compaction geomechanics mbdci compaction geomechanics: mechanisms, screening maurice b. dusseault

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Compaction Geomechanics: Compaction Geomechanics: Mechanisms, ScreeningMechanisms, Screening

Maurice B. Dusseault

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Compaction as a DriveCompaction as a Drive

Compaction occurs whenever the net stress increases ()

Magnitude depends of the rock stiffness, fabric (some rocks have a quasi-stable ) Important in high-porosity sandstones Important in North Sea Chalk (e.g. Ekofisk) Important in Diatomite (California mainly) “Recompaction” is important in cyclic steam

stimulation (porosity cycling) Compaction geomechanics is fundamental

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The Case of EkofiskThe Case of Ekofisk

3000 m deep 7 km wide Very thick reservoir High porosity chalks

from 25 to 50% Overpressured by 1.7 Large drawdowns are

feasible – large Δσ′v Large compactions… Would we plan for

these nowadays?

EKOFISK

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Reservoir CompactionReservoir Compaction

Triggered by reduction in pore pressure Important drive mechanism in high 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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Major 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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Screening StrategiesScreening 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 casesIs compaction

beneficial?

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Compaction Effects ICompaction Effects I

Compaction = ƒ(ij´, E, , ...)

Is p always = ij´? 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 modeled quite well

z

ij, p

Cc (E, )

z ~ Ccv´

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Compaction Effects IICompaction Effects II

Compaction and depletion can change both normal stresses and shear stresses

If “sharp” gradient of compaction occurs, (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

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

p region

no p yet

initially after some Q

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z Delay Through Archingz Delay Through Arching

overburden stresses “flow” around the p, V zone

drawdownzones

compaction impeded

in this phase, v’is not equal to p

arching occursuntil drawdown zones

interact at thereservoir scale

full subsidence response delayed

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Full CompactionFull Compaction

stresses now “flow” without arching around zones

fullsubsidencedevelops full

compactiontriggered

when zones meet,arching is destroyed,

full compaction occurs

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Compaction sustains production, and can change the production profile substantially

Q -

tot

al f

ield

time

predicted, assuming v = p

actual Q withdelayed z

economiccutoff Q

predicted life

actual life

oil lost?

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Negative Effects of Negative Effects of ijij

Productivity can decline with an increase in 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

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Positive Effects of Positive Effects of 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 Chalk, a volume change as much as –10% can take place, all oil production

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

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Mechanisms I (Sandstones)Mechanisms I (Sandstones)

Pore pressure is reduced by production (p) The vertical effective stress, v, rises A sand of high compressibility will begin to

compact to a lower porosity This maintains drive pressures in liquid-

dominated reservoirs, may cause subsidence Also, a direct fluid expulsion occurs (V) Water can be expelled from adjacent shales

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Elastic CompressionElastic Compression

leads to increased contactforces, f´n

The contact area increases, porosity drops This is a function of compressibility If elastic, V is recoverable (-V = +V)

Ap

fn

E,

p Ap - A

fn

fi

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Inelastic CompressionInelastic Compression

leads to increased contactforces, f´n

Grain rearrangement takes place, drops Perhaps a bit of grain contact crushing In high sands, this is an irrecoverable V

Ap

fn

E,

p Ap - A

fn

fi

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Elastic and Inelastic StrainElastic and Inelastic Strain

porosity

log(v)

1 MPa 10 MPa 100 MPa

compaction curve

rebound curve

irrecoverablecompaction

Depletion = +

Injection = -

elastic behavior

elastic behavior

inelastic behavior

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Mechanisms II (Sandstones)Mechanisms II (Sandstones)

For small p, grain rearrangement is most important; V not recoverable. Also,

Contacts compress elastically, recoverable At intermediate p, grain contacts deform

elastoplastically, strain is not recoverable At high p (high ), grain splitting,

crushing, and even creep occurs, especially in lithic and arkosic sandstones

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Processes and CompactionProcesses and Compaction

porosity

log(v)1 MPa 10 MPa 100 MPa

rebound curve

irrecoverablecompaction

grain crushing at high

grain rearrangementat intermediate

elastic compression at low

elastic recovery

low high

0.30

0.25

0.20

elastoplastic grain contact behavior

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Mechanisms, Chalk (I)Mechanisms, Chalk (I)

North Sea Chalks (and a few other materials) exist in a high-porosity state (>35-40%)

This state is “quasi-stable” and exists because of cementation between grains

If grains crush or cement ruptures, massive compaction occurs (>10 m at Ekofisk)

This is triggered by the increase in v, and also by increased stress difference (1 - 3)

Shear destroys cement, triggers compaction

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Why Does Chalk Collapse?Why Does Chalk Collapse?

