Module C-2: Stresses Around a Borehole - II Argentina SPE

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Module C-2 Stresses Around a Borehole - II
Argentina SPE 2005 Course on Earth Stresses and
Drilling Rock Mechanics Maurice B.
Dusseault University of Waterloo and Geomec a.s.
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Stress Trajectories
sv
stress trajectories are lines which represent the
flow of stresses through the solid body
sHMAX
shear stresses cannot pass through a fluid,
however, compressive stresses can (i.e. a fluid
pressure in a borehole)
sHMAX
on the boundary of the opening, t is zero and sr
pw (pressure)
sv
Example of a horizontal well
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Stress Trajectories

• These are plots of how the principal stresses
  flow around a hole or reservoir
• If the trajectories are closely spaced, the
  compressive stresses are large
• If they are sparse, stresses are lower
• They provide a good visualization of how the
  stresses are distributed
• For more detail and analysis, we plot them along
  a radial line from the borehole (see previous
  Module for examples)

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Typical Borehole Instability Issues

• Pack-offs
• Excessive tripping and reaming time
• Excessive mud losses (fracturing losses)
• Stuck pipe and stuck or wedged BHAs
• Loss of equipment and costly fishing trips
• Sidetracks, often several in the same hole
• Cannot get casing to bottom
• Poor logging conditions, cleaning trips
• Poor cementing conditions, large washouts
• These are all related in some way to rock failure
  and sloughing

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Yield of Rock Around a Borehole
sHMAX
Axial borehole fractures develop during drilling
when MW is higher than sq (surges, yield). (This
is related to ballooning as well.) Swelling or
other geochemical filtrate effects (strength
deterioration, cohesion loss) lead to rock yield
High shear stresses cause shear yield,
destroying cohesion (cementation), weakening the
rock
shmin
Borehole pressure pw MW ? z
High sq
Low sq
Shear yield Tensile yield
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Borehole Stability and Rock Failure

• The rock can yield somewhat around a borehole but
  drilling can continue. Why?
• The yield process relieves high stresses, so the
  yield zone stops propagating
• If we can still trip and drill ahead, the
  borehole fulfils its function it has not
  failed
• But, the rock around the borehole has yielded and
  lost its cohesive strength
• This distinction is very important
• Rock yield does not mean borehole loss
• Mud support pressure can sustain the hole, even
  if the hole is surrounded by yielded (fragmented)
  rock

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Cat-Scan of Hole Yield

• This is a tomographic reconstruction of a hollow
  cylinder test
• The dark lines are higher-porosity shear bands
  around the hole
• The central part of the hole is filled with
  spalled rubble
• This is evidence of typical borehole yield in a
  symmetrical stress field

Equal far-field stresses - sh
Intact portion
Sheared region
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Are Breakouts Serious?
Breakouts are evidence that there is a stress
difference in the plane normal to the hole. They
also indicate that the rock in the breakout area
has surpassed its strength. However, they are
not a sign of impending full collapse unless they
grow in an uncontrolled manner. Rock mechanics
analysis can predict the onset of breakouts and
yield, but less successful in predicting complete
opening collapse. Collapse is a complex
structural response affected by many factors
including stresses, strength, fabric of the rock,
drilling and tripping practices, and so on
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Geochemical Effects

• Swelling or shrinkage can occur because of
  geochemical effects in shales
• Geochemical changes lead to swelling or
  shrinkage!
• This ?V changes the tangential stresses (?s?)
• Swelling always leads to problems
• Rock yield from high hoop stresses
• Deterioration of cohesion from chemistry changes
  and small volume changes
• Squeezing of borehole, mudrings, poor mud
• Shrinkage can also reduce strength because any ?V
  helps degrade grain-to-grain cohesion
• Modest shrinkage or no shrinkage are best

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What is a Washout?

