Transcript Flow equations in various cases

Compaction Geomechanics
Compaction Geomechanics:
Mechanisms, Screening
Maurice B. Dusseault
Compaction 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


Compaction geomechanics is fundamental
The Case of Ekofisk




3000 m deep
7 km wide
Very thick reservoir
High porosity chalks

Compaction Geomechanics




 from 25 to 50%
Overpressured by 1.7
Large drawdowns are
feasible – large Δσ′v
Large compactions…
Would we plan for
these nowadays?
EKOFISK
Reservoir Compaction


Compaction Geomechanics

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…
Major 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
Compaction Geomechanics



Study the cost-benefit gain to the company
Implement mitigation measures in advance
Screening Strategies
Screening and Analysis
Compaction Geomechanics
First-order screening
(geology + cases)
Geomechanics assessment
-geophysical logs
-petrophysical evaluation
-stress history
Impact assessment
-well modeling
-reservoir modeling
Mitigation options
-pressure maintenance
-facilities redesign
-production strategies
Risk assessment
-second-order screening
-production predictions
Cost-benefit analysis
Cost of options
$ - ¥ - Bs
-Monitoring cases
Decision making
-experience base
-learning and teaching
Is compaction
beneficial?
Compaction 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
Compaction Geomechanics



Thus, compaction moves out
from production wells, arching
delays full z development
This can be modeled quite well
z
ij, p
Cc (E, )
z ~ Ccv´
Compaction Effects II


Compaction Geomechanics


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
Reservoir Compaction

Delay of compaction always occurs in early
time, before p zones intersect (aspect ratio)
Compaction Geomechanics
initially
after some Q
p region
no p yet
z Delay Through Arching
full subsidence response delayed
in this phase, v’
is not equal to p
Compaction Geomechanics
drawdown
zones
arching occurs
until drawdown zones
interact at the
reservoir scale
compaction impeded
overburden stresses “flow” around the p, V zone
Full Compaction
Compaction Geomechanics
full
subsidence
develops
full
compaction
triggered
when zones meet,
arching is destroyed,
full compaction occurs
stresses now “flow” without arching around zones
Reservoir Compaction

Compaction sustains production, and can
change the production profile substantially
Q - total field
Compaction Geomechanics
oil lost?
predicted, assuming v = p
actual Q with
delayed z
predicted life
actual life
economic
cutoff Q
time
Negative Effects of ij

Productivity can decline with an increase in
ij , both in normal and shear stresses
Fracture aperture diminution (+++)
 Pore throat constriction (+)

Compaction Geomechanics




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
Positive Effects of ij

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
Compaction Geomechanics



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
Mechanisms I (Sandstones)



Compaction Geomechanics



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 liquiddominated reservoirs, may cause subsidence
Also, a direct fluid expulsion occurs (V)
Water can be expelled from adjacent shales
Elastic Compression



Compaction Geomechanics

 leads to increased contact forces, f´n
The contact area increases, porosity drops
This is a function of compressibility
If elastic, V is recoverable (-V = +V)
fn
Ap
p
fi
E,
Ap - A
fn
Inelastic Compression



Compaction Geomechanics

 leads to increased contact forces, f´n
Grain rearrangement takes place,  drops
Perhaps a bit of grain contact crushing
In high  sands, this is an irrecoverable V
fi
fn
Ap
p
E,
Ap - A
fn
Elastic and Inelastic Strain
porosity
elastic behavior
inelastic
behavior
Injection = -
compaction curve
irrecoverable
compaction
Compaction Geomechanics
Depletion = +
elastic behavior
rebound curve
1 MPa
10 MPa
100 MPa
log(v)
Mechanisms II (Sandstones)


Compaction Geomechanics


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
Processes and Compaction
porosity
0.30
elastic compression at low 
grain rearrangement
at intermediate 
elastic
recovery
Compaction Geomechanics
irrecoverable
compaction
0.25
elastoplastic grain
contact behavior
rebound
curve
grain crushing
at high 

0.20
1 MPa
low 
10 MPa
100 MPa
high 
log(v)
Mechanisms, Chalk (I)


Compaction Geomechanics



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
Why Does Chalk Collapse?
Hollow, weak grains (coccoliths)
Compaction Geomechanics
Weak cementation
(dog-tooth calcite)
Weak, cleavable grain
mineral (CaCO3)
Mechanisms, Chalk (II)



Compaction Geomechanics


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
North Sea Chalk Collapse
porosity
threshold stress
Compaction Geomechanics
irrecoverable strain
collapse of fabric
compression curve
rebound curves
1 MPa
10 MPa
100 MPa
log(v’)
Geological History!