Hollow, weak grains (coccoliths)

Weak, cleavable grain mineral (CaCO3)

Weak cementation (dog-tooth calcite)

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Mechanisms, Chalk (II)Mechanisms, Chalk (II)

Threshold stress from cementation in Chalk Grains are also hollow and weak Once collapse happens, the Chalk can even

become “liquefied” locally The stresses are transferred to adjacent rock

and the process can propagate far The whole reservoir compacts when the

collapse is at the interwell scale

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North Sea Chalk CollapseNorth Sea Chalk Collapse

porosity

log(v’)1 MPa 10 MPa 100 MPa

compression curve

threshold stress

irrecoverable strain

rebound curves

collapse of fabric

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Geological History!Geological History!

Diagenesis = pressure solution, densification, grain-to-grain cementation

Cementation can preserve a rock at a very high porosity (collapsible, like Chalk)

Densification and pressure solution can make the rock stiffer at the same value

Overcompaction (deep burial history) can make the rock stiffer, little compaction

Geological history is a vital factor

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Cementation, CompactionCementation, Compaction

porosity

log(v’)

“virgin”compression curve

apparent threshold’

stiffresponse

collapse if cementis ruptured

normaldensificationcementation effect

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Diagenetic DensificationDiagenetic Densification

porosity

log(v’)

diageneticporosity loss

@ constant ’

stiffresponse


present state

apparent threshold ’

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Precompaction EffectPrecompaction Effect

porosity

log(v’)

present state


apparent threshold ’

stiffresponse

Deep burial followed by uplift and erosion lead to precompaction

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““Threshold” DrawdownThreshold” Drawdown

Usually, some threshold drawdown must occur before significant compaction starts

There are three effects responsible:

These are hard to quantify without careful geological studies and laboratory testing

Short-term well testing can be misleading!

•The sand may be geologically pre-densified•There may be a cementation to overcome (Chalk)•The p may not yet be at the reservoir scale (arching)

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Cementation, DiagenesisCementation, Diagenesis

timetemperature

chemistry

cementation,25-32%

pressuresolution,25-32%

initial state, 35% porosity

porosity reduction

stresses

Both solution and cementation reduce porosity, increase stiffness

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Additional MechanismsAdditional Mechanisms

Compaction can lead to loss of some permeability in natural fractures

Grain crushing can occur as well, k Depletion loss of lateral stress, increase

in mean , increase in shear stress Increase in shear stress usually causes k Compaction can release fines from strata Are there other effects in your reservoirs?

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Fracture Aperture, Fracture Aperture, kk

Fracture aperture is sensitive to n

Permeability is highly sensitive to aperture Shear displacement and asperity crushing

can develop with

asperities

p p + peffective aperture

n

It appears that a homo-geneous constitutive macro-scopic law is required for good predictions in analysis

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Fracture Permeability LossFracture Permeability Loss

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

This is to a degree a compaction effect (loss of aperture as net stress increases)

It also retards compaction (e.g. fractures in Chalk)

It is important in coal, North Sea Chalk, but not in sandstones

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Grain Crushing and Grain Crushing and k k

Depletion or differential volumetric strain causes high high f´n on individual grains

Weak (lithic) or cleavable (felspathic) grains crush and fragment

Pore throats then become smaller or blocked by fragments, k drops

feldspar quartz

feldspar crushed, quartz intact

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Depletion Effect on Depletion Effect on hh

h stress trajectories h concentration

Z

h alongwellbore

initialh

finalh

far-

field

str

ess

es

wellbore

Operational consequences: -low pfrac in reservoir -higher pfrac above reservoir

Zone after production (p)

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Shear, Fracture OpeningShear, Fracture Opening

v

h

Zone of pressure decline, -p

Pressure decline leads to an increase in the shear stressThis leads to shearing, which causes fractures and fissures to openThis leads to increases in permeability, better reservoir drainage

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Pore Blockage MechanismsPore Blockage Mechanisms

Mineral deposition

Fines migration can block pores

Geochemical effects!