• When shale yields (high s?q), it weakens and
  tends to fragment
• If filter cake is poor, s?r is low (no support
  for the shale fragments) ? sloughing
• Washouts develop all around the borehole, roughly
  symmetric (made worse by fissility)

gage ri
Washouts, no strong orientation
Stresses flow around borehole
gage
shmin
breakouts
yielded shale
sHMAX
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Borehole Wall Features Failure
0
90
180
270
360

• Axial fractures (high MW) are not rock failure
  and deterioration
• Breakouts are evidence of rock shear failure
• Large washouts as well, leading to problems
• Natural fractures are not usually a problem,
  except if they are high-angle and can slip
• This case is more common than thought

axial fractures
breakouts
washout
Natural fracture traces
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Sandstone Mudcake, ?p Support
pressure
Excellent support
pw
MW
p(r), steady-state, no mud-cake
Dp across mudcake
po
borehole
p(r) with mudcake
distance (r)
mudcake
sandstone
limited solids invasion depth
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Filter Cake in Sandstones
sHMAX

• Filter cake is made of clays, polymers, etc.
• Very low permeability
• Sand k is much larger than cake k
• Allowing the pressure difference to give a direct
  support stress
• Therefore sands almost never slough, but
• Differential sticking is an issue in sandstones

po
Filter cake
shmin
pw
Damaged rock held in place by ve mud support
The positive support pressure in a sandstone is
usually close to pw po because permeability is
high
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Shale Mudcake, ?p Support
pressure
pw
MW
p(r), steady-state, _at_ t 8 now, no more
mud-cake effect!
shale
mudcake?
This is a time-dependent process
po
p(r) initially, _at_ t 0. This is an excellent
support condition
borehole
distance (r)
Because no mudcake can form on a shale, slow
pressure penetration takes place, and the support
pressure effect is slowly destroyed
shale
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Filter Cake in Shales
sHMAX

• Intact shale k is much lower than cake k
• A true filter cake cannot form on the borehole
  wall
• Initially, support is good
• But, with t, it decays
• Rock yields microfissures
• pw penetrates more fully into the damaged region
• Dp support is lost leading to sloughing,
  breakouts
• A time-dependent process!

po
Support lost with time
shmin
pw
Damaged rock is not held in place by mud pressure
and high k
The support pressure in shale is a function of
time
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Cake Efficiency Management

• Using OBM in intact shale gives excellent
  efficiency, good Dp support, reducing the shear
  stresses in the borehole wall
• In fractured shale, OBM often ineffective
• Filtrate penetrates the small fractures
• No Dp across wall can be sustained (no cake)
• These shales easily slough on trips, connections
• When using WBM
• Gilsonite, dispersed glycol, fn.-gr. solids can
  help plug small induced microfissures
• This helps maintain good Dp across the wall
• But! Geochemical effects can take place.

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Damage Effect on ?p Support
pressure
no Dp for wall support
mud pressure
pw
B(damaged borehole)
A(intact borehole)
transient pressure curves
p(r) curves with time
po
formation pressure
pressure gradient drops with time
borehole
distance (r)
low permeability shale, no mudcake
shale
High sq leads to rock damage. This permits
pressure penetration, loss of radial mud support.
It is time-dependent, and reduces stability.
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Thermal Destabilization
shear stress
Shear strength criterion for the rock around the
borehole
initial conditions
heating leads to borehole destabilization
Y
s?r
To
po
s?q
mud support
??i,j
T DT
Ds?q
normal stress
s?q Ds?q
When the stress state semicircle touches the
strength criterion, it is assumed that this is
the onset of rock deterioration (not necessarily
borehole collapse)
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Thermal Alterations of ???
These curves show the hoop stress calculated
using an assumption of heating and an assumption
of cooling. Clearly, heating a borehole
increases the magnitude of the stress, and leads
to hole problems. Cooling the borehole is
generally always beneficial to stability.
tangential stress - s?q
Except for heating, most processes reduce the
s?qmax value at the borehole wall
s?q (r) for heating
s?qmax
s?q (r) for cooling
Initial s?h
To
radius
Tw
borehole
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What Happens with Hot Mud?

