AFES Masterclass Overburden Correction Evidence …€¦ · AFES Masterclass Overburden Correction Evidence from Log Data Ken Russell Geomechanics & Sonic Scanner Support – Europe

AFES MasterclassOverburden Correction

Evidence from Log Data

Ken RussellGeomechanics & Sonic Scanner Support – Europe Africa

19th May 2010

Aberdeen

Mechanical Earth Model

Strength

20 400

0 70

UCS

Friction Angle

Earth Stress & Pore Pressure

MPa0 200Stress W N E

Stress Direction Sh1Poisson’s

Ratio

Young’s Modulus 100

Elastic

0

10

“A numerical description of subsurface rock properties and stresses for use in predicting reservoir behaviour”

2

UCS F

Pp S h S H S V

fault ?

Regional Trend

PR E

A test that fractures the formation then measures Fracture Closure Pressure:

� Driller’s Leak-Off Test

� Drill Stem Test – mini-DST

� Wireline Dual Packer – mini-frac

Horizontal Stress Magnitudes – Direct Measurement

3

� Wireline Dual Packer – mini-frac

σh

Breakdown pressure

Re-opening pressurePropagation pressure

Initial shut-in pressure

Closure pressure

Tensile strength

Horizontal Stress Magnitudes – Inferred from Image Data

MinimumStress

σθ min

4

Hydraulic Hydraulic Hydraulic Hydraulic

FractureFractureFractureFractureShearShearShearShear

FailureFailureFailureFailure

MaximumStress

PmudPmudPmudPmud

PpPpPpPp

σθ min

σθ max

σr

HPHT- North Sea – Central Graben

Overpressure Mechanisms:

� Undercompaction- Trapped fluids - with increasing depth of

5“Pressure Study of the North Sea Central Graben”

– GeoPressure Technology

burial

� Clay diagenesis – Smectic to Illite- Illite occupies larger volume

� Hydrocarbon generation- Kerogen conversion to oil/gas with increase in temperature and pressure

Effective Stress in HP Reservoirs

σx’ = σx - α Pp (α = Biot’s constant)

Dep

th

σx

Normally pressured

Dep

th

σx

HP Reservoir

6

Dep

th

Pressure

Pp

σx’D

epth

Pressure

Pp

σx’

HP reservoirs have low initial effective stress

Compaction of weak carbonate (Ekofisk)

Porosity

Pore (fabric) collapse

7

log(σv’)1 MPa 10 MPa 100 MPa

Threshold stress

Pore (fabric) collapse

Triaxial compression

Acoustic Core tests

8

Confiningpressure

Confiningpressure

Jacket

Acousto-elasticity:� Non-linear relationship of stress to acoustic velocity

Earth Stresses – Far-field vs. Wellbore

σt

σa

σHσh

σV

1000

2000

3000

4000

5000

Eff

ec

tive

Str

ess

es

(psi

)

SigV

SigHSigh

SigTSigA

SigR

9

σr

Far-field stresses:VerticalMin horizontalMax horizontal

Wellbore stresses:TangentialAxialRadial

0

1000

0 5 10 15 20Radius (in)

Eff

ec

tive

Str

ess

es

(psi

)

SigR

After Tom Bratton

Stress Loading – Acoustic Response

Lab Stress Loading Wellbore Stress Loading

3000

4000

5000

Eff

ec

tive

Str

ess

es

(psi

)

SigH

SigV

SigT

SigA

10

0

1000

2000

3000

0 5 10 15 20Radius (in)

Eff

ec

tive

Str

ess

es

(psi

)Shear Stress

SighSigH

SigR

After Tom Bratton

Calibration Points

3000

4000

5000

Eff

ec

tive

Str

ess

es

(psi

)

SigV

SigT

SigA

Wellbore Stress LoadingDipole Radial profile of velocity

11

0

1000

2000

3000

0 5 10 15 20Radius (in)

Eff

ec

tive

Str

ess

es

(psi

)

Shear Stress

SighSigH

SigR

After Tom Bratton

Direct Estimate of Shmax & Shmin from sonic data

12 “Estimation of Formation Stresses Using Borehole Sonic Data” – Sinha et al, Paper F, SPWLA 49th Annual Logging Symposium May 25-28, 2008.