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Most Sensitive CasesMost Sensitive Cases

Highly fractured reservoirs Reservoirs with asphaltene precipitation,

scale deposition, or fines migration potential

Tectonically stressed reservoirs (high ) Reservoirs with crushable grains or

collapsible fabric (North Sea chalk, coal) Thermal shock in unconsolidated sands (?) Other cases?

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Porosity vs DepthPorosity vs Depth

mud

clay

mud-stone

shale

0 0.25 0.50 0.75 1.0

clay & shale,“normal” line

sands &sandstones

effect ofoverpressures

on porosity

depth

porosity

4-6 km

The specific details ofthese relationships area function of basin age,diagenesis, heat flow ...

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Subsidence MacroMechanicsSubsidence MacroMechanics

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Subsidence BowlSubsidence Bowl

subsidence bowl, L

depth, Z

width, W

extension

compression

compaction, T

zmax

(Vertical scale is greatly exaggerated)

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Subsidence 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 + 2Zsin If W > 2Z, zmax approaches T (angle of draw) usually 25 to 45 degrees In cases of complex geometry and stacked

reservoirs, numerical approaches required

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Casing ImpairmentCasing Impairment Either loss of pressure integrity, or

excessive deformations (dogleg, buckling) Problem in massively compacting reservoirs

These are vexing and difficult problems More compliant casing and cement?

•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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Casing Shearing!!Casing Shearing!!

It is a major problem in all compacting reservoir cases

Will deal with this in greater detail in another presentation, as it is important for many new production technologies: CHOPS Thermal methods Etc.

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Measuring CompactionMeasuring Compaction

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Measuring CompactionMeasuring Compaction

In the reservoir… Radioactive bullets, casing collar logs,

gravimeter logs, behind-the-casing precision logs, magnetic devices, extensometers, strain gauges on casing, and other devices…

At the surface… (subsidence) Precision surveys, aerial photos, differential

GPS, InSAR, depth gauges (offshore sea floor subsidence), tiltmeters, and other methods…

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Radioactive BulletsRadioactive Bullets

The zone of interest is selected Before casing, radioactive bullets are fired

into the adjacent strata Casing is placed, garbage removed A baseline gamma log is run (slowly!) At intervals, logging is repeated, and the

difference in gamma peaks is measured Strain = L/L, accuracy of about 1-2 cm

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Casing Collar LogsCasing Collar Logs

Casing moves with the cement and the rock The casing collar makes a thicker steel zone This can be detected accurately on a log

sensitive to the effect of steel (magnetic) Logs are run repeatedly, strain = L/L Short casing joints can be used for detail If casing slips, results not reliable If doglegged, can’t run the log

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Borehole ExtensometersBorehole Extensometers

Wires anchored in the casing Brought to surface, tensioned Attached to a transducer or to

a mechanical measuring tool Reading taken repeatedly Resistant to doglegging Logs can’t be run in the hole Other instruments installed

wire 1wire 3

wire 2

W

anchor 3

anchor 1

anchor 2

sheaves

casing

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Other Borehole MethodsOther Borehole Methods

Strong magnets outside fibreglass casing are used (fibreglass just over the interest zone)

Strain gauges bonded to the casing, inside or outside (best), leads to surface

Gravity logs (downhole gravimeter) Other behind-the-casing logs which are

sensitive to the lithology changes Tilmeters can be placed in boreholes

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Surface Surface z Measurementsz Measurements

Differential GPS can give accuracies about one cm on land, not as good offshore

Precision aerial photos with stable targets give down to perhaps one cm, a bit less

Surface monument array with surveying can give precisions of less than a millimetre

Tiltmeters measure inclination extremely precisely, give electronic readout

Other methods?

