Compaction Geomechanics: Mechanisms, Screening

1
Compaction Geomechanics Mechanisms, Screening

• Maurice B. Dusseault

2
Compaction as a Drive

• Compaction occurs whenever the net stress
  increases (???????)
• Magnitude depends of the rock stiffness, fabric
  (some rocks have a quasi-stable f)
• 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

3
The 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 ?s'v
• Large compactions
• Would we plan for these nowadays?

EKOFISK
4
Reservoir 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

5
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
• Study the cost-benefit gain to the company
• Implement mitigation measures in advance

6
Screening Strategies
Screening and Analysis
First-order screening
(geology cases)
Geomechanics assessment
Impact assessment
Mitigation options
-geophysical logs
-well modeling
-pressure maintenance
-petrophysical evaluation
-reservoir modeling
-facilities redesign
-stress history
-production strategies
Risk assessment
Cost-benefit analysis
Cost of options - - Bs
-second-order screening
-production predictions
Is compaction beneficial?
Decision making
-Monitoring cases
-experience base
-learning and teaching
7
Compaction Effects I

• Compaction ƒ(??ij, E, ?, ...)
• Is ?p always ??ij?
• No, compaction is not uniform
• Dp 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 Cc??v
8
Compaction 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

9
Reservoir Compaction

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

initially
after some Q
?p region
no ?p yet
10
Dz Delay Through Arching
full subsidence response delayed
arching occurs until drawdown zones interact at
the reservoir scale
in this phase, ??v is not equal to ?p
drawdown zones
compaction impeded
overburden stresses flow around the ?p, ?V zone
11
Full Compaction
full subsidence develops
when zones meet, arching is destroyed, full
compaction occurs
full compaction triggered
stresses now flow without arching around zones
12
Reservoir Compaction

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

oil lost?
predicted, assuming Ds?v Dp
actual Q with delayed Dz
predicted life
actual life
Q - total field
economic cutoff Q
time
13
Negative Effects of ???ij

• 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

14
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
• In high f Chalk, a volume change as much as 10
  can take place, all oil production
• Compaction of high f shales can also expel water
  into the reservoir, displacing more oil

15
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

16
Elastic Compression

• ??? leads to increased contact forces, fn
• The contact area increases, porosity drops
• This is a function of compressibility
• If elastic, DV is recoverable (-DV DV)

f?i
f?n
E,?
Ap
Ap - A
?p
?f?n
17
Inelastic Compression

• ??? leads to increased contact forces, fn
• Grain rearrangement takes place, f drops
• Perhaps a bit of grain contact crushing
• In high f sands, this is an irrecoverable DV

f?i
f?n
E,?
Ap
Ap - A
?p
?f?n
18
Elastic and Inelastic Strain
porosity
Depletion Ds? Injection -Ds?
elastic behavior
inelastic behavior
compaction curve
irrecoverable compaction
elastic behavior
rebound curve
1 MPa
10 MPa
100 MPa
log(s?v)
19
Mechanisms II (Sandstones)

• For small Dp, grain rearrangement is most
  important ?V not recoverable. Also,
• Contacts compress elastically, recoverable
• At intermediate Dp, grain contacts deform
  elastoplastically, strain is not recoverable
• At high Dp (high Ds?), grain splitting, crushing,
  and even creep occurs, especially in lithic and
  arkosic sandstones

20
Processes and Compaction
porosity
elastic compression at low s?
grain rearrangement at intermediate s?
elastic recovery
irrecoverable compaction
elastoplastic grain contact behavior
rebound curve
grain crushing at high s?
Ds?
1 MPa
10 MPa
100 MPa
log(s?v)
high ??
21
Mechanisms, Chalk (I)

• North Sea Chalks (and a few other materials)
  exist in a high-porosity state (gt35-40)
• This state is quasi-stable and exists because
  of cementation between grains
• If grains crush or cement ruptures, massive
  compaction occurs (gt10 m at Ekofisk)
• This is triggered by the increase in ??v, and
  also by increased stress difference (?1 - ?3)
• Shear destroys cement, triggers compaction

22
Why Does Chalk Collapse?
Hollow, weak grains (coccoliths)
Weak cementation (dog-tooth calcite)
Weak, cleavable grain mineral (CaCO3)
23
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

24
North Sea Chalk Collapse
porosity
threshold stress
irrecoverable strain
collapse of fabric
compression curve
rebound curves
1 MPa
10 MPa
100 MPa
log(sv)
25
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 s? value
• Overcompaction (deep burial history) can make the
  rock stiffer, little compaction
• Geological history is a vital factor

