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CONSOLIDATION AND CREEP OF A MULTIPHASE HIGH POROUS

CHALK

Priol Grégoire, priol@ntnu.noDirection: De Gennaro V., Delage P.

Ecole Nationale des Ponts et Chaussées (ENPC-CERMES)

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Problematic

The weakening effects due to a modification of the water content

1. In a oil/water system

2. In a air/water system

Subsidence of sea-bed in the North Sea oilfields

Stability and durability of quarry, or natural slope

Ageing of chalk massif

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Presentation plan

Introduction:

Concept tools:

Experiments:

Conclusion:

Waterflooding, compaction and subsidence in Ekofiskoilfields (chalk reservoir)

Similarity with unsaturated soils

Retention properties, suction and capillary pressure in chalk,

Load stages odometer tests

Time dependent behaviour,

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General presentation

Production: 1971-20501986: Injection of sea water

waterfloodingSubsidence: 40 cm/year

Evolution of oil pressure:from 49 MPa to 24 MPa

2000: Subsidence (10 meters)

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Schematic profile

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Suction : so = uo - uw

Soil skeleton

OilWater

Lixhe chalk (Belgium)

Cretaceous (35 million years)Upper Campanian (Hod formation)

Plates of coccolithes (1~10 microns)

n = 38% ~ 41%, rpores= 0.37 µm

Similarity with unsaturated soils

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Testing procedures for unsaturated soils allowing to control suction:

•Overpressure method•Osmotic technique•Mercury Intrusion Porosimetry (MIP)

Oil-water suction so = uo-uw

Capillary and physico-chemical effects between chalk, water and oil

wettability of the chalk

Experimental techniques of suction control

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The osmotic method

The osmotic method is based on the used of semi permeable membranes which permit to reach suction levels below 1500 kPa

Polythene sheet

PEG solution

Soil sample

Semi-permeable membrane

Magnetic stirrer

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Vapour phase

Pompe pneumatique

Atmosphère à humiditérelative contrôlée

Solution saturée

Echantillon

Sels

Dessicateur étanche

20 °C +/- 1°C

4,297K2SO4

8255Mg(NO3)2

Suction (MPa)Humidity (%)Salt

Table 1 : Various types of salt used

wwowo aRTs ln−=−= µµ

0vv uuHR =

Suction control by managing the relative humidity (HR)

HR is controlled viathe salt nature

The technique allows higher suction levels (up to 100 MPa)

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Axis translation method

GDS Eau

Contrôle de la pression d'eau

GDS HuileContrôle de la pression d'huile

Pierre poreuse

EchantillonPierre céramique

-500 0 500 1000 1500

Pression d'huile (kPa)

-1000

-500

0

500

1000

1500

Suc

cion

(kPa

)

pw= 0 kPa

pw= 200 kPa

pw= 500 kPaControl separately of the two pressures (and exchange volumes by mean of a ceramic porous stone that is hydrophilic and lipophobic,

The water pressure is kept constant and positive, in order to work in a larger suction path (<400kPa)

Drainage: water driving by oil

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Retention curves

0 0.2 0.4 0.6 0.8 1Srw

0.01

0.1

1

10

100

1000

10000

100000

succ

ion

(MP

a)

ImbibitionDrainage

0 0.2 0.4 0.6 0.8 1Srw

0.01

0.1

1

10

100

1000

10000

100000

succ

ion

(kP

a)

ImbibitionDrainage

Oil/water System Air/water system

Suc

tion

(kP

a)

Suc

tion

(kP

a)

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0 20 40 60 80 100

WATER SATURATION, Srw (%)

0.001

0.010

0.100

1.000

10.000

SUC

TIO

N, s

(MPa

)

Retention curves of Lixhechalk (oil-water)

OSMOTIC TECHNIQUE(imbibition)

MERCURY INTRUSION POROSIMETRY (drainage)

OVERPRESSURE (drainage)

Retention curve, synthesis

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Viscous mechanical behaviour