� Waveform data exhibiting “crossover” due to imbalanced stress is inverted for horizontal stress ratio.

Conclusion� Understanding how changes in effective stress during

production affect both porosity and permeability is essential to develop an effective drilling, completion and production strategy.

13“We can save 700 Lira by not taking soil tests”

www.senergyworld.com

Compressibility from Core ☺

Phil McCurdy and Colin McPhee

☺and from logs too

2

Why is compressibility important???

Hydrocarbon recoveryReservoir depletion causes increase in effective stressPore volume compacts and adds energy to reservoirPore volume compressibility used in material balance calculations

Porosity and permeability reductionReduction in porosity and permeability with increasing effective stress on depletionProductivity reduction in depleting reservoirs

Compaction and subsidence (weak sands & HPHT)Compaction can lead to casing and tubular failuresCompaction can lead to surface subsidenceCompaction linked to compressibility

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

0500100015002000250030003500400045005000

Inferred Reservoir Pressure, psi

Per

mea

bilit

y M

ultip

lierIn-Situ

3

Compressibility terms and calculations

Compressibility units10-6psi-1 referred to as “microsips”

Grain compressibility, Cma or CgCg ~ 0.16 – 0.20 microsips

Bulk Modulus, Krelated to rock stiffnessinverse of compressibility

Bulk Compressibility, CbCbc –constant pore pressure and changing confining pressure

Cbp - under constant confining pressure and changing pore pressure (depletion)

)21(3 ν−=

EKK

Cb 1=

PpPcVb

VbCbc

⎭⎬⎫

⎩⎨⎧∂∂

=1

PcPpVb

VbCbp

⎭⎬⎫

⎩⎨⎧∂∂

=1

Vb = bulk volume Pc= confining pressure Pp = pore pressure

Cup for a “microsip”

4

Compressibility terms and calculations

Bulk and Grain Compressibility

As Cg is small in comparison, Cbc ≈ CbpPore Volume Compressibility, Cf (Dake) or Cp

Cpc - isostatic pore volume compressibility under constant pore pressure and changing confining pressure

Cpp – isostatic pore volume compressibility under constant confining pressure and changing pore pressure (depletion)

As Cg is small in comparison, Cpp ≈ Cpc

⎥⎦

⎤⎢⎣

⎡ −=

φCgCbcCpc

PpPcVp

VpCpc

⎭⎬⎫

⎩⎨⎧∂∂

=1

PcPpVp

VpCpp

⎭⎬⎫

⎩⎨⎧∂∂

=1

CgCbcCbp −=

CgCpcCpp −=

i.e. pore volume compressibility is 3 to 5 times higher than bulk compressibility

5

Measurement Conditions

Reservoir (Triaxial)three principal stressesuniaxial loading

SCAL Labsisostatic loadingradial stress = axial stress

Rock Mechanics Labsbiaxial loadingradial stress < axial stress

Axial

Radial

σv = σz

σhmax = σx

σhmin = σy

6

Compressibility terms and calculations

Isostatic and Uniaxial Compressibility, Cpuuniaxial loading assumes reservoir formations behave elasticallyand are boundary constrained in horizontal direction

assumes strain is entirely verticalassumes no tectonic strain during burial loading

Cpu defined as uniaxial pore volume compressibility under producing conditions (from Teeuw)

For example, Biot factor (α) = 1 and ν = 0.3 then Cpu = 0.62*Cpp

( )( ) ⎥⎦

⎤⎢⎣

⎡−+

=ννα

131CppCpu

Reservoir has stiff lateral restraints

7

Typical Lab Presentation

( )( ) ⎥⎦

⎤⎢⎣

⎡−+

=ννα

131CppCpu Note neither α nor υ are measured!