Compaction Geomechanics



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
Cementation, Compaction
porosity
apparent threshold ’
Compaction Geomechanics
normal
densification
cementation effect
collapse if cement
is ruptured
“virgin”
compression curve
stiff
response
log(v’)
Diagenetic Densification
porosity
apparent threshold ’
Compaction Geomechanics
diagenetic
porosity loss
@ constant ’
“virgin”
compression curve
present state
stiff
response
log(v’)
Precompaction Effect
porosity
apparent threshold ’
Compaction Geomechanics
“virgin”
compression curve
present state
stiff
response
log(v’)
Deep burial followed by uplift and erosion lead to precompaction
“Threshold” Drawdown


Usually, some threshold drawdown must
occur before significant compaction starts
There are three effects responsible:
Compaction Geomechanics
•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)


These are hard to quantify without careful
geological studies and laboratory testing
Short-term well testing can be misleading!
Cementation, Diagenesis
stresses
Compaction Geomechanics
time
temperature
chemistry
initial state,
35% porosity
porosity
reduction
Both solution and cementation reduce porosity, increase stiffness
pressure
solution,
25-32%
cementation,
25-32%
Additional Mechanisms


Compaction Geomechanics




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?
Fracture Aperture, k



Fracture aperture is sensitive to n
Permeability is highly sensitive to aperture
Shear displacement and asperity crushing
can develop with 
Compaction Geomechanics
n
effective aperture
p
asperities
It
p + p
appears that a homogeneous constitutive macroscopic law is required for good
predictions in analysis
Fracture Permeability Loss


Compaction Geomechanics


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
Grain Crushing and k


Compaction Geomechanics

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 crushed, quartz intact
feldspar
quartz
Depletion Effect on h
wellbore
h concentration
Compaction Geomechanics
far-field stresses
h stress trajectories
final
h
Z
initial
h
h along
wellbore
Zone after production (p)
Operational consequences:
-low pfrac in reservoir
-higher pfrac above reservoir
Shear, Fracture Opening
v
Zone of pressure
decline, -p
Compaction Geomechanics
h
Pressure decline leads to an increase in the shear stress
This leads to shearing, which causes fractures and fissures to open
This leads to increases in permeability, better reservoir drainage

Pore Blockage Mechanisms
Geochemical effects!
Compaction Geomechanics
Fines migration
can block pores
Mineral
deposition
Most Sensitive Cases


Compaction Geomechanics




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?
Porosity vs Depth
0
0.25
0.50
0.75
1.0
porosity
sands &
sandstones
mud
clay & shale,
“normal” line
clay
Compaction Geomechanics
mudstone
effect of
overpressures
on porosity
4-6 km
depth
shale
The specific details of
these relationships are
a function of basin age,
diagenesis, heat flow ...
Compaction Geomechanics
Subsidence MacroMechanics
Subsidence Bowl
(Vertical scale is greatly exaggerated)
subsidence bowl, L
compression
Compaction Geomechanics
extension

zmax
compaction, T
depth, Z
width, W
Subsidence Magnitude



Compaction Geomechanics




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
Casing Impairment

Compaction Geomechanics

Either loss of pressure integrity, or
excessive deformations (dogleg, buckling)
Problem in massively compacting reservoirs
•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


These are vexing and difficult problems
More compliant casing and cement?
Casing Shearing!!

Compaction Geomechanics

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.

Compaction Geomechanics
Measuring Compaction
Measuring Compaction

In the reservoir…

Compaction Geomechanics

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…
Radioactive Bullets



Compaction Geomechanics



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
Casing Collar Logs



Compaction Geomechanics




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
Borehole Extensometers



Compaction Geomechanics




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 3
wire 2
wire 1
sheaves
W
anchor 3
casing
anchor 2
anchor 1
Other Borehole Methods


