Effects of Diagenesis on Compaction of Reservoir Rocks · Effects of Diagenesis on Compaction of Reservoir Rocks ... Determine Effects of Diagenesis on Compaction ... – Explore

Effects of Diagenesis on Compactionof Reservoir Rocks

Euro-conference of Rock Physics and Geomechanics, Erice, Sicily, 25-30 September 2007

Dave Olgaard*, Steve Cameron§, John Dunsmuir§, Amy Herhold§, Hubert King§, MJ Gooch*

ExxonMobil: §Corporate Strategic Research, New Jersey, *Upstream Research Co., Houston, Texas, USA

(http://landsat.org)

1

Why worry about compaction?

• 20 – 23 MPa reduction in reservoir pressure• > 4 m of seafloor subsidence• > US$1G to raise platform• compaction + fracture perm. ⇒

extended lease, added resource

Ground elevation

Porous silica (diatomite) reservoirs

Porous carbonate reservoirs

1925

1955

1977

Bakersfield, CaliforniaGround water withdrawal

9m / 50 yrsOil production-induced rates can be higher! Ekofisk, North Sea

Xu & Nur (2001)

Tuefel et al. (1991)

Ground elevation

2

Borehole acoustic televiewer images

Wellbore failure

3

GENESIS, deepwater GOM

Chevron, SPE 84415

Start production

Permeability reduction by depletion

4

Reservoir

Overburden, Sv

Horizontal stress, Sh

Pore Pressure, Pr

Porosity, φ

Effective Stress, σ = S-Pr FUNDAMENTAL IMPACT ON ROCK PROPERTIES

RESERVOIR PRESSUREDEPLETION ↓

EFFECTIVE STRESSINCREASE ↑

e.g. compaction-induced casing deformation

e.g. pore collapse grain crushing

Stress path, K = ∆σh / ∆σv1 ≥ K ≥ 0(σv - σh)

increasingvolumechange

shapechange

• Geomechanics is an essential element of the dynamic reservoir environment

INDUCED STRAIN IN ROCKReservoir

Overburden, Sv


Pore Pressure, Pr

Porosity, φ

Effective Stress, σ = S-Pr

Reservoir

Overburden, Sv


Pore Pressure, Pr

Porosity, φ

Effective Stress, σ = S-Pr FUNDAMENTAL IMPACT ON ROCK PROPERTIES

RESERVOIR PRESSUREDEPLETION ↓

EFFECTIVE STRESSINCREASE ↑

e.g. compaction-induced casing deformation

e.g. pore collapse grain crushing

Stress path, K = ∆σh / ∆σv1 ≥ K ≥ 0(σv - σh)

increasingvolumechange

shapechange

• Geomechanics is an essential element of the dynamic reservoir environment

INDUCED STRAIN IN ROCK

Basic mechanics during reservoir production

5

Diagenesis and Compaction

Definition: Diagenesis – Physical and chemical changes in a sediment that convert it into a rock. Am. Geol. Inst.(1976)

Outline:• Motivation for research• Typical “porosity / density profiles” (mechanical and chemical effects)• Current research project (emphasis on carbonates)

– State-of-the-Art– Fabrication of synthetic oolitic grainstones– Compaction behavior (porosity versus effective stress)– Quantitative imaging (X-ray microtomography)– Next steps

6

Reservoir Quality Prediction: porosity profiles

Mostly mechanical compaction 2

Chemical compaction dominant

70-100°C2-3 km1

(clastics)

5 km

Dep

th

Porosity

1 Carbonates and evaporites compact chemically at shallower depth than silicates.2 Mineral strength: quartz & feldspar ≥carbonates > gypsum > salt



70-100°C2-3 km1

(clastics)

5 km

Dep

th

Porosity



70-100°C2-3 km1

(clastics)

5 km

Dep

th

Porosity


General



70-100°C2-3 km1

(clastics)

5 km

Dep

th

Porosity




70-100°C2-3 km1

(clastics)

5 km

Dep

th

Porosity



70-100°C2-3 km1

(clastics)

5 km

Dep

th

Porosity


General

7

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

Sandstone

Bulk density

dept

h be

low

sea

floor

Shale

SandstoneEvaporites

dept

h be

low

sea

floor

Salt

2.2 g/cm 3

Bulk density

dept

h be

low

sea

floor

Salt

2.1 g/cm 3

Bulk density

dept

h be

low

sea

floor

Salt

2.2 g/cm 3

Bulk density

dept

h be

low Salt

2.1 g/cm

Bulk density

Evaporites

dept

h be

low

sea

floor

Salt

2.2 g/cm 3

Bulk density

dept

h be

low

sea

floor

Salt

2.1 g/cm

Bulk density

dept

h be

low

sea

floor

Salt

Bulk density

dept

h be

low Salt

Bulk density

dept

h be

low

sea

floor

Salt

Bulk density

dept

h be

low

sea

floor

Salt

Bulk density

Salt

Bulk densityBulk density

Siliciclastics

3

Basin Modeling: density profiles

8

Carbonate Compaction: Sediment Type & Alteration

• Carbonate compaction curves:– Natural carbonates: variety of rock types, both mechanical and chemical alteration

– Experimental: effects of chemical diagenesis not included in mechanical properties

• Current data predicts a wide range of outcomes – which is correct?