• The rock in the borehole wall is heated
• Thermal expansion takes place
• This attracts stress to the expanding zone
  around the well
• The peak stress rises right at the borehole wall,
  and yield and sloughing is likely
• For cooling, the rock shrinks this allows the
  stress concentration to be displaced away from
  the borehole, helping stability
• Cooling occurs at and above the bit
• Heating occurs farther uphole

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Heating and Cooling in the Hole
T
cooling in tanks
Heating occurs uphole, cooling downhole. The
heating effect can be large, exceptionally
30-35C in long open-hole sections in areas with
high T gradients. Heating is most serious at
the last shoe. The shale expands, and this
increases s?q, often promoting failure and
sloughing. At the bit, cooling, shrinkage, both
of which enhance stability. Commercial software
exists to draw these curves
mud up annulus
casing
heating
shoe
geothermal temperature
open hole
T
mud down pipe
drill pipe
BHA
mud temperature
-T
cooling
depth
bit
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Expansion and Borehole Stresses
D
See Module C
This is the standard elastic case of borehole
stress redistribution
lost s
elastic rocks resistribute the lost stress
D
High s?q near the hole
This is the case of rock heating when the mud is
hotter than the formation
elastic rocks redistribute thermal stresses as
well
expanding rocks
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Thermal Stresses Around Boreholes

• Heat transfer conductive or convective
• Conductive low permeability rock shale, salt
• Convective high permeability rocks sandstone
• The stress distributions are different for these
  cases, and conduction is much slower
• Heating increases s??, and shear failure is more
  likely ( sloughing)
• Cooling reduces hoop stresses, and short axial
  fracturing is more likely
• In general, the effects of axial fracturing on
  stability are not substantial

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Effect of Rock Yield on ???
These curves show s?q calculated assuming that
rock yield occurs once a limit stress has been
exceeded. One curve is for a very simple model
of yield, the other for a more complex case. In
all yield cases, the stress concentration is
reduced, and the peak pushed away from the
borehole.
tangential stress - s?q
Except for heating, most processes reduce the
s?qmax value at the borehole wall
s?q (r)
s?qmax
Initial s?h
Kirsch elastic solution Yield solution A Yield
solution B
radius
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Rock Yield and Borehole Stresses

• When rock yields, it loses some of its load
  carrying capacity, thus shedding stress
• This stress is pushed out into the rock mass, and
  may cause adjacent rock to fail
• This reduces the magnitude of the hoop stresses
  around the hole
• Therefore, yield is evidence of the rock trying
  to find a stable equilibrium
• If the damaged (weakened) rock can be held in
  place, the hole becomes stable
• If not, sloughing occurs yield propagates

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Drilling-Induced Fractures
shift of peak stress site
stress
reduction in s?qmin
s?q, damaged
s?q
s?q, intact
s?r
damaged zone
po
fractures are propagated during drilling and
trips when effective mud pressures exceed sq
borehole, pw
sHMAX
radius
limited depth fractures
shmin
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Induced Axial Fractures

• Near the borehole, yield causes a reduction in
  the hoop stress, sq
• The MW may exceed sq near the wall
• When this happens, a short hydraulic fracture
  opens up, but it terminates against the zone of
  higher sq
• This can be exacerbated by high surges, high ECD,
  etc.
• If this is significant, it leads to ballooning
  or breathing of the well

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Borehole Shear Displacement

• Vincent Maury (1987, Elf-Aquitaine)
• High angle faults, fractures can slip and cause
  pipe pinching
• Near-slip earth stresses condition
• High MW causes pw charging
• Reduction in s?n leads to slip
• BHA gets stuck on trip out
• Probably more common than we realize we never
  check for it, its effect is subtle on logs
  because drilling destroys evidence
• Raising MW makes it worse! Lower MW

pw
s?n
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Lessons Learned

• The hoop stress around the borehole can be
  counteracted by good MW support
• In sands, no problem, in shales, problems
• Stresses around the borehole can be affected by a
  number of factors
• Geochemical effects that lead to shrinkage,
  swelling, loss of cohesion
• Thermal effects of heating or cooling
• Rock damage effects, breakouts
• Axial fractures are related to stresses
• Even slip of old fault planes or joints