Compaction Geomechanics



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
Surface z Measurements


Compaction Geomechanics



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?
GPS - Fixed Monuments Visits
Compaction Geomechanics
Antenna
Monument
InSAR - IOL - Cold Lake
+285 mm
+200
mega-row
CSS
-210
Compaction Geomechanics
+100
+260
+130 mm
-165
Vertical
displacements
(mm)
over 86 days
heave
km
subsidence
mod. Stancliffe & van der Kooij, AAPG 2001
Belridge Field, CA - Subsidence
Compaction Geomechanics
30-40 cm per year
Compaction Geomechanics
Belridge Rate - Δz/Δt
0.0 in./yr
12.5
25.0
over 18 months
Shell Oil Canada – Peace River
Surface
uplift / tilt
data
Multilateral
CSS
Compaction Geomechanics
reservoir inversion grid
with 50x50m grid cells
ref. Nickle’s New Technology Magazine, Jan-Feb 2005
NAM, Netherlands - Ameland
Compaction Geomechanics
- Gas Field
- 3350m reservoir
depth
- 22cm subsidence
Measurement Parameters


Compaction Geomechanics




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.
Compaction Analysis


Compaction Geomechanics




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
Compaction Geomechanics
Discussion of Some Case Histories
Case Histories




Compaction Geomechanics



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
Ekofisk (I)



Compaction Geomechanics




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)
Ekofisk (II)



Compaction Geomechanics





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
Ekofisk (III)
2.3 billion $


Compaction Geomechanics



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
Ekofisk (IV)



Compaction Geomechanics




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
Ekofisk Continues ....



Compaction Geomechanics




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
Maracaibo, Venezuela (I)




Compaction Geomechanics



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
Location
Compaction Geomechanics
MARACAIBO
Lago de
Maracaibo
N
Maracaibo Setting

MARACAIBO

Subsidence area
N
CABIMAS
Lago de
Maracaibo
TIA JUANA
LAGUNILLAS
I
Compaction Geomechanics
IX
X
MENE
GRANDE
III
VIII
IV
VI
V

BACHAQUERO
II
XIV XII

V

XI
VII
XIII
Central
development
areas

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)
Maracaibo, Venezuela (II)


Compaction Geomechanics



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
Ravenna, Niigata…


Compaction Geomechanics



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
Wilmington, California (I)



Compaction Geomechanics




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
Wilmington. California


Compaction Geomechanics



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
Wilmington, California (II)


Compaction Geomechanics



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
Little-Compacting Cases

Compaction Geomechanics



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
Surface Heave from ΔT & Δp
Surface heave – Δz –
above a SAGD project
Compaction Geomechanics
320 mm +Δz
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, ,
How Much Compaction?

Compaction Geomechanics

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
Will the Reservoir Compact?

Compaction Geomechanics



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.)
Prediction by Comparison

Compaction Geomechanics

Other case histories are carefully studied
Quantitative comparisons are 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?

A probability estimate is made
Reservoir & Overburden



Compaction Geomechanics



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)
Geometry Effects


Compaction Geomechanics




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
Modeling Compaction

Compaction Geomechanics


Best approach is a fully coupled flowgeomechanics 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”
Coupling 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
Compaction Geomechanics


Also gives overburden shear stresses changes
Compaction Geomechanics
Stress Trajectories, -V Case
Overburden Arching

Delay of compaction always occurs in early
time, before p zones intersect (aspect ratio*)
Compaction Geomechanics
initially
stress arching
after some Q
p region
no p yet
*aspect ratio is W/H; if W>~3H, arching is disappearing
Reduced Lateral Stresses
wellbore
h concentration
Compaction Geomechanics
far-field stresses
h stress trajectories
final
h
Z
initial
h
h along
wellbore
Zone of high drawdown
Operational consequences:
low pfrac in reservoir
higher pfrac above reservoir
Prediction of ij


Compaction Geomechanics

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!
Coupled Modeling


Compaction Geomechanics

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...
Iterative Coupled Models




Compaction Geomechanics




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)
Mitigating 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
Compaction Geomechanics




Avoidance & management require modeling
Under-reaming & no cement delays distress
Better sealing cements to reduce p migration
Under-Reaming to Reduce Shear
sand stratum
Compaction Geomechanics
interface slip
casing cemented, but not
in the under-reamed zone
under-reamed zone
bedding plane slip
shale stratum
Under-Reaming of Hole
100
Unprotected Wells
(155 total)
15" Under-ream
(5 total)
26" Under-ream
(147 total)
Compaction Geomechanics
Percent of Total
90
80
70
60
Wilmington
50
40
30
20
10
0
Undamaged
Damaged
Failed
Risk Mitigation Approaches

Pressure maintenance
Water injection
 Gas injection
 CO2 sequestration, and use as an enhanced oil
recovery approach
Compaction Geomechanics





Structural design of platforms
Judicious placement of wellbore to reduce
the incidence of casing shear
Special completion techniques
Monitor, monitor, monitor…
Modeling Compaction