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GPS - Fixed Monuments VisitsGPS - Fixed Monuments Visits

Antenna

Monument

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mod. Stancliffe & van der Kooij, AAPG 2001

+285 mm

+200

-210

+260

+130 mm

-165

km

+100 Vertical displacements

(mm)

over 86 days

subsidence

heave

InSAR - IOL - Cold LakeInSAR - IOL - Cold Lake

mega-row

CSS

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Belridge FieldBelridge Field, CA, CA - Subsidence- Subsidence

30-40 cm per year

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Belridge Rate - Belridge Rate - ΔΔz/z/ΔΔtt

over 18 months

0.0 in./yr

12.5

25.0

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Shell Oil Canada – Peace RiverShell Oil Canada – Peace River

ref. Nickle’s New Technology Magazine, Jan-Feb 2005

Surface

uplift / tilt

data

reservoir inversion grid

with 50x50m grid cells

Multi-lateral

CSS

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NAM, NetherlandsNAM, Netherlands - Ameland - Ameland

- Gas Field

- 3350m reservoir depth

- 22cm subsidence

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Measurement ParametersMeasurement Parameters

Precision must be acceptable (5% of zmax) No systematic errors if possible (random only) The number of measurement stations must be

chosen carefully, depending on goals If inversion needed, array designed rigorously Array must extend beyond reservoir limits to

capture the subsidence bowl Stable remote benchmark needed, etc.

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Compaction AnalysisCompaction Analysis

Prediction, measurement, and analysis is almost a “solved” problem nowadays

Good data remain essential Better coring and lab work needed Screening criteria should always be applied Can use subsidence to monitor processes Casing/cement design to resist compaction

and shear collapse can be greatly improved

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Discussion of Some Case HistoriesDiscussion of Some Case Histories

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Case 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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Ekofisk (I)Ekofisk (I)

3000 m deep Chalk reservoir, very thick Exceptionally high porosities, 48-49% at the

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

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Ekofisk (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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Ekofisk (III)Ekofisk (III)

Redevelopment decision in 1994, S = 6.4 m Pressure maintenance tried in 1980’s, but it

seemed quite ineffective, in use now 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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Ekofisk (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 Good compaction drive continues (z > 9 m) Max z now thought to be greater than 15 m Field may produce more than twice as much

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

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Ekofisk 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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Maracaibo, Venezuela (I)Maracaibo, Venezuela (I)

Moderate depth UCSS, thick sequence 30-35% in situ Lithic to arkosic strata Geologically quite young In a monotonically sedimenting basin, no

tectonic compression, no unloading po slightly above hydrostatic No cementation, no pre-compaction

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LocationLocation

MARACAIBO

Lago deMaracaibo

N

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Maracaibo SettingMaracaibo Setting

Sandstone reservoirs Late Cretaceous to

Tertiary Normally pressured High porosity for the

present burial depth Clay cement usually Asphaltene present in

heavier oils (<30°API)

CABIMAS

LAGUNILLAS

MARACAIBO

I

XIX

VI

V

XIII

III

IV

XIV IIXII

Lago deMaracaibo

VIII

XIVVII

BACHAQUERO

TIA JUANA

MENE GRANDE

Central development

areas

NSubsidence area

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Maracaibo, Venezuela (II)Maracaibo, Venezuela (II)

Subsidence up to 6.5 m, broad bowl Adjacent to the coast, extensive dykes had

to be constructed Some visible tension cracks developed at

the surface, on subsidence bowl crest Thermal recovery methods seem not to

have triggered new subsidence of consequence

Casing loss has been moderate

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Ravenna, Niigata…Ravenna, Niigata…

Italy, Japan, some other places Intermediate depth gas sands, only water

present as a second phase, no oil High porosity (>30%), arkosic sands,

several stacked reservoirs, water influx Compaction in the reservoirs, plus water

was expelled from bounding silts and clays Serious problem was subsidence, as these

are both coastal cities

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Wilmington, California (I)Wilmington, California (I)

Intermediate depth, many stacked reservoirs Great aggregate producing thickness UCSS, porosity > 30%, arkosic Extensional tectonics (LA Basin) No cementation, no geological history of

deeper burial followed by erosive unloading Medium weight oil, liquid reservoir drive Large area, but edges relatively smooth

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Wilmington. CaliforniaWilmington. California

Bowl shaped Shear of casings

occurred mainly on the shoulders of the subsidence bowl

Few shears in the middle, where z greatest

Few on flanks Associated

earthquakes

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Wilmington, California (II)Wilmington, California (II)

