katahara clayeffects pressureworkshop2014 edited for release to spe

8/11/2019 Katahara ClayEffects PressureWorkshop2014 Edited for Release to SPE

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Effect of clay content on shale

properties with application to pressureanalysis (edited for public release)

Keith Katahara

Hess Corporation

SPE/AAPG/SEG Pore Pressure Workshop

San Antonio, TX, USA, 11-12 March 2014


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Outline

Clay content inferred from well logs

Pressure mechanism: Fluid-Expansion vs. Load-Transfer

Conclusion

2


3/25

Neutron Porosity Density Porosity Crossplot

3

DPHI DEPTH

NPHI

QUARTZ CLAY


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Neutron Porosity Density Porosity Crossplot

4

DPHI DEPTH

NPHI

SANDS

SHALES


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Clay

Water

Quartz

DensityPorosity

Neutron Porosity

Clay asfractionof solid

Clay as fraction of solids from NPHI and DPHI


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Clay

Water

Quartz

DensityPorosity

Neutron Porosity

Clay asfractionof solid

Clay as fraction of solids from NPHI and DPHI

Clay point depends on(usually unknown)mineralogy, well-logenvironmental

corrections,


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Outline

Clay content inferred from well logs

Pressure mechanism: Fluid-Expansion vs. Load-Transfer

Conclusion

7


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Well 3, shales

WELL: AAPG Compendium

ZONE: 0. 000 - 20000.000 FTDATE: 28 Jun 2000 @ 17:14

SONIC, us/ft

70 17080 90 100 110 120 130 140 150 160

D

EN

SITY

,g/cc

2

2.6

2.0

5

2.1

2.1

5

2.2

2.25

2.3

2.3

5

2

.4

2.4

5

2.5

2

.55

D

EPTH

,ft

4800

12900

615

0

7500

8850

10200

11550

Normal

Pressure

High Pressure

SONIC DT (1/Vel)

DE

NSITY

DEPTH

Public-domainGulf of Mexico

well


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Well 3, shales


ZONE: 0. 000 - 20000.000 FTDATE: 28 Jun 2000 @ 17:14

SONIC, us/ft

70 17080 90 100 110 120 130 140 150 160

D

EN

SITY

,g/cc

2

2.6

2.0

5

2.1

2.1

5

2.2

2.25

2.3

2.3

5

2

.4

2.4

5

2.5

2

.55

D

EPTH

,ft

4800

12900

615

0

7500

8850

10200

11550

SONIC DT (1/Vel)

DE

NSITY

DEPTH

Smectite-rich

Illite-rich


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Well 3, shales


ZONE: 0. 000 - 20000.000 FTDATE: 28 Jun 2000 @ 17:14

SONIC, us/ft

70 17080 90 100 110 120 130 140 150 160

D

EN

SITY

,g/cc

2

2.6

2.0

5

2.1

2.1

5

2.2

2.25

2.3

2.3

5

2

.4

2.4

5

2.5

2

.55

D

EPTH

,ft

4800

12900

6150

7500

8850

10

200

11550

SONIC DT (1/Vel)

DE

NSITY

DEPTH

If pressure drains

off and effectivestress increases,

the load-transfer

model says that

overpressuredshales will move

along the dark

red dashed line.

The fluid-

expansion model

says shales will

move along theorange solid line.


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GoM sonic-density xplot with ~clay volume in color

11SONIC DT

DE

NSITY

Clay

volu

me

Same well as previous plot with transitionzone removed, and with clay proxy in color.

The overpressured section has claycontours that are parallel to thecompaction trends. The point of this talk isthat this clay contour pattern is generallynot consistent with fluid expansion.


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Shale Loading/Compaction

12

This diagram schematically shows how sediments

compact. Velocity and density both increase withincreasing effective stress. Compacting shales tend tofall on a linear sonic-density crossplot, moving from lowerright to upper left with compaction. Sonic slowness is

1/velocity.


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Unloading: Fluid-expansion or Load-transfer

13

If pore pressure is increased, at constant confining stress,

velocity decreases significantly, but density decreasesvery little. So on a sonic-density crossplot a shale pointwill move off to the right of the original trend.


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Reloading: Fluid-expansion

14

If overpressure is due to fluid expansion, then if effective

stress increases after the initial unloading, velocity anddensity will reverse until they reach the originalcompaction curve. They will then move along the originalcompaction curves. The reloading paths are shown in

green.


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Reloading: Load-Transfer

15

The load-transfer concept is that if effective stress

increases after unloading, the shales will move along anew compaction trend. The reloading path is shown ingreen.


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Effective stress variation on sonic-density crossplot

16

Sonic Slowness

Density

Sonic Slowness

Density

Fluid-Expansion Load-Transfer

The solid green reloading arrows must cut acrosscontours of constant effective stress foroverpressured shales, as indicated on the next slide.


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Sonic-Density-Effective Stress Contours

17

Sonic Slowness

Density

Sonic Slowness

Density


Eff. Stress Contours Eff. Stress contours


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Sonic-Density-Clay patterns

18

Sonic Slowness

Density

Sonic Slowness

Density


CLAY contours ? CLAY contours

For fluid-expansionoverpressure, clay contours willdepend on the magnitude ofoverpressure, as indicated on

the next slide.

For load-transfer (or for simplecompaction) clay contours willbe parallel to the compactiontrend.


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Sonic-Density-Clay patterns

19

Sonic Slowness

Density

Sonic Slowness

Density



High P

Normal P

Normal P

This fluid-expansion scenario isfor a zone that isoverpressured at its center butis normally pressured at its

margins.


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Example

Data shown at the workshop is not reproduced here. Schematicdiagrams showing observed shale-property patterns are shown instead.

Well A is at normal pressure as verified by formation test data over athick interval with alternating sands and shales.

Well B is highly pressured. Shale vertical effective stress in one thicksection is known because mud weights for a series of connection-gasobservations closely matches several formation test points in thatsection.


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Example: effective stress contours

22

Sonic Slowness

Density

Sonic Slowness

Density


Eff. Stress Contours Eff. Stress contours

Effective stress contours look like this in the highlypressured section of Well B. The pattern does notmatch the fluid expansion scenario at left.


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Example: schematic clay contours

23

Sonic Slowness

Density

Sonic Slowness

Density



Normal P

High P

Clay contours are parallel to the compaction trend in both high andnormally-pressured sections. Clay contours line up nicely betweenWell B with high pressure and Well A with normal pressure. The

patterns do not match the expected fluid-expansion scenario at left.

Well A

Well BWell B

Well A


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Conclusion

Clay content can help distinguish between fluid-expansion andload-transfer (or disequilibrium compaction) mechanisms.

Sonic-density-clay and sonic-density-stress patterns in shales inthe example wells A and B are not consistent with fluid-

expansion.

The observed crossplot patterns indicate that the overpressuredshales are on a compaction trend. This is consistent with eitherload-transfer unloading, or with disequilibrium compaction as

pressure mechanisms. (Circumstantial evidence, not shown here, indicates that the

load-transfer version of smectite-illite transformation is asignificant cause of overpressure in Well B. But this point was

questioned during Q&A at the workshop.)


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END

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I am grateful to Hess for permission to present, and to MarkAlberty and Glenn Bowers for enlightening discussions.

katahara clayeffects pressureworkshop2014 edited for release to spe

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