Compaction Geomechanics


Best approach is a fully coupled flowgeomechanics 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”
Prediction of ij


Compaction Geomechanics

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!
Coupled Modeling


Compaction Geomechanics

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...
Iterative Coupled Models




Compaction Geomechanics




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)
Analyzing Special Compaction
Joints
Telescoping
Joint
Telescoping
joint
9,743’
9,818’
210 ft
Compaction Geomechanics
9980’
Screen, basepipe,
Screen, base
-pipe, couplings
couplings
Sump
Packer
Sump
packer
450 ft
FracPacPacker
10,192’
for  = 0.2,
Cp = 910-6 psi-1
Δp = 2600 psi
ΔH/H = 1%
Compaction Geomechanics
Mathematical Modeling of Strains
Modeling a 600′ Compacting Section
FLAC3D 2.10
Step 47000 Model Perspective
04:24:56 Sun Oct 20 2002
Center:
X: 9.062e+000
Y: 4.610e+001
Z: -1.201e+005
Dist: 1.991e+004
Rotation:
X: 3.862
Y: 359.989
Z: 359.828
Mag.: 13.7
Ang.: 22.500
Compaction Geomechanics
Block Group
Base_Pipe
Gravel
Casing
cement
Shale
Sand
Screen
Coupling
Terralog Technologies USA, Inc.
Arcadia, CA 91006
Job Title: KW1: 600' Wellbore Modeling
View Title:
Elastoplastic Zone Generation
FLAC3D 2.10
Step 205616 Model Perspective
11:40:46 Fri Nov 01 2002
Center:
X: 1.241e+001
Y: 4.610e+001
Z: -1.202e+005
Dist: 1.991e+004
Rotation:
X: 0.000
Y: 0.000
Z: 0.000
Mag.:
369
Ang.: 22.500
Compaction Geomechanics
Contour of es_plastic
Magfac = 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
Terralog Technologies USA, Inc.
Arcadia, CA 91006
Job Title: KW1: 600' Wellbore Modeling
View Title:
The Design Paths
Compaction Geomechanics
Reservoir
Deformation
Analytical
Analytical Estimate
Analysis
Tool
Tool
Well
Well
Performance
Performance
Comparison
Comparison
Tool
Tool
Well
Deformation
Limits
Common
Common
Design
Design
Analysis
Analysis
Database
Simple
Simple
Decision
Decision
Analysis
Tool
Proprietary
Proprietary
Reservoir
Analysis
Analysis
Proprietary
Proprietary
Decision
Decision
Analysis
Analysis
Optimum Well Design
Proprietary
Proprietary
Well
WellDamage
Damage
Analysis
Analysis
Combining the Elements
Compaction Geomechanics
Well Damage Analysis
and Design Process
Proprietary
Reservoir
Analysis
Proprietary
Well Damage
Analysis
Optimum Well Design
Decision Analysis Techniques
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.
Compaction Geomechanics
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
Net Sand/Gross ratio
Sand Compaction (1/psi)
Shale Compaction (1/psi)
Pressure Drawdown (psi)
Well Inclination (deg from
vertical)
0%
10%
20%
30%
40%
50%
60%
Percent Maximum Axial Strain
70%
80%
90%
100%
Example of Decision Analysis
Model Input Parameters
Completion #1 Cost
Completion #2 Cost
Completion #3 Cost
Avg Daily Production
Price per barrel
Replacement Time
Abandonment Cost
180000
195000
220000
30
20
60
20,000
dollars
dollars
dollars
bbls/day
dollars
days
dollars
Well Damage Risk
Completion #1 Added Cost
Initial damage risk
Risk reduction
Risk reduction
0.10
0.50
0.75
Probability x Consequences =
Risk Cost
0.1
Damage Cost
Risk Cost
180000
36000
20,000
$18,000
$3,600
$2,000
$0
Replacement Well
Lost Production
Abandonment Cost
0
Compaction Geomechanics
Total Risk Cost
Well Damage Risk
Completion #2 Added Cost
0.05
Replacement Well
Lost Production
Abandonment Cost
195000
36000
20,000
15000
Total Risk Cost
Well Damage Risk
Completion #2 Added Cost
0.025
Replacement Well
Lost Production
Abandonment Cost
220000
36000
20,000
40000
Total Risk Cost
Simple Decision Analysis Example
$23,600
$9,750
$1,800
$1,000
$15,000
$27,550
$5,500
$900
$500
$40,000
$46,900