• Odometer tests: Strain rate effects and creep effects– CRS Tests,– Stage loading tests,

• Triaxial tests: study of the “3D” behaviour– Effects of the pores fluid,– Suction effects on the yield surface,– Loading rate effects on the yield surface,

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Constant Rate of Strain tests (1/4)

Barre fixe

Capteur de force5 Tonnes

Comparateurs

Piston

Echantillon

Pierre poreuseDéplacement du plateau inférieur contrôlé

au moyen d'une presse pneumatiqueà vitesse de déplacement constant (1 à 50 µm/min)

e.g. Leroueil, Sheahan

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CRS Test (2/4)

10 100 1000 10000 100000σV (kPa)

-0.08

-0.06

-0.04

-0.02

0

Déf

orm

atio

n vo

lum

ique

CRS TestsEau 1µm/minEau 5µm/minEau 10µm/min Eau 50µm/min

Vol

umet

ric s

train

WaterWaterWaterWater

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CRS Tests (3/4)

1E-008 1E-007 1E-006 1E-005 0.0001Vitesse de déformation (s-1)

1000

10000

100000

Lim

ite é

last

ique

(kP

a)

SecHuile200 kPaEau

122,20,0454,499Dry

1,3316,660,0604,451Oil

2,0410,90,0924,516s=200 kPa

2,449,250,1084,462Water

ratiom’1/m’A

Tableau 2: Parameters of the Leroueil law (1985)

Variation of the slope according to the wettability

( ) ( )1log1log εσ &m

Ap ′+=′

Yie

ld s

tress

(kP

a)

Strain rate (s-1)

DryOil

Water

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CRS Test (4/4)

s

σ

dε/dt

Strain rate effects on the suction-yield stress hardening relationship

LC Curve , Alonso et al. (1990)

LC (dε/d

t)

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The odometer test

Several odometer test have been performed by submitting chalk samples to series of load. Notably, one was suction controlled (200 kPa); and attention was mainly paid on consolidation and creep.

F=F0

Sample

Porous stone

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Consolidation theory

av : compressibility 1-4 10-6 kPa-1

k: permeabilities ranges between 2-5 10-8 m.s-1 (water) and 6.10-9

(oil). ( )ke1aγhT

t vw2

v

+=

Thanks to the above equation, the dissipation time ranges about 1- 100 seconds. It seems likely that the low compressibility of the soil skeleton (bonding) and the permeability of the soft rock are sufficient in chalk to prevent excess pore fluid pressure generation (Lade and de Boer 1997).

No significant generated pore pressure, mainly diffused strain corresponds to creep

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Creep model

0 20 40 60 80 100Temps (j)

0.88

0.92

0.96

1

e/e 0

1E-010

1E-009

1E-008

1E-007

stra

in ra

te

Stage at 14.5 MPaexperimental curveslope (20 points)

αβ −= tee .0

cstettee

+−−=⎟⎟⎠

⎞⎜⎜⎝

⎛)ln(ln 0

0

α

β represents the instantaneous strain,α controls the slope of strain vs time curve

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Parameters’ evolutions

α and β are quite bilinear, and represent well the visco-elastoplastic behaviour.

0 1 2 3Rapport de la Contrainte

sur la contrainte de pré consolidation

0

0.001

0.002

0.003

0.004

0.005A

lpha

0.88

0.9

0.92

0.94

0.96

0.98

1

Bet

a

abβ

β

α

α

σ/σe

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Stress – strain relations

• Results are well ordered with suction (and wettabilitycharacteristics),

• The yield stress is suction dependent

100 1000 10000 100000Axial stress (kPa)

-0.12

-0.08

-0.04

0

Axia

l stra

in

Water saturatedOil saturatedMix saturated s=200kPaDry sample

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Fluids effects

0.8 1.2 1.6 2 2.4Normalized yield stress

0

0.2

0.4

0.6

0.8

1

Wet

tabi

lity

0

5000

10000

15000

20000

25000

Suc

tion

(kP

a)

Fluid wettabilitySample suction (kPa)

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Influence of fluids on creep (1/3)

No significant modification in creep is observed according to the over stress

0 1 2 3 4Οverstress ratio σ/σe

0

0.004

0.008

0.012

0.016

0.02

Cre

ep ra

te p

aram

eter

α

Dry sampleOil saturatedMix saturated s = 200 kPaWater saturated

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Influence of fluids on creep (2/3)

0 400000 800000 1200000 1600000 2000000Time (s)

0.975

0.98

0.985

0.99

0.995

1

e/e 0

Oil saturateds=200 kPaWater saturated

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0 1 2 3 4Οverstress ratio σ/σe

0

0.004

0.008

0.012

0.016

0.02

Cre

ep ra

te p

aram

eter

α

Dry sampleOil saturatedMix saturated s = 200 kPaWater saturated

Influence of fluids on creep (2/3)

0 20 40 60 80Time (days)

0.92

0.94

0.96

0.98

1vo

id ra

tio e

/e0

Axial stress: 19.8 MPaOil saturated (initially)'Water saturated'

Intantaneous collapseunder

water infiltration

α=0,0164

α=0,0047

Water injection divided σe by 2

α increases by a factor of 5

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Mechanism of water injection

100 1000 10000 100000Axial stress (kPa)

-0.12

-0.08

-0.04

0

Axia

l stra

in

Water saturatedOil saturatedMix saturated s=200kPaDry sample

Water injection

Strength decrease

Creep (strain)

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Application to an other chalk (1/3)

100 1000 10000 100000Contrainte verticale (kPa)

0.75

0.8

0.85

0.9

0.95

1In

dice

des

vid

es n

orm

é e/

e 0

Essai secEssai s=1500kPaEssai saturé

•Detritic chalk withglauconite,

•Density: 2,74 Mg/m3

•Porosity: 37%

•Average pore radius: 700 nm:

Craie d’Estreux

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Application to an other chalk (2/3)

0 1 2 3 4 5σ/σe

0

0.005

0.01

0.015

0.02

0.025

α

EausecSuccionSuction

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Application to an other chalk (3/3)

0 1 2 3 4σ/σe

0

0.005

0.01

0.015

0.02

0.025α

EstreuxLixhe The behaviour is very

close to the oilfied chalk one,

Viscosity seems to be strongly connected to water content.

Dry chalk is less viscous.

In both system (oil/water, air/water), chalk can potentially collapse due to physicochemical mechanisms

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CONCLUSION

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Conclusions (1)

• Retention properties of chalk have been clearly identified for the couple oil and water,

• As for clays, retention is not only governed by capillarity,

• This results should be taken carefully, because chalk used in this study has not known oil before (it is not the case in the reservoir which would change wettability).

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Conclusions (2)

• Odometer s test confirmed the collapsible behaviourof oilfield chalk submitted to water injection

• Fluids do not seem to have a influence in the viscous behaviour considering that: creep rate remains equal taken into account of the over stress ratio,

• These last remarks warn us against comparing suction controlled tests at different loading rate despite a good drainage and good suction control,

• Also, several tests in an air/water saturated chalk have confirmed the chalk sensitivity to water, and the coupling between creep and suction .

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Thank you for your kind attention

Further information: gregoire.priol@ntnu.no

De Gennaro V., Delage P., Priol G., Collin F. & Cui Y.-J. 2004. On the collapse behaviour of oil reservoir chalk, Géotechnique 54 n°6 pp. 415-420.

De Gennaro V., Delage P., Priol G., Sorgi C., Collin F. (2005). Multiphase viscous behaviour of two different outcrop chalks, XIème IACMAG, Turin,

Priol G., De Gennaro V., Delage P., Sorgi C., Candel Hernandis J.V. (2004) Influence des fluidessur le comportement différé de la craie, XXIIème Rencontres universitaires de génie Civil, Marne-la-Vallée.