Schmoker and Halley (1982) [75-100% limestone]Log analysis

Goldhammer (1997) [ooid, peloid, skeletal grains]Loose sedimentGoldhammer (1997) [mud]Loose sediment, transition +/- 40%

Need a method to predict compaction as function of sediment type and diagenetic alteration

0

1000

2000

3000

4000

0 10 20 30 40 50 60 70

Porosity (%)

Dep

th (m

)

9

Moldic Porosity in Ooid Grainstones

• Marine oolites are important reservoir rocks– E.g. Ghawar Field, Saudi Arabia, (Flugel, 2004)

• Diagenesis of ooids inverts original intergranular porosity to moldic porosity• Por. & perm. depend on both environment of deposition & diagenesis

• Similar total porosity as original sediment, but very different permeability.• How does the inversion affect the porosity-depth curve?

• A-C conversion occurs in fresh water lens, only 1-2 m below sea surface– I.e., diagenesis with little load; e.g. experimentally tractable

• Fabrication of moldic porosity previously established– Challenge: Make samples large enough for geomechanics tests

AragoniteCalcite

Porosity Reaction rim

10

Research: Determine Effects of Diagenesis on Compaction of Synthetic Oolitic Grainstones

• Synthetic rocks decouple effects

– Adjust kinetics (t/T) to control diagenesis

– Study effect of diagenetic environment without overprinting

– Focus on matrix rather than vugs present in natural samples

Natural ooid sand

Heat to accelerate kinetics

Create diagenetically-altered rocks

Acquire compaction curves from uniaxial strain experiments

Poro

sity

Effective Stress

Natural grainstone (U. Miami)

Synthetic grainstone (current)

Follow microstructure with X-ray microtomography + thin section imaging

11

Samples Fabricated in Autoclave

LA-329/330

Aragonite Ooids

X-ray microtomography

500 µm

25 mm

12

Typical Mechanical Strength Tests

∆Pc = 0

∆σa

∆εr > 0

axialstress

coreplug

porepressure

confiningpressure

σa

Pc Pp

Triaxial stress

RockStrength

∆σa

∆εr = 0

∆Pc

Uniaxial strain

RockCompressibility

Triaxial Load Frame

13

Typical Stress Paths

Reservoir Stress Path - Depletion (Plastic compaction)

0

1000

2000

3000

4000

5000

6000

0 1000 2000 3000 4000 5000 6000

p' (effective mean stress )

q (d

iff s

tress

, σv

- σh

)

Elastic zone

Dilatent zone

Plastic zone

Uniaxial strain path

Near wellbore path

Hydrostatic

Peak Failure Line

Triaxial

(σ’v+ σ’h1+ σ’h2) / 3P*

Reservoir Stress Path - Depletion (Plastic compaction)

0

1000

2000

3000

4000

5000

6000

0 1000 2000 3000 4000 5000 6000

p' (effective mean stress )

q (d

iff s

tress

, σv

- σh

)

Elastic zone

Dilatent zone

Plastic zone


Near wellbore path

Hydrostatic

Peak Failure Line

Triaxial

(σ’v+ σ’h1+ σ’h2) / 3P*


Near wellbore path

Hydrostatic

Peak Failure Line

Triaxial

(σ’v+ σ’h1+ σ’h2) / 3P*P*

σ1

σ3

σ1

σ3

σ1

σ3

Triaxial Compression

p’

Hydrostatic CompactionHydrostatic Compaction

q (s

tres

s di

ffere

nce)

14

0.38

0.42

0.46

0.50

0.54

0.58

0.62

0.07 0.7 7 70

Axial Effective Stress (MPa)

Void

Rat

io

Unconsolidated sediment

Typical drained uniaxial strain test

elastic

plastic

unload-reload

unload

Fabricating synthetic oolitic grainstones

16

30 days14 daysCements missing due to screen contact

~500 microns

~500 microns

Uncemented Grains

Laboratory - Simulated Diagenesis

180 daysScreen removal pulled open dissolved ooids

30 daysCements missing due to screen contact

Time at 180 ºC: 2 days

Uncemented Grains Moldic Porosity

~500 microns

~500 microns

~500 microns

~500 microns

>95%15%% A -C reaction: 30%<5%

30 days30 days14 daysCements missing due to screen contactCements missing due to screen contact