26
Cementation, Compaction
porosity
apparent threshold Ds
normal densification
collapse if cement is ruptured
cementation effect
virgin compression curve
stiff response
log(sv)
27
Diagenetic Densification
porosity
apparent threshold Ds
diagenetic porosity loss _at_ constant s
virgin compression curve
present state
log(sv)
28
Precompaction Effect
porosity
apparent threshold Ds
virgin compression curve
present state
stiff response
log(sv)
Deep burial followed by uplift and erosion lead
to precompaction
29
Threshold 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 Dp may not yet be at the reservoir scale
  (arching)

30
Cementation, Diagenesis
stresses
pressure solution, 25-32
time temperature chemistry
initial state, 35 porosity
cementation, 25-32
Both solution and cementation reduce porosity,
increase stiffness
31
Additional 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 t
• Increase in shear stress usually causes k?
• Compaction can release fines from strata
• Are there other effects in your reservoirs?

32
Fracture Aperture, ???, ?k

• Fracture aperture is sensitive to ??n
• Permeability is highly sensitive to aperture
• Shear displacement and asperity crushing can
  develop with ??

???n

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

effective aperture
p
p ?p
asperities
33
Fracture 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

34
Grain Crushing and ?k

• Depletion or differential volumetric strain
  causes high ??, high fn on individual grains
• Weak (lithic) or cleavable (felspathic) grains
  crush and fragment
• Pore throats then become smaller or blocked by
  fragments, k drops

35
Depletion Effect on sh
wellbore
?h stress trajectories
?h concentration
far-field stresses
?h along wellbore
Zone after production (Dp)
final ?h
Operational consequences -low pfrac in
reservoir -higher pfrac above reservoir
initial ?h
Z
36
Shear, Fracture Opening
Zone of pressure decline, -Dp
s?v
s?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

37
Pore Blockage Mechanisms
Geochemical effects!
Fines migration can block pores
Mineral deposition
38
Most 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?

39
Porosity vs Depth
porosity
sands sandstones
mud
clay
clay shale, normal line
mud- stone
shale
The specific details of these relationships are a
function of basin age, diagenesis, heat flow ...
effect of overpressures on porosity
4-6 km
depth
40
Subsidence MacroMechanics
41
Subsidence Bowl
(Vertical scale is greatly exaggerated)
subsidence bowl, L
compression
extension
?zmax
?
compaction, T
depth, Z
width, W
42
Subsidence Magnitude

• If W lt Z, arching, little subsidence (lt25 T)
• If Z lt W lt 2Z, partial arching (25-75 of T)
• If W gt 2Z, minimal arching (gt75 of T)
• Bowl width L W 2Zsin?
• If W gt 2Z, ?zmax approaches T
• ? (angle of draw) usually 25 to 45 degrees
• In cases of complex geometry and stacked
  reservoirs, numerical approaches required

43
Casing 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

44
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.

45
Measuring Compaction
46
Measuring 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

47
Radioactive 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

48
Casing 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, cant run the log

49
Borehole 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 cant be run in the hole
• Other instruments installed

wire 3
wire 1
sheaves
wire 2
W
anchor 3
casing
anchor 2
anchor 1
50
Other 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

51
Surface Dz 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?

52
GPS - Fixed Monuments Visits
Antenna
Monument
53
InSAR - IOL - Cold Lake

• mega-row
• CSS

285 mm
200
-210
100
Vertical displacements (mm) over 86 days
260
130 mm
-165
km
heave
subsidence
mod. Stancliffe van der Kooij, AAPG 2001
54
Belridge Field, CA - Subsidence
30-40 cm per year
55
Belridge Rate - ?z/?t

• over 18 months

56
Shell Oil Canada Peace River
Multi-lateral CSS
Surface uplift / tilt data
reservoir inversion grid with 50x50m grid cells
ref. Nickles New Technology Magazine, Jan-Feb
2005
57
NAM, Netherlands - Ameland

• - Gas Field
• - 3350m reservoir depth
• - 22cm subsidence

58
Measurement 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.

59
Compaction 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

60
Discussion of Some Case Histories
61
Case 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

62
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 gt 3D)

63
Ekofisk (II)

• Lengthy well tests failed to detect compaction
• Subsidence assumed minor because of depth
• Casing shearing became a problem in 1980s
• 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 90s

64
Ekofisk (III)
2.3 billion

• Redevelopment decision in 1994, S 6.4 m
• Pressure maintenance tried in 1980s, 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

65
Ekofisk (IV)

• However, it is a fabulous reservoir!
• 100 of initial predicted production was
  surpassed in early 1990s!
• Life predicted to 2011, extended 30 years
• Good compaction drive continues (Dz gt 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

66
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

67
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

68
Location
MARACAIBO
N
Lago de
Maracaibo
69
Maracaibo Setting

• Sandstone reservoirs
• Late Cretaceous to Tertiary
• Normally pressured
• High porosity for the present burial depth
• Clay cement usually
• Asphaltene present in heavier oils (lt30API)

