Compaction Geomechanics MBDCI Compaction Geomechanics: Mechanisms, Screening Maurice B. Dusseault.
-
Upload
lucas-golden -
Category
Documents
-
view
265 -
download
16
Transcript of Compaction Geomechanics MBDCI Compaction Geomechanics: Mechanisms, Screening Maurice B. Dusseault.
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Compaction Geomechanics: Compaction Geomechanics: Mechanisms, ScreeningMechanisms, Screening
Maurice B. Dusseault
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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…
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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?
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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´
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Reservoir 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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Full CompactionFull Compaction
stresses now “flow” without arching around zones
fullsubsidencedevelops full
compactiontriggered
when zones meet,arching is destroyed,
full compaction occurs
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Reservoir CompactionReservoir Compaction
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?
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Why Does Chalk Collapse?Why Does Chalk Collapse?
Hollow, weak grains (coccoliths)
Weak, cleavable grain mineral (CaCO3)
Weak cementation (dog-tooth calcite)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Cementation, CompactionCementation, Compaction
porosity
log(v’)
“virgin”compression curve
apparent threshold’
stiffresponse
collapse if cementis ruptured
normaldensificationcementation effect
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Diagenetic DensificationDiagenetic Densification
porosity
log(v’)
diageneticporosity loss
@ constant ’
stiffresponse
“virgin”compression curve
present state
apparent threshold ’
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Precompaction EffectPrecompaction Effect
porosity
log(v’)
present state
“virgin”compression curve
apparent threshold ’
stiffresponse
Deep burial followed by uplift and erosion lead to precompaction
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
““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)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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?
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Pore Blockage MechanismsPore Blockage Mechanisms
Mineral deposition
Fines migration can block pores
Geochemical effects!
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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?
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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 ...
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Subsidence MacroMechanicsSubsidence MacroMechanics
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Subsidence BowlSubsidence Bowl
subsidence bowl, L
depth, Z
width, W
extension
compression
compaction, T
zmax
(Vertical scale is greatly exaggerated)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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.
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Measuring CompactionMeasuring Compaction
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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…
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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?
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
GPS - Fixed Monuments VisitsGPS - Fixed Monuments Visits
Antenna
Monument
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Belridge FieldBelridge Field, CA, CA - Subsidence- Subsidence
30-40 cm per year
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Belridge Rate - Belridge Rate - ΔΔz/z/ΔΔtt
over 18 months
0.0 in./yr
12.5
25.0
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
NAM, NetherlandsNAM, Netherlands - Ameland - Ameland
- Gas Field
- 3350m reservoir depth
- 22cm subsidence
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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.
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Discussion of Some Case HistoriesDiscussion of Some Case Histories
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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 $
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
LocationLocation
MARACAIBO
Lago deMaracaibo
N
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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.)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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?
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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”
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Stress Trajectories, -Stress Trajectories, -V CaseV Case
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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!
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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...
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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…
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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”
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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!
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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...
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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)
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Screen, base-pipe, couplings
9,743’
9,818’
9980’
10,192’Sump Packer
Screen, base-pipe, couplings
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
Mathematical Modeling of StrainsMathematical Modeling of Strains
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
ProprietaryReservoir Analysis
ProprietaryReservoir Analysis
Reservoir
Estimate
Reservoir Deformation
Estimate
Well Deformation
Limits
ProprietaryWell Damage Analysis
ProprietaryWell Damage
Analysis
ProprietaryDecisionAnalysis
ProprietaryDecisionAnalysis
Optimum Well Design
The Design PathsThe Design Paths
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
ProprietaryReservoir Analysis
ProprietaryWell Damage Analysis
Optimum Well Design
Well Damage Analysis and Design Process
ProprietaryReservoir Analysis
ProprietaryReservoir Analysis
ProprietaryWell Damage Analysis
ProprietaryWell Damage Analysis
Optimum Well Design
Well Damage Analysis and Design Process
Combining the ElementsCombining the Elements
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Co
mp
acti
on
Geo
mec
han
ics
MBDCI
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
Total Risk Cost $27,550
Replacement Well 220000 $5,500Well Damage Risk 0.025 Lost Production 36000 $900
Abandonment Cost 20,000 $500Completion #2 Added Cost 40000 $40,000
Total Risk Cost $46,900
Simple Decision Analysis ExampleSimple Decision Analysis Example
Probability x Consequences = Probability x Consequences = Risk CostRisk Cost
Example of Decision AnalysisExample of Decision Analysis