~500 microns

~500 microns

~500 microns

~500 microns

Uncemented Grains

Laboratory - Simulated Diagenesis

180 daysScreen removal pulled open dissolved ooidsScreen removal pulled open dissolved ooids

30 days30 daysCements missing due to screen contactCements missing due to screen contact

Time at 180 ºC: 2 days2 days

Uncemented Grains Moldic Porosity

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

~500 microns

>95%15%% A -C reaction: 30%<5%

17

Morphology of Lab DiagenesisRock Resembles Natural Samples

Autoclave Experiment7 days at 180 C

Pleistocene Grainstonefrom West Caicos

Natural DiagenesisLab Diagenesis

Holocene Ooid SandSchooner Cays, Bahamas

[Budd+Land, J. Sed. Pet., 1990]

Partially-dissolved

rim

Blocky calcite


Autoclave Experiment7 days at 180 CAutoclave Experiment7 days at 180 C








Partially-dissolved

rim

Blocky calcite







Partially-dissolved

rim

Blocky calcite

Morphology of lab diagenesisrock resembles natural samples

Autoclave Experiment7 days at 180 C7 days at 180 C








Partially-dissolved

rim

Blocky calcite







Partially-dissolved

rim

Blocky calcite

Optical thin sections with










Partially-dissolved

rim

Blocky calcite








Partially-dissolved

rim

Blocky calcite











Partially-dissolved

rim

Blocky calcite










Partially-dissolved

rim

Blocky calcite









Partially-dissolved

rim

Blocky calcite









Partially-dissolved

rim

Blocky calcite

Autoclave Experiment7 days at 180 C7 days at 180 C








Partially-dissolved

rim

Blocky calcite

Lab / natural diagenesis

Morphology of lab diagenesisrock resembles natural samples

18

Cemented Ooid Grainstones, CT-scans

14 day 30 day 90 day

19

Ooid Thin Sections

30 day

14 dayFresh

90 day

500 µm

Uniaxial Strain Compaction Tests& Analyses

21

Synthetic Oolite Deformation & Analysis

Cement ooidsf( t, 180ºC, Ωaragonite)

Image Cemented CoreX-ray Microtomography (CT)

Compact Core

Re-image CoreCT, thin-section

Determine Degree ofReaction w/XRD

Unaltered ooids

14 day 15%

30 day 30%

90 day 50%

Samples Studied

time A→C reacted

22

Compaction of Unaltered Aragonite Ooids

Uniaxial Strain Results

Unaltered ooids

Effective Axial Stress (MPa)

Poro

sity

(%)

25%

30%

35%

40%

45%

0.07 0.7 7 70

23

Uniaxial strain test results

25%

30%

35%

40%

45%

0.07 0.7 7 70

Effective Axial Stress ( MPa)

Poro

sity unreacted

30% 15% calcite

>50%

25%

30%

35%

40%

45%

0.07 0.7 7 70

Effective Axial Stress ( MPa)

Poro

sity unaltered

30% 15% calcite

≥50%

24

Published carbonate porosity profiles

0

1000

2000

3000

0% 10% 20% 30% 40% 50% 60% 70% 80%

Porosity (%)D

epth

(m)

70-100% limestone

ooid, peloid &skeletal grains

carbonate mud

25

Comparison to Published I

0

500

1000

1500

2000

20% 30% 40% 50% 60%

Porosity D

epth

(m)

Unreacted

15% calcite

70-100% limestone

mixed grains

carbonate mud

26

Comparison to Published II

0

500

1000

1500

2000

20% 30% 40% 50% 60%

Porosity

Dep

th (m

)

Unreacted

15% calcite

30%

>50%

70-100% limestone

mixed grains

carbonate mud

27

Quantitative X-ray Microtomography

• Registration (pre versus post-compaction)– Fiducial markers required to register pre and post compaction samples

+ AutoCorrelation Function analysis for unregistered samples+ 2D visual inspection of local grain changes+ Potential for cross-correlation analysis for local changes

• Imaging difficulties– Calibrate CaCO3 X-ray opacity– Sample prep. techniques and pore fluid affect opacity – Damage in post-compaction cores

28

Fiducial Markers in ~50% calcite Sample

• Initial marker position is difficult to control

• Simple Z scaling registers fiducial markers

• Compaction is homogeneous

Pre-Post Compaction position

y = 0.8553x + 0.1907

0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000

Pre Compaction cm

Post

Com

pact

ion

cm

AfterLinear (After)

3D marker positionsPre-Compaction(Y)

Post-Compaction(R)

3D marker positions after 1D axial image rescaling

29

3D AutoCorrelation Function

• Computed using FFTs in 3D ACF(u,v,w) = F(u,v,w)F*(u,v,w)– Measures probability that two points separated by r will both lie in the same phase in

binarized pore-grain image (requires resolved grains)– Similar interpretation in calibrated (porosity) gray scale image– Qualitatively, 3D picture of the “averaged” grain environment

Images are not calibrated, but can make qualitative assessments.– ACF arbitrarily rescaled to max = 1 and min = 0– Interpreted as the average density profile around a grain– Interpretation uncertainties in mixed diagenetic structures

It is not necessary to register images to compare the ACFs

2D real space image of ooid grain pack

2D real space ACF of ooid grain pack

30

ACF Results, Compacted Fresh Ooids (17%)

• Small change in axial compression direction.– Decrease in 1st neighbor distance from 570 to 540um– Slight decrease in intergranular contrast

• Interpretation– Some reordering of ooid grains to accommodate strain , little or no

crushing or fines in pore space

Normalized ACF

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

0.1200

0.1400

0.1600

0.1800

0.2000

0 0.0336 0.0672 0.1008 0.1344

R(cm)

Nor

mal

ized

Inte

nsity

Axial-Post

R1-Postcom

pact

ion

2D Slice from 3D image

2D Slice from 3D ACF

31

90 day: Compacted 15%

• ;lafglsd

Normalized ACF

0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

0.3500

0.4000

0.000 0.017 0.034 0.050 0.067 0.084 0.101

R(cm)

Nor

mal

ized

Inte

nsity

Axial-Pre

R1-Pre

Axial-Post

R1-Post

• Significant precompaction alteration of ooid structure. Weak 1st

coordination shell.• Post compaction decrease in radial 1st shell distance, slight increase in

contrast• Axial post compression suggests crushed dissolution products and

cements.

32

Compaction Mechanism: Based on Images

• No Conversion– Stress accommodated

by grain reorientation• Low Conversion

– Stress accommodated by compaction of dissolution rim

• High conversion– Stress accommodated

by compaction of reacted rims and calcite cements

• Full conversion?– Stress also

accommodated by calcite framework crushing?

33

Model

Lab Diagenesis

• Local, pore -scale, simultaneous dissolution and precipitation• Aragonite partially dissolves from outer surface inward• Calcite grows on outer surface •• Framework of leftover aragonite needles allows structural inversion

Synthesizing diagenetic rocks in the laboratory allows systematic, quantitative investigation

Summary I: Frabricated Poldic Porosity

Rim: Aragonite nonoparticles dissolve first, leaving needles

34

Summary II: Evolution of Mechanical Behavior

0

500

1000

1500

2000

20% 30% 40% 50% 60%

Porosity

Dep

th (m

)Unreacted

15% calcite

30%

>50%

70-100% limestone

mixed grains

carbonate mud


25%

30%

35%

40%

45%

0.07 0.7 7 70


Poro

sity unreacted

30% 15% calcite

>50%


25%

30%

35%

40%

45%

0.07 0.7 7 70


Poro

sity unreacted

30% 15% calcite

>50%

Uniaxial Strain Compaction Results

Improved Porosity –Depth Profiles

Students?

Help!?

36

Hint…

Ooid grainstone with intergranular porosity

Ooid grainstone with moldic porosity

Ooid grainstone with intergranular porosity


diagenesis

37

Summary III: Texture / Mechanical Evolution

Menéndez, Zhu & Wong (1996)

Compaction of Granular vs. Foam TexturesBerea sandstone (Peff =10 MPa)

σ1σ1

Metal Foam, Micro-CT images(hydrostatic pressure)

Ooid grainstone with intergranularporosity


Ooid grainstone with intergranularporosity


diagenesis

Gioux et al. (2000)

Borrowedfrom T-f

Wong

38

What Next?

• Rock Synthesis– Move beyond moldic porosity to other pore/rock types?

+ Other key carbonate reservoir types+ Cements in siliciclastics

• Geomechanical tests:– Explore other stress paths & physical properties (e.g., AE, vp & vs,

electrical…)– Evolution of permeability

• X-ray Microtomography analysis– Continue to improve techniques– Quantify grain properties

+ Grain type and degree of conversion+ Identify importance to compaction behavior

– Quantify Rock Deformation+ E.g. non-uniform compaction: track local grain displacements and

morphology changes using cross correlation• Seek more direct link to grain-scale geomechanics modeling

39(http://landsat.org)

Thank You!

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Effects of Diagenesis on Compaction of Reservoir Rocks · Effects of Diagenesis on Compaction of Reservoir Rocks ... Determine Effects of Diagenesis on Compaction ... – Explore