E-1 .5E-2 0 -.5E-2 -.1E-1 -.15E-1 -.2E-1 -.25E-1 -.3E-1 -.35E-1 -.4E-1

Compaction IN 1995 in which the result of injection is shown

.2E-1 .15E-1 .1E-1 .5E-2 0 -.5E-2 -.1E-1 -.15E-1 -.2E-1 -.25E-1 -.3E-1 -.35E-1 -.4E-1

Example 2

UKNS, Seismic Stress Sensitivity (ECLIPSE, VISAGE, H WU software)

Features of a 2D flow model grid embedded for coupled geomechanical simulation

Overburden

Well

Faults Caprock Sideburden

Gas , Water in the flow model grid

Displaced shape of the geomechanical model

Surface subsidence Typical location of shear strain on faults Differential compaction across faults in reservoir

(VISAGE Output)

Mean effective stress distribution at the end of the simulation

Unperturbed stress field (constant gradient) Apparent deepening of reservoir due to decreasing pore pressure Perturbed stress field above and below reservoir Localized effects at faults

(VISAGE Output)

Time-lapsed compressional acoustic impedance

Changes in overburden/caprock due to stress redistribution Changes in reservoir Top of caprock due to pore pressure decline Initial gas-water contact Changes in reservoir due to fluid movement

(VISAGE Output)

Initial Modelling: Before Production Begins

Time Lapse Model: Saturation Changes Only

Time Lapse Model: Saturation + Stress

Time-lapsed seismic trace model

Reflector at top of caprock Reservoir top Reservoir base Perturbations at reflector event due to fluid change effects Pull-up in reflector event due to stress 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 17 th August?

K

Hysteresis

Increasing Stress

K

Hysteresis

Increasing Stress

K

Hysteresis

Increasing Stress

UBD site history very important Effective Stress around the wellbore

Failure Level

Drilling Completion Production Time

Multi Disciplinary Tasks assembling data for Model

Building the Geomechanical Model

*Structure and anisotropy analysis from Seismic *Geomechanical Core Analysis *Published and proprietary studies Basin process simulations *Genetic Units expertise *Log analysis *Geomechanics of fracture genesis Analogue studies Creation of the Geomechanical Model Stress-Sensitive Coupled Modelling Deliverables Characterise Structural Setting of the Reservoir Characterise Reservoir Rocks Characterise Reservoir Faults & Fractures feedback to improve characterisation Reservoir Geomechanical Model Stress-Sensitive Reservoir Modelling and Coupled Simulations (Ground movements, Fluid Flow and 4D seismic) Better Decisions Reservoir Management feedback to improve characterisation