Transcript Geoscience and Rock Mechanics

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.
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)
circular
opening,
pw
sHMAX
sv
Example of a horizontal well
on the boundary of the
opening, t is zero and
sr = pw (pressure)
Stress Trajectories
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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)
Typical Borehole Instability Issues
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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
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.)
shmin
Borehole pressure
= pw = MW  z
High sq
Low sq
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
Shear yield
Tensile yield
Borehole Stability and Rock Failure
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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
Cat-Scan of Hole Yield
This is a tomographic
Equal far-field stresses - sh
reconstruction of a
hollow cylinder test
Intact portion
 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
Sheared region
typical borehole yield
in a symmetrical
stress field

Are Breakouts Serious?
sMAX
smin
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…
Geochemical Effects

Swelling or shrinkage can occur because of
geochemical effects in shales
 Geochemical
changes lead to swelling or shrinkage!
 This ΔV changes the tangential stresses (Δσ’θ)

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 ΔV 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)

Stresses
“flow”
around
borehole
gage = ri
shmin
breakouts
sHMAX
gage
Washouts,
no strong
orientation
yielded shale
Borehole Wall Features & Failure
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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 highangle and can slip
This case is more
common than thought
0
90
180
270
axial fractures
breakouts
washout
Natural fracture traces
360
Sandstone Mudcake, p Support
pressure
Excellent support
MW
p across
mudcake
pw
p(r), steady-state,
no mud-cake
po
p(r) with mudcake
borehole
mudcake
sandstone
limited solids
invasion depth
distance (r)
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
shmin
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po
Filter cake
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
Shale Mudcake, p Support
pressure
MW
p(r), steady-state, @ t = ∞
now, no more mud-cake effect!
mudcake?
pw
shale
This is a time-dependent process
po
p(r) initially, @ t = 0. This is an
excellent support condition
borehole
shale
distance (r)
Because no mudcake can form on a shale, slow
pressure penetration takes place, and the support
pressure effect is slowly destroyed
Filter Cake in Shales
sHMAX
Intact shale k is much
lower than cake k…
Support lost with time A true filter cake cannot
form on the borehole wall
 Initially, support is good
 But, with t, it decays…
 Rock yields = microfissures
pw
 pw penetrates more fully
into the damaged region
 p support is lost leading
to sloughing, breakouts…
 A time-dependent process!
shmin

po
Damaged rock is not
held in place by mud
pressure and high k
The support pressure in
shale is a function of time
Cake Efficiency Management
Using OBM in intact shale gives excellent
efficiency, good p support, reducing the
shear stresses in the borehole wall
 In fractured shale, OBM often ineffective:
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 Filtrate
penetrates the small fractures
 No p across wall can be sustained (no cake)
 These shales easily slough on trips, connections
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When using WBM
 Gilsonite,
dispersed glycol, fn.-gr. solids can
help plug small induced microfissures
 This helps maintain good p across the wall
 But! Geochemical effects can take place.
Damage Effect on p Support
pressure
no p for wall support
pw
mud pressure
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)
shale
low permeability shale, no mudcake
High sq leads to rock damage. This permits pressure penetration, loss of
radial mud support. It is time-dependent, and reduces stability.
Thermal Destabilization
shear stress
Shear strength criterion for
the rock around the borehole
heating leads to
borehole destabilization
Y
sr
initial
conditions
To
po
sq
mud
support
si,j
T + T
sq
normal
stress
sr
sq
sq + s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…)
Thermal Alterations of sq
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
sq (r) for heating
sq]max
Except for heating, most
processes reduce the sq]max
value at the borehole wall
sq (r) for cooling
Kirsch elastic solution
thermoelastic heating (convection)
thermoelastic cooling (convection)
To
Tw
borehole
Initial sh
radius
What Happens with Hot Mud?
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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
Heating and Cooling in the Hole
T
cooling
in tanks
mud up
annulus
Heating occurs uphole, cooling
downhole. The heating effect can
be large, exceptionally 30-35°C in
long open-hole sections in areas
with high T gradients.
casing
heating
+T
mud
down
pipe
mud
temperature
open
hole
drill
pipe
-T
At the bit, cooling, shrinkage, both
of which enhance stability.
BHA
cooling
depth
Heating is most serious at the last
shoe. The shale expands, and this
increases sq, often promoting
failure and sloughing.
shoe
geothermal
temperature
bit
Commercial software exists to draw
these curves
Expansion and Borehole Stresses
See Module C
D
“lost” s
“elastic” rocks resistribute the “lost” stress
This is the standard
elastic case of borehole
stress redistribution
D
High sq near the hole
“elastic” rocks redistribute thermal stresses as well
expanding
“rocks”
This is the case of rock
heating when the mud is
hotter than the formation
Thermal Stresses Around Boreholes
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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 σθ, 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 sq
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
sq]max
sq (r)
Kirsch elastic solution
Yield solution A
Yield solution B
Except for heating, most
processes reduce the sq]max
value at the borehole wall
Initial sh
radius
Rock Yield and Borehole Stresses
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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
Drilling-Induced Fractures
stress
shift of peak stress site
reduction in sq]min
sq,
damaged
sq
sq, intact
sr
damaged zone
po
σHMAX
borehole,
pw
fