7 B OK Compaction Risks · 7-B Compaction Risks Compaction Geomechanics – II Compaction and...

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© M B D C I © M B D C I 7-B Compaction Risks Compaction Risks Compaction Risks Maurice Dusseault

Transcript of 7 B OK Compaction Risks · 7-B Compaction Risks Compaction Geomechanics – II Compaction and...

Page 1: 7 B OK Compaction Risks · 7-B Compaction Risks Compaction Geomechanics – II Compaction and depletion will change both normal stresses and shear stresses If a “sharp” gradient

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Compaction RisksCompaction Risks

Maurice Dusseault

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ksReservoir CompactionReservoir Compaction

� Triggered by reduction in pore pressure

� Important drive mechanism in high φ cases (Maracaibo, Ekofisk, Wilmington, ...)

� But, problems develop with compaction ...

•Casing collapse in the reservoir

•Surface subsidence from deep compaction

•Casing shears above the reservoir (Ekofisk)

•Reservoir simulator predictions are contentious

•Large stress redistributions, microseismicity…

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ksThe Case of EkofiskThe Case of Ekofisk

� 3000 m deep

� 7 km wide

� Very thick reservoir

� High porosity chalks� φ from 25 to 50%

� Overpressured by 1.7

� Large drawdowns are feasible – large ∆σ′v

� Large compactions…

� Would we plan for these nowadays?

EKOFISK

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ksCompaction as a DriveCompaction as a Drive

� Compaction occurs whenever the net stress increases (σ′↑, φ↓)

� Magnitude depends on the rock stiffness, fabric (some rocks have a quasi-stable φ)� Important in high-porosity sandstones

� Important in North Sea Chalk (e.g. Ekofisk)

� Important in Diatomite

� “Recompaction” is important in cyclic steam stimulation (porosity cycling)

� Geomechanics of compaction are fundamental to the risk management process

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highσh

region

flanks -- shoulders -- crest -- shoulders -- flanks

compaction

highτ

region

slip planesin overburden

Bedding Plane Slip & Casing ShearBedding Plane Slip & Casing Shear

lowσh

region

normal faultmechanism

Large compaction: seismicity, casing shear, casing bucking

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ksMajor Design StepsMajor Design Steps

� Identify physical compaction mechanisms

� Identify susceptible reservoirs �Based on experience in other reservoirs

�Based on geophysical data (logs, seismics)

�Based on core examination and lab tests

�Based on geomechanics analysis

�Based on monitoring information

� Study the cost-benefit gain to the company

� Implement mitigation measures in advance

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ksCorporate Screening StrategiesCorporate Screening Strategies

Screening and Analysis

Risk assessment-second-order screening-production predictions

Geomechanics assessment-geophysical logs

-petrophysical evaluation-stress history

Decision making-experience base

-learning and teaching

Cost-benefit analysis

Impact assessment

-well modeling-reservoir modeling

Cost of options$ - ¥ - Bs

Mitigation options-pressure maintenance-facilities redesign-production strategies

First-order screening(geology + cases)

Monitoring cases

-casing shear risk

Is compaction beneficial?

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ksCompaction Geomechanics Compaction Geomechanics –– I I

� Compaction = ƒ(∆σij´, E, ν, ...)

� Is ∆p always = ∆σij´?

� No, compaction is not uniform�∆p is not uniform in reservoir

�Overburden arching takes place

� Thus, compaction moves out from production wells, arching delays full ∆z development

� This can be simulated quite well, with coupled models…

∆z

∆σij, ∆p

Cc (E, ν)

∆z ~ Cc∆σv´

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ksCompaction Geomechanics Compaction Geomechanics –– IIII

� Compaction and depletion will change both normal stresses and shear stresses

� If a “sharp” gradient of compaction occurs, ∆τ(shear stress) can be quite large

� This can cause shearing, grain crushing, loss of cohesion, liquefaction (Chalk), etc.

� These factors affect the permeability, often negatively, but shearing can also increase k

� It is essential to study case histories to assess these factors, even semi-quantitatively

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ksReservoir CompactionReservoir Compaction

� Delay of compaction always occurs in early time, before ∆p zones intersect (aspect ratio)

∆p region

no ∆p yet

initially after some Q

Arching means that compaction can be missed in early time!

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ks∆∆z Delay Through Archingz Delay Through Arching

overburden stresses “flow” around the ∆p, ∆V zone

drawdownzones

Compaction is impeded by arching during early drawdown

in this phase, ∆σv’is not equal to ∆p

arching occursuntil drawdown zones

interact at thereservoir scale

subsidence response is delayed

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ksFull CompactionFull Compaction

stresses now “flow” without arching around zones

fullsubsidencedevelops

fullcompaction

is now triggered

when zones meet,arching is destroyed,

full compaction occurs

∆z

This demonstrates that geomechanics must be incorporated in risk assessment in compacting and subsiding cases

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ksReservoir CompactionReservoir Compaction

� Compaction sustains production, and can change the production profile substantially

Q -

tota

l fie

ld

time

predicted, assuming ∆σ′v = ∆p

actual Q withdelayed ∆z

economiccutoff Q

predicted life

actual life

oil lost?

Cost-benefit analyses require compaction assessment

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ksNegative Effects of Negative Effects of ∆σ∆σ′′ijij� Productivity can decline with an increase in

∆σ′ij , both in normal and shear stresses�Fracture aperture diminution (+++)

�Pore throat constriction (+)

� The relative importance depends on the rock mechanics properties of the reservoir

� Casing shear or buckling can occur

� Surface subsidence can take place

� These can be costly if unexpected. If understood, they are factored into risk study…

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ksCasing DeformationCasing Deformation

“Wedging” Shear

Courtesy Trent Kaiser, noetic Engineering

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ksPositive Effects of Positive Effects of ∆σ∆σ′′ijij� Changes in the effective stress can trigger

changes in the porosity�Compaction can be substantial (UCSS, chalk)

�Compaction serves to sustain the pressure

�Much more oil is driven to the wellbores

� In high φ Chalk, a volume change as much as –10% can take place, all oil production

� Compaction of high φ shales can also expel water into the reservoir, displacing more oil

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ksAsk a Geologist!Ask a Geologist!

timetemperature

chemistry

cementation,25-32%

pressuresolution,25-32%

initial state, 35% porosity

porosity reduction

stresses

Solution and cementation reduce porosity, increase stiffness

Part of the risk assessment process for compaction and subsidence is assessment

of reservoir compaction potential. This requires clarification of geological history

and comparing the microfabric of the material to other known cases. Consult an

experienced geologist for more help.

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ksAdditional MechanismsAdditional Mechanisms

� Compaction can lead to loss of some permeability in natural fractures, especially important in high φ fractured carbonates

� Grain crushing can occur in some UCSS cases, therefore k↓, reducing flow capacity

� Depletion → loss of lateral stress, increase in mean σ′, increase in shear stress τ

� Increase in shear stress often causes k↑ in jointed strata!

� Compaction can release fines from strata…

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ksFracture Aperture, Fracture Aperture, ∆σ∆σ′′, , ∆∆kk

� Fracture aperture is sensitive to σ′n� Permeability is highly sensitive to aperture

� Shear displacement and asperity crushing can develop with ∆τ

asperities

p p + ∆peffective aperture

∆σ′n

Reservoir engineers want a homogeneous constitutive

macroscopic law linking k to ∆p. Really, a coupled

geomechanics analysis is required, linking k to ∆σ′.

∆σ′n

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ksFracture Permeability LossFracture Permeability Loss

� In many cases, fracture permeability decreases by a factor of 1.5 to 3

� This is a compaction effect (loss of aperture as net stress increases is a form of compaction)

� If we have folded or inclined NFCR’s, it becomes more complex: some fractures open, other fractures may close

� It is important in coal, North Sea Chalk, fractured gas shales, but not in sandstones

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ksSo, Compaction Risks Include:So, Compaction Risks Include:

� Casing shear and costly well re-drilling

� Platform subsidence offshore, ground subsidence onshore

� Induced seismic events (Ekofisk 2001 event, Wilmington 1953-1955 events)

� Unexpected production behavior�Compaction maintaining drive forces

�Fractures closing or dilating

� And other effects…

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Subsidence MacroMechanicsSubsidence MacroMechanicsPredicting the Effects at the Surface and Predicting the Effects at the Surface and

in the Reservoirin the Reservoir

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ksSubsidence BowlSubsidence Bowl

θ

subsidence bowl, L

depth, Z

width, W

extension

compression

compaction, T

∆zmax

(Vertical scale is greatly exaggerated)

Consequences of compaction must be included $-risk analysis

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ksSubsidence MagnitudeSubsidence Magnitude

� If W < Z, arching, little subsidence (<25% T)

� If Z < W < 2Z, partial arching (25-75% of T)

� If W > 2Z, minimal arching (>75% of T)

� Bowl width: L = W + 2Zsinθ� If W > 2Z, ∆zmax approaches T

� θ (angle of draw) usually 25 to 45 degrees

� In cases of complex geometry and stacked reservoirs, numerical approaches required

� Consequences: offshore or onshore??

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ksCasing ImpairmentCasing Impairment

� Either loss of pressure integrity, or excessive deformations (dogleg, buckling)

� Problem in massively compacting reservoirs

� These are vexing and difficult problems

� Is more compliant casing and cement best?

•Compaction can distort and even buckle casing•Threads can pop open, casing can be ovalized•Triggering of faults shear casing which pass through•Overburden flexure causes shear planes to develop•Casings cannot withstand much shear

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ksCasing DogCasing Dog--Leg DevelopmentLeg Development

tubi

ng

cased, cemented wellbore

reservoir

shale

Single shearslip plane

Distributedshear zone

In compaction risk assessment, costs of casing shear are statistically included through a geomechanics coupled model

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ksMitigation of Compaction Risks Mitigation of Compaction Risks

� Pressure maintenance�Water injection or gas injection

�CO2 sequestration, and also used as an enhanced oil recovery approach (the future…)

� Structural design of platforms in offshore cases

� Repair and replace surface infrastructure

� Judicious placement of wellbore to reduce the incidence of casing shear

� Special completion techniques

� Monitor, monitor, monitor…

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More Discussion of Ekofisk and RiskMore Discussion of Ekofisk and Risk

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ksCase HistoriesCase Histories

� Maracaibo in Venezuela

� Groeningen in Netherlands

� Niigata in Japan (gas)

� Ekofisk in the North Sea (Norway Sector)

� Ravenna in Italy (gas)

� Many examples elsewhere as well

� Good examples in the hydrogeological and geotechnical literature are interesting

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ksHollow Coccoliths, Little CementHollow Coccoliths, Little Cement

Ris

nes,

R.,

Mad

land

, M.V

., H

ole,

M. &

Kw

abia

h, N

.K.,

2005

. Wat

erw

eake

ning

of

cha

lk –

Mec

hani

cal e

ffect

s of

wat

er-g

lyco

l mix

ture

s. J

ourn

al o

f Pet

role

um

Sci

ence

and

Eng

inee

ring,

48,

21-

36.

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ksRock Mechanics ImplicationsRock Mechanics Implications

� During development (1969-1975), engineers failed to predict massive subsidence

� When noted in 1981-83, maximum subsidence was greatly underestimated (~6-7 m)

� Also, engineers failed for many years to understand the water-weakening phenomenon

� Some consequences:�Platform jacking by 6.3 m in 1988-89

�Complete platform redevelopment in 1999-2002

�Far greater OOIP and production rate

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ksProductionProduction--Injection HistoryInjection History

Start water injection

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ksEkofisk (I)Ekofisk (I)

� 3000 m deep Chalk reservoir, very thick� Exceptionally high porosities, ~50% at the

top, 30-35% at the base� Overpressured, σv = 65 MPa, po = 54 MPa� Moderate lateral stresses, extensional regime� Chalk slightly cemented� Overlying shales overpressured� Large width with respect to depth (W > 3D)� Water-weakening of Chalk promotes ∆z

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ksEkofiskEkofisk……

10 km

Reservoir structure

Subsidence bowl

salt

Porosity, φ ≈50% in this

zone

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ksEkofisk (II)Ekofisk (II)

� Lengthy well tests failed to detect compaction

� Subsidence assumed minor because of depth

� Casing shearing became a problem in 1980’s

� Wells had to be redrilled, some twice

� Subsidence first noted from platform legs

� 4.2 m in 1987, predicted max of 6.2 m

� Platforms raised, 1987 ($US485,000,000.00)

� Subsidence exceeded 6.0 m in early 90’s

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ksMassive Reservoir Compaction Massive Reservoir Compaction

� Seafloor subsidence

� Leads to wave risks for platforms

� Casing shearing (some wells redrilled several times)

� Wonderful drive sustaining process!

1973

1984

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ksEkofisk (III)Ekofisk (III)

� Redevelopment decision in 1994, S = 6.4 m

� Pressure maintenance tried in 1980’s, but it seemed quite ineffective, but in use now since 1988

� Repressurized since 1990’s

� More casing shear, most wells redrilled twice

� Numerical analysis showed 80-85% of compaction was appearing as subsidence

� Microseismic activity in overburden, along zones where casing was shearing regularly

2.3 billion $

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ksEkofisk (IV)Ekofisk (IV)

� However, it is a fabulous reservoir!� 100% of initial predicted production was

surpassed in early 1990’s!� Life predicted to 2011, extended 30 years� Some compaction drive continues (∆z > 9 m),

but at ~30-35 cm/yr only� Max ∆z now thought to be greater than 11 m� Field may produce more than twice as much

oil as initially thought!� Ekofisk has been a major learning experience

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ksCasing Shear at EkofiskCasing Shear at Ekofisk……

Source: EPOKE, Phillips Petroleum, No 7, 1999

6262000

6263000

6264000

6265000

6266000

6267000

6268000

6269000

6270000

6271000

510000 511000 512000 513000 514000 515000 516000

E-W CoordinatesN

-S C

oord

inat

es

Overburden

Reservoir

Distribution of overburden shear damage and producing interval compaction damage

at Ekofisk(SPE 71695)

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ksEkofisk Continues ....Ekofisk Continues ....