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Additional Material Relevant to Stresses Around a
Borehole
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Review of Stresses and Boreholes

• In situ stresses
• sv (Vertical/overburden stress) (or Sv)
• sh (Two horizontal stresses),, shmin and sHMAX
  (sometimes you will see Sh, Shmin, SHMAX
• (sh - po) K(sv - po)
• In other words ??h K??v
• K ƒ n/(1- n) if no tectonics
• But, n is not constant it varies with f (depth)
• Fracture gradients (shale vs. sand)
• Eatons curve
• Ballooning/fracturing (clean sand fractures first
  in most stress regimes!)

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MORE REVIEW

• Depleted sands
• Fracture gradient is lower than expected
• A hesitation squeeze can increase PF
• LCM injection, drilling with LCM solids
• Stress concentration around a wellbore
• Gravity dominated stress system - GoM
• Tectonic system high compression or extension
  (Rocky Mtn. Foreland, North Sea Central Graben)
• Borehole breakouts are evidence of large
  differences in stresses Ds is large
• Breakouts vs. hole washouts not the same
• These issues should be well understood

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In RM, We Can Calculate Strength

• Rock Strength (next Modules)
• Failure in shear
• Failure in tension
• Borehole stability calculations (example)
• Minimum pressure for hole collapse
• Pw(3.shmax-shmin)/2(1 - sin??) Pressin??
• - So.cos ??
• Co 2Sotan (45 ??/2) (shear strength)
• We want to calculate stability, and use logs,
  etc. to make assessments, predictions

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Borehole Stability Philosophy

• Calculate stresses, compare to strengths
• Check for yield (rock failure)
• In many cases we must live with yield
• Breakouts, sloughing, etc.
• Careful surveillance to manage it
• If we avoid yielding the rock it is stronger
• If we reduce the hoop stress less yield
• If we increase support Dp less yield
• We do the best we can, but there is much
  uncertainty.

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E Q U A T I O N S

• Effective (??) vs. Total stress (S or s)
• ?? (S - po) or (s - po) Pore press. po
• Gravity dominated basin
• Sv or ?v ? Overburden weight (known)
• ?h ?vn/(1- n) (estimate)
• Sh - po n/(1- n)Sv - po
• Here, n is Poissons ratio, see next section
• Remember that this is just an estimate
  measurements are always preferred

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E Q U A T I O N S (Contd.)

• Eaton Pilkingtons Correlation to estimate
  stresses, developed for the GoM
• Sh - po KSv - po
• K-gt Stress Factor, empirically derived
• Sv-gt Overburden total stress sv
• Sh-gt Minimum horizontal total stress shmin
• (Also called fracture gradient, PF)
• SHMAX sHMAX Shmin in relaxed basins
• Different in tectonically stressed cases

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E Q U A T I O N S (Contd.)

• The General Stress System
• ??v (Sv - po) or (sv - po)
• ??HMAX (SHMAX - po) or (sHMAX - po)
• ??hmin (Shmin - po) or (shmin - po)
• Tangential stress at the borehole wall
• Vertical well case (best direction for drlg in a
  relaxed basin or offshore continental margin case
  where sHMAX shmin lt sv)
• Parallel to vertical wellbore (assuming pw po)
• ??qmax 3??HMAX - ??hmin
• ??qmin 3??hmin - ??HMAX

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E Q U A T I O N S (Contd.)

• Stress at the borehole wall (Contd.)
• Horizontal well cases
• Well parallel to maximum horizontal direction
• ??qmax 3??v - ??hmin
• ??qmin 3??hmin - ??v
• Well parallel to minimum horizontal direction
• ??qmax 3??HMAX - ??v
• ??qmin 3??v - ??HMAX

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E Q U A T I O N S (Contd.)

• Borehole Stability (Contd.)
• Pressure for vertical borehole fracture
  breakdown
• pw (3shmin) - sHMAX - po To
• To - Rock tensile strength, psi
• We have to try to estimate and measure these
  rock parameters, but going from lab to field in
  this case seems not possible