8

Core Test Methods

DirectMeasure change in pore volume as a function of increasing effective stress

Effective stress method – SCAL labsIncrease σ to increase σ’

Simulated depletion method – SCAL labsReduce Pp to increase σ’

Uniaxial (K0) Test – Rock Mechanics labsReduce pp to increase σ’Instrument core to determine strains

IndirectFrom E and υ from triaxial tests

pisoiso pασσ −='

9

Direct Measurements – SCAL Lab

Effective Stress MethodSCAL lab method (porosity/FF at overburden)pore pressure constant, radial pressure increasedeffective stress increased by increasing confinementpore volume by squeeze-out

Simulated Depletion Methodraise stresses and pore pressure to reservoir valuestotal stress (Pc) constant – Pp reduceddepletionisostatic pore volume compressibility (SCAL)

⎭⎬⎫

⎩⎨⎧=

⎭⎬⎫

⎩⎨⎧∂∂

='

11δσδVp

VpPcVp

VpCpc

Pp

⎭⎬⎫

⎩⎨⎧=

⎭⎬⎫

⎩⎨⎧∂∂

='

11δσδVp

VpPpVp

VpCpp

Pc

10

Uniaxial Ko Test

Sample instrumented with axial and radial strain gaugesSample loaded to same total vertical (axial) and total horizontal (radial) stresses as in reservoirPore pressure increased to reservoir valuePore pressure reduction

vertical stress stays the samehorizontal stress adjusted to maintain zero radial strainrock mechanics labs onlyuniaxial pore volume compressibility (K0) ∆pp

∆σh

Core Compaction

εh = 0

0

1

=⎭⎬⎫

⎩⎨⎧∂∂

=radial

PpVp

VpCpu

ε

11

Example PV calculation – SCAL data

13.00

13.20

13.40

13.60

13.80

14.00

14.20

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Effective Hydrostatic Pressure (psi)

Pore

Vol

ume

(ml)

DataModel

Initial Reservoir Pressure

Depleted Reservoir Pressure

( )di

diihyd

VpVpVp

cf''

)(1)( σσ −

−=

12

Stress Hysteresis

Effective Stress Methodinitial loading cyclemicrocracks in plug closehigher pore volume reductionOK for φ stress correction

Simulated Depletion Methodextended loading cycle

load to initial conditions (cracks close)depletion stage (Cp from matrix pore volume compaction)more reliable pore volume compressibility data

Uniaxial KO Methodpotentially most reliable dataclosest representation of stresses/pressures during depletion

GAUGEROSETTE

13

Stress Hysteresis Example

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Effective Overburden Stress (psi)

Pore

Vol

ume

Com

pres

sibi

lity

(x10

-6ps

i-1)

1A2A3A4A5A6A7A8A1D2D3D4D5D6D7D8D

Suffix A: Effective Stress MethodSuffix D: Simulated Stress Mrthod

14

Indirect Method

Triaxial dataDetermine E and υ over equivalent deviatoric stress range associated with depletion

KCbc 1

=)21(3 ν−

=EK⎥

⎦

⎤⎢⎣

⎡ −=

φCgCbcCpc

15

Compressibility from Logs

DSI LogsDTS (∆ts), DTCO (∆tc)

Obtain dynamic (elastic) moduli

)21(3 ν−=

EK

Poisson’s Ratio, ν

Shear Modulus, G (psi)

Young’s Modulus, E (psi)

Bulk Modulus, Kb (psi)

Bulk Compressibility, Cbc (psi-1)

Pore Volume Compressibility, Cpc (psi-1)

( )( ) 1/

1/21

2

2

−∆∆

−∆∆

cs

cs

tt

tt

2101034.1

s

b

tx

∆ρ

( )ν+12G

bK1

ρb in g/cc

∆t in µsecs/ft

⎥⎦

⎤⎢⎣

⎡ −=

φCgCbcCpc

16

Scaling Dynamic and Static Moduli

Dynamicelastic and perfectly reversible

Static (core)large strainsirreversible

Scalingstatic ε < dynamic εEsta = 0.15 - 0.5 Edyn

− νsta = 0.8 - 1.2 νdyn

17

Compaction and Subsidence

Compactionchange in reservoir thickness (Hres) as a result of depletion (Geertsma)

Compaction coefficient

Casing compressive strain

Subsidence (Bruno)

( )CbCm βνν

−⎥⎦⎤

⎢⎣⎡−+

= 111

31

CbCg

=β

)( finaliresm PPHCH −=∆Depth, D

Thickness, HH

Subsidence

Compaction

Reservoir Radius, R

Mud Line

Depth, D

Thickness, HH

Subsidence

Compaction

Reservoir Radius, R

Mud Line

( )[ ] pDRHDRHCS resresm ∆++++−−= 5.0225.022 )()()1(2 ν

pCmc ∆+= )2cos1(5.0 θε

18

Conclusions

Common techniques for measuring compressibility and situations that they are most suited to

AFES Overburden Correction Masterclass

Permeability

Craig Lindsay

Core Specialist Services Ltd.