Sudsidence reached 9.5 m Minor earthquakes triggered, and in one

case, > 100 casings simultaneously sheared On the sea coast = great problems with

naval shipyards, inundation Railway tracks buckled, fissures opened,

buildings cracked sometimes, etc. Pressure maintenance in 1960’s

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MBDCI

Little-Compacting CasesLittle-Compacting Cases

Groeningen, Holland - competent rock Deeper oil sands, Alberta - low overall

stresses + geological pre-compaction and mild diagenesis, but no cementation

Faja del Orinoco, Venezuela - thick quartzose sands, similar to Alberta, so compaction will not be substantial

We can also learn from these cases

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Surface Heave from Surface Heave from ΔΔT & T & ΔΔpp

Surface heaves cannot be explained by ΔT & Δp alone: there must be shear dilation taking place. Therefore, there are massive changes in the reservoir properties – k, Cc, ,

Surface heave – Δz – above a SAGD project

1 km

320 mm +Δz

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How Much Compaction?How Much Compaction?

Depends on compressibility, Z, p, p, 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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Will the Reservoir Compact?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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Prediction 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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Reservoir & OverburdenReservoir & Overburden

Compaction delay due to reservoir arching Later, arching destroyed, subsidence starts If W>Z, 85-90% h transmitted to surface Strain transmittal to the surface is essentially

instantaneous (if there is no arching) Geometry is very important (next slides) Overburden distortion leads to massive

and shear potential (next slides)

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Geometry EffectsGeometry Effects

Everything depends on aspect ratios (W,L,Z) A deep narrow sand will cause no z A wide reservoir (W > 1.5Z) will always

transmit compaction to the surface as z The subsidence bowl is wider than the width

of the compacting reservoir If very wide, zmax approaches hmax Simple models OK, but complex geometries

and stacked reservoirs: numerical models

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Modeling CompactionModeling Compaction

Best approach is a fully coupled flow-geomechanics simulation (FEM or DD), giving all stresses and strains directly

Next best approach is a reservoir simulator coupled to a stress-strain FEM or DD model, iterating between them to solve z

A simple but limited approach is to get p from a simulator, calculate V, then project the V to surface using “nucleus-of-strain”

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Coupling Stresses, FlowCoupling Stresses, Flow

The assumption v´ = p is usually wrong It ignores redistribution of stresses in the

reservoir and through overburden stiffness Thus, a full stress-flow solution is needed:

Calculate p, use in a model (one step) A model iteratively coupled to flow model A fully-coupled finite element approach Use of DD + flow model for is most efficient

Also gives overburden shear stresses changes

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Stress Trajectories, -Stress Trajectories, -V CaseV Case

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Overburden ArchingOverburden Arching

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

p region

no p yet

initially after some Qstress arching

*aspect ratio is W/H; if W>~3H, arching is disappearing

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Reduced Lateral StressesReduced Lateral Stresses

Zone of high drawdown

h stress trajectories h concentration

Z

h alongwellbore

initialh

finalh

far-

field

str

ess

es

wellbore

Operational consequences: low pfrac in reservoir higher pfrac above reservoir

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Prediction of Prediction of ijij

A flow-coupled geomechanics model is required to correctly solve for ij and p}

FEM, FEM + FD, DD + FD, Hybrid models using analytical solutions + FEM, DEM …

Material constitutive behavior is critical Non-linear E (granular and fractured media)? Potential for shear of weak rocks, fractures? Fabric changes & yield (grains, shearing …)?

Boundary conditions and initial conditions!

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Coupled ModelingCoupled Modeling

Coupling requires that the volume changes from be analyzed along with p

Only limited closed-form solutions exist Coupling can be achieved numerically by

(at least) two different approaches: The complete coupled differential equations are

written and solved, usually by FEM Or, an iterative approach can be used

Latter is instructive, as it shows principles...