� Casing shearing not fully ceased or cured� Will high-angle flank faulting develop?� Redrilling wastes injection (Where? How?)� Surface strains and subsea pipelines: will

there be impairment of these facilities� Oil storage facilities relocated?� Can we reasonable predict these events?� I believe petroleum geomechanics has

advanced enough so that we can predict

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ksRisk: Will the Reservoir Compact?Risk: Will the Reservoir Compact?

� All reservoirs compact, but how much?

� Best is to test truly undisturbed core samples in the laboratory under representative uniaxial and triaxial loading conditions

� Failing this, a detailed comparison to other cases of compaction is carried out (logs, etc.)

� Predictions of compaction can be expected only to be +/-25% at best (sampling problems, long-term creep, etc.)

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ksRisk: How Much Compaction?Risk: How Much Compaction?

� Depends on compressibility, Z, p, ∆p, φ� Qualitative screening criteria (geology!)

•If porosity > 25% (> 35% is virtually certain)

•If the reservoir is geologically young (little diagenesis)

•If it is at its maximum burial depth (no over-compaction)

•If the mineralogy is arkosic or lithic (weak grains)

•If ∆p will be large, and particularly if overpressured•Mainly in extensional regimes and continent margin basins

•If largely uncemented by SiO2 or CaCO3 •If reservoir width > depth to reservoir (no arching)

•Other criteria are probably of little relevance

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ksValhall DispositionValhall Disposition

From: Zoback MD, Zinke JC. Production-induced Normal Faulting in the Valhall and Ekofisk Oil Fields. Pure Appl.

Geophys. 159 (2002) 403-420

Ekofisk and Valhall are similar Chalk reservoirs, formed because of deep-seated salt doming creating an anticline.EKOFISK

Geography

VALHALL

A

A′

Deep salt ridge

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ksPrediction by ComparisonPrediction by Comparison

� Other case histories are carefully studied

� Quantitative comparisons are made:

� A probability estimate is made

•Geological setting, thickness, etc.•Porosity from cores and density logs•Comparison of seismic velocities (vP, vS)•Study of diagenetic fabric and stress history•Geometry and scale of the reservoir wrt depth•Mineralogy and lithology of the sediment•Stresses, pressures, drawdowns, timing•Other factors?

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ksMonitoring Compaction, ShearMonitoring Compaction, Shear

pumpV, t, r

data acquisition

injectionwell

monitorwell

process zone

monitorwell

pressuregeophonetiltmeter

V

t

r

volumes

time, rate

density

You cannot manage risk properly without monitoring and use of monitoring data to update risk and cost assessment

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ksMitigating Casing Shear RiskMitigating Casing Shear Risk

� Stronger cement and casing are not useful

� There are several possible approaches�Pressure maintenance in the reservoir (H2O)

�Avoid placing wells in zones of high shear

�Manage reservoir development strategies to reduce incidence: e.g. choose a lower shear stress method

�Create a more compliant casing-rock system

� Avoidance & management require modeling

� Under-reaming & no cement delays distress

� Better sealing cements may reduce p migration

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ksSimulation of Casing ShearingSimulation of Casing Shearing

47Courtesy - TTI

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ksDistortion of CasingDistortion of Casing

48

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ksMathematical Modeling of StrainsMathematical Modeling of Strains

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ksUnderUnder--Reaming to Reduce ShearReaming to Reduce Shear

interface slip

casing cemented, but notin the under-reamed zone

shale stratum

sand stratum

bedding plane slip

under-reamed zone

casi

ng

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ksUnderUnder--Reaming of HoleReaming of Hole

0

10

20

30

40

50

60

70

80

90

100

Undamaged Damaged Failed

Per

cent

of T

otal

Unprotected Wells(155 total)

15" Under-ream (5 total)

26" Under-ream (147 total)

Wilmington

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FracPacPacker

Sump Packer

Telescoping Joint

Screen, base-pipe, couplings

9,743’

9,818’

9980’

10,192’

210

ft

450

ftSump Packer

Telescoping Joint

Screen, base-pipe, couplings

9,743’

9,818’

9980’

10,192’Sump Packer

Screen, base-pipe, couplings

9,743’

9,818’

9980’

10,192’

210

ft

450

ft

210

ft

450

ft

Analyzing Special Compaction JointsAnalyzing Special Compaction Joints

for ν = 0.2, Cp = 9×10-6 psi-1

∆p = 2600 psi ∆H/H = 1%

Telescoping joint

Screen, basepipe, couplings

Sump packer

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ksCompaction Risk MitigationCompaction Risk Mitigation

� Good predictions are necessary�Geology, analogous cases, simulation, testing…

� Avoid compaction altogether?� It is a good drive mechanism, avoidance costs $$

� Live with it, but be prepared

� Most important is to do our geomechanics right, and to examine the full spectrum of effects, negative + positive ones…