MARACAIBO
Subsidence area
CABIMAS
TIA JUANA
LAGUNILLAS
I
BACHAQUERO
II
XIV
XII
X
IX
MENE GRANDE
III
VIII
IV
VI
V
XI
V
VII
XIII
Central development areas
70
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

71
Ravenna, Niigata

• Italy, Japan, some other places
• Intermediate depth gas sands, only water present
  as a second phase, no oil
• High porosity (gt30), 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

72
Wilmington, California (I)

• Intermediate depth, many stacked reservoirs
• Great aggregate producing thickness
• UCSS, porosity gt 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

73
Wilmington. California

• Bowl shaped
• Shear of casings occurred mainly on the shoulders
  of the subsidence bowl
• Few shears in the middle, where Dz greatest
• Few on flanks
• Associated earthquakes

74
Wilmington, California (II)

• Sudsidence reached 9.5 m
• Minor earthquakes triggered, and in one case, gt
  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 1960s

75
Little-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

76
Surface Heave from ?T ?p
Surface heave ?z above a SAGD project
320 mm ?z
1 km
Surface heaves cannot be explained by ?T ?p
alone there must be shear dilation taking place.
Therefore, there are massive changes in the
reservoir properties k, Cc, ?,
77
How Much Compaction?

• Depends on compressibility, Z, p, Dp, f
• Qualitative screening criteria (geology!)

• If porosity gt 25 (gt 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 Dp 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 gt depth to reservoir (no
  arching)
• Other criteria are probably of little relevance

78
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.)

79
Prediction 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?

80
Reservoir Overburden

• Compaction delay due to reservoir ? arching
• Later, arching destroyed, subsidence starts
• If WgtZ, 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)

81
Geometry Effects

• Everything depends on aspect ratios (W,L,Z)
• A deep narrow sand will cause no ?z
• A wide reservoir (W gt 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

82
Modeling 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

83
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
• Also gives overburden shear stresses changes

84
Stress Trajectories, -?V Case
85
Overburden Arching

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

stress arching
initially
after some Q
?p region
aspect ratio is W/H if Wgt3H, arching is
disappearing
86
Reduced Lateral Stresses
wellbore
?h stress trajectories
?h concentration
far-field stresses
?h along wellbore
Zone of high drawdown
final ?h
Operational consequences low pfrac in
reservoir higher pfrac above reservoir
initial ?h
Z
87
Prediction of ???ij

• 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!

88
Coupled 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...

89
Iterative Coupled Models

• Pressures are solved for a single time step
• ?p pi1 - 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)

90
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
• Avoidance management require modeling
• Under-reaming no cement delays distress
• Better sealing cements to reduce p migration

91
Under-Reaming to Reduce Shear
casing cemented, but not in the under-reamed zone
sand stratum
interface slip
under-reamed zone
casing
bedding plane slip
shale stratum
92
Under-Reaming of Hole
100
Wilmington
90
80
70
Percent of Total
60
50
40
30
20
10
0
Undamaged
Damaged
Failed
93
Risk 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

94
Modeling 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

95
Prediction of ???ij

• 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!

96
Coupled 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...

97
Iterative Coupled Models

• Pressures are solved for a single time step
• ?p pi1 - 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)

98
Analyzing Special Compaction Joints
9,743
9,743
Telescoping joint
9,818
9,818
Telescoping Joint
450 ft
450 ft
450 ft
9980
9980
for ? 0.2, Cp 9?10-6 psi-1 ?p 2600 psi
?H/H 1
Screen, basepipe, couplings
210 ft
210 ft
210 ft
Screen, base
-
pipe, couplings
Screen, base
-
pipe, couplings
10,192
10,192
Sump packer
Sump Packer
Sump Packer
99
Mathematical Modeling of Strains
100
Modeling a 600' Compacting Section
101
Elastoplastic Zone Generation
102
The Design Paths
Common
Well
Well
Common
Analytical
Analytical
Performance
Design
Performance
Design
Analysis
Analysis
Comparison
Analysis
Comparison
Analysis
Tool
Tool
Tool
Database
Database
Tool
Simple
Simple
Decision
Decision
Analysis Tool
Proprietary
Proprietary
Proprietary
Proprietary
Proprietary
Proprietary
Decision
Well Damage
Well Damage
Decision
Reservoir
Reservoir
Analysis
Analysis
Analysis
Analysis
Analysis
Analysis
Optimum Well Design
103
Combining the Elements
104
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. In some instances the appropriate
action is not to change completion design and
simply accept and account for damage risk in
economic projections.
105
Example of Decision Analysis
Probability x Consequences Risk Cost
Simple Decision Analysis Example