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Iterative Coupled ModelsIterative Coupled Models

Pressures are solved for a single time step {p} = {pi+1 - pi} calculated in flow model Assume = p, solve a FEM model Calculate {V} for all reservoir point Use {V} as flow model source-sink terms Get new {p} and iterate until error is small Take another time step and continue (Robust and rapid convergence)

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Mitigating Casing ShearMitigating Casing Shear

Stronger cement and casing are not useful There are three possible approaches

Avoid placing wells in zones of high shear Manage reservoir development to reduce

incidence Create a more compliant casing-rock system

Avoidance & management require modeling Under-reaming & no cement delays distress Better sealing cements to reduce p migration

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Under-Reaming to Reduce ShearUnder-Reaming 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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Under-Reaming of HoleUnder-Reaming of Hole

0

10

20

30

40

50

60

70

80

90

100

Undamaged Damaged Failed

Per

cen

t o

f T

ota

l

Unprotected Wells(155 total)

15" Under-ream (5 total)

26" Under-ream (147 total)

Wilmington

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Risk Mitigation ApproachesRisk Mitigation Approaches

Pressure maintenance Water injection Gas injection CO2 sequestration, and use as an enhanced oil

recovery approach Structural design of platforms Judicious placement of wellbore to reduce

the incidence of casing shear Special completion techniques Monitor, monitor, monitor…

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Modeling CompactionModeling Compaction

Best approach is a fully coupled flow-geomechanics simulation (FEM or DD), giving all stresses and strains directly

Next best approach is a reservoir simulator coupled to a stress-strain FEM or DD model, iterating between them to solve z

A simple but limited approach is to get p from a simulator, calculate V, then project the V to surface using “nucleus-of-strain”

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Prediction of Prediction of ijij

A flow-coupled geomechanics model is required to correctly solve for ij and p}

FEM, FEM + FD, DD + FD, Hybrid models using analytical solutions + FEM, DEM …

Material constitutive behavior is critical Non-linear E (granular and fractured media)? Potential for shear of weak rocks, fractures? Fabric changes & yield (grains, shearing …)?

Boundary conditions and initial conditions!

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Coupled ModelingCoupled Modeling

Coupling requires that the volume changes from be analyzed along with p

Only limited closed-form solutions exist Coupling can be achieved numerically by

(at least) two different approaches: The complete coupled differential equations are

written and solved, usually by FEM Or, an iterative approach can be used

Latter is instructive, as it shows principles...

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Iterative Coupled ModelsIterative Coupled Models

Pressures are solved for a single time step {p} = {pi+1 - pi} calculated in flow model Assume = p, solve a FEM model Calculate {V} for all reservoir point Use {V} as flow model source-sink terms Get new {p} and iterate until error is small Take another time step and continue (Robust and rapid convergence)

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FracPacPacker

Sump Packer

Telescoping Joint

Screen, base-pipe, couplings

9,743’

9,818’

9980’

10,192’21

0 ft

450

ft

Sump Packer

Telescoping Joint


9,743’

9,818’

9980’

10,192’Sump Packer


9,743’

9,818’

9980’

10,192’21

0 ft

450

ft

210

ft

450

ft

Analyzing Special Compaction Analyzing Special Compaction JointsJoints

for = 0.2, Cp = 910-6 psi-1 Δp = 2600 psi ΔH/H = 1%

Telescoping joint

Screen, basepipe, couplings

Sump packer

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Mathematical Modeling of StrainsMathematical Modeling of Strains

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FL AC3D 2.10

Terralog Technologies USA, Inc .Arcadia, CA 91006

Step 47000 M odel Perspective04:24:56 Sun O ct 20 2002

Center: X: 9.062e+000 Y: 4.610e+001 Z: -1.201e+005

Rotation: X: 3.862 Y: 359.989 Z: 359.828

Dis t: 1.991e+004 M ag.: 13.7Ang.: 22.500

Job T itle : KW 1: 600 ' W ellbore M odeling

View T itle :

B lo c k G ro u pBase_PipeG ravelCas ingcem entShaleSandScreenCoupling

Modeling a 600′ Compacting Modeling a 600′ Compacting SectionSection

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FLAC3D 2.10

Terralog Technologies US A , Inc .A rcadia, CA 91006

S tep 205616 M odel P erspec tive11:40:46 Fri Nov 01 2002

Center: X : 1.241e+ 001 Y : 4.610e+ 001 Z: -1.202e+ 005

Rotation: X : 0.000 Y : 0.000 Z: 0.000

Dis t: 1.991e+ 004 M ag.: 369A ng.: 22.500

Job T itle : KW 1: 600 ' W ellbore M odeling

View T itle :