Synopsis

• Permeability definitions

• Effect of overburden on permeability

• Other factors

• Examples • Examples

- Unconsolidated core

- Tight gas

- High pore pressure

• Conclusions

You can’t squeeze blood from

a stone – or can you?

Permeability Definitions

• Fluid flow in porous media, units milliDarcy (md)

>50 mD = good, 1 – 50 mD low < 1m D = tight

• Routine Core analysis – ambient or overburden air permeability (hydrostatic), e.g. Ka 75 mDpermeability (hydrostatic), e.g. Ka 75 mD

• Special Core Analysis (SCAL) – effective permeability at overburden (hydrostatic), e.g. Ko@Swi at 3000 psi OB

• Effective permeability – single mobile phase e.g. Oil permeability at immobile water saturation (Swi)

• Relative Permeability – 2 mobile phases

Permeability at Overburden

O

v

e

r

b

P

e

r

m

e

a

• General expectation

• Increased

overburden stress =

reduced & more

Increased

Ambient Permeability

b

u

r

d

e

n

a

b

i

l

i

t

y

reduced & more

tortueous flow path

= lower permeability

• Low ambient

permeability =

greater reduction at

overburden

Decreased

Permeability at Overburden including

Fluid Effects

O

v

e

r

b

P

e

r

m

e

a

• Immobile fluid

phase e.g. Swi, Sor,

Sgr

• Endpoints, typically

Ko@Swi

Increased

?

?

Ambient Permeability

b

u

r

d

e

n

a

b

i

l

i

t

y

Ko@Swi

• Wettability

• Effect typically = >

Overburden

• Variable and

unpredictable effects

Decreased

?

All Permeability's are Equal?

BLAXTER SST.

Average Ka 284 mD

Average Ko 269 mD

Average Kw 93 mD

1.

10.

100.

1000.

In-s

itu P

erm

eabi

lity,

mD

Kw

Kg@Swi

North Sea Clastic Gas Reservoir• Combined impact of overburden &

fluid system

•Non-linear relationships

• Kg@Swi & Overburden >= Ka at

ambient (Ka >1 mD)

• Kw << Kg@Swi – low brine mobility

0.001

0.01

0.1

0.001 0.1 10. 1000.

Ambient Air Permeability (Ka) , mD.

In-s

itu P

erm

eabi

lity,

mD

• Kw << Kg@Swi – low brine mobility

Slip Boundary Condition

In the presence of very small amounts of a wetting

phase the permeability of a non-wetting phase can

be > the absolute permeability of the rock.

Special case: Unconsolidated core

Shaken not stirred? Poor core handling at wellsite & during transportation

Log porosity constant at 35% (+-1%). Overburden permeability ~ 80% ambient??

Special case: Tight Gas

Rotliegend Sandstone

Ambient Conditions:

Average Ka = 0.1 mD

Average φ = 8%

Overburden:

Average Kg@Swi =

0.002 mD

Average = 6.5%

Cannot easily

measured!

Special case: Tight Gas

Rotliegend Sandstone

Ambient Conditions:

Average Ka = 0.1 mD

Average φ = 8%

Overburden:

Average Kg@Swi =

0.002 mD

Average = 6.5%

Cannot easily

measured!

Special case: Tight Gas

CER, Holditch & Assoc 1991

Special case: High Pore Pressure

Gulf of Mexico:

• Depth > 20,000 ft

• Pore pressure > 20,000 psi

• Initial overburden 1,000 psi

• Permeability at e.g. 1000 psi • Permeability at e.g. 1000 psi

net stress & ambient pore

pressure

• Pore pressure supports grains

• True permeability may be 5-

25% (up to 50%) >

Ambient (Shafer et al).

Conclusions

• Overburden has significant impact on

permeability

• Overburden acts in conjunction with fluid • Overburden acts in conjunction with fluid

effects – must be accounted for too

• Effects variable and unpredictable must be

measured not assumed

• Make sure lab is suitably equipped!

Thank you

Any Questions?