C ontour o f es _p las t ic M agfac = 0.000e+ 000 Gradient Calculation

7.5988e-010 to 5.0000e-003 5.0000e-003 to 1.0000e-002 1.0000e-002 to 1.5000e-002 1.5000e-002 to 2.0000e-002 2.0000e-002 to 2.5000e-002 2.5000e-002 to 3.0000e-002 3.0000e-002 to 3.5000e-002 3.5000e-002 to 4.0000e-002 4.0000e-002 to 4.5000e-002 4.5000e-002 to 5.0000e-002 5.0000e-002 to 5.5000e-002 5.5000e-002 to 6.0000e-002 6.0000e-002 to 6.5000e-002 6.5000e-002 to 6.7369e-002

Interval = 5.0e-003

Elastoplastic Zone GenerationElastoplastic Zone Generation

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Well PerformanceComparison Tool

CommonDesign AnalysisDatabase

AnalyticalAnalysisTool

Simple Decision

ProprietaryReservoir Analysis

ProprietaryWell Damage Analysis

ProprietaryDecisionAnalysis

Well PerformanceComparison Tool

Well PerformanceComparison

Tool

CommonDesign AnalysisDatabase

CommonDesign Analysis

Database

AnalyticalAnalysisTool

AnalyticalAnalysis

Tool

Simple Decision

Simple Decision

Analysis Tool



Reservoir

Estimate

Reservoir Deformation

Estimate

Well Deformation

Limits


ProprietaryWell Damage

Analysis



Optimum Well Design

The Design PathsThe Design Paths

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Optimum Well Design

Well Damage Analysis and Design Process





Optimum Well Design

Well Damage Analysis and Design Process

Combining the ElementsCombining the Elements

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An economic decision tree model can be applied to compare the costs and benefits of alternative well designs, while taking into account the inherent uncertainties in geomechanical model input data, well damage location, and effectiveness of various mitigation strategies.

In some instances the appropriate action is not to change completion design and simply accept and account for damage risk in economic projections.

Input Parameters P10 P50 P90

Formation Depth (ft below mudline) 1.40E+04 1.60E+04 1.80E+04

Areal Extent (acres) 2.00E+03 4.00E+03 6.00E+03

Gross Formation Thk. (ft) 3.00E+02 4.00E+02 6.00E+02

Net Sand/Gross ratio 6.50E-01 7.50E-01 9.00E-01

Sand Compaction (1/psi) 1.10E-06 3.30E-06 6.60E-06

Shale Compaction (1/psi) 1.00E-07 3.30E-07 1.00E-06

Poisson's Ratio 1.50E-01 2.50E-01 3.50E-01

Pressure Drawdown (psi) 2.00E+03 2.50E+03 3.00E+03

Well Inclination (deg from vertical) 1.00E+01 2.00E+01 4.50E+01

Overburden Young's Modulus (psi) 5.00E+05 1.00E+06 1.50E+06

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Net Sand/Gross ratio

Sand Compaction (1/psi)

Shale Compaction (1/psi)

Pressure Drawdown (psi)

Well Inclination (deg fromvertical)

Percent Maximum Axial Strain

Decision Analysis TechniquesDecision Analysis Techniques

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Model Input ParametersCompletion #1 Cost 180000 dollars Initial damage risk 0.10Completion #2 Cost 195000 dollars Risk reduction 0.50Completion #3 Cost 220000 dollars Risk reduction 0.75Avg Daily Production 30 bbls/dayPrice per barrel 20 dollarsReplacement Time 60 daysAbandonment Cost 20,000 dollars

Damage Cost Risk Cost

Replacement Well 180000 $18,000Well Damage Risk 0.1 Lost Production 36000 $3,600

Abandonment Cost 20,000 $2,000Completion #1 Added Cost 0 $0

Total Risk Cost $23,600

Replacement Well 195000 $9,750Well Damage Risk 0.05 Lost Production 36000 $1,800

Abandonment Cost 20,000 $1,000Completion #2 Added Cost 15000 $15,000


Replacement Well 220000 $5,500Well Damage Risk 0.025 Lost Production 36000 $900

Abandonment Cost 20,000 $500Completion #2 Added Cost 40000 $40,000


Simple Decision Analysis ExampleSimple Decision Analysis Example

Probability x Consequences = Probability x Consequences = Risk CostRisk Cost

Example of Decision AnalysisExample of Decision Analysis

compaction geomechanics mbdci compaction geomechanics: mechanisms, screening maurice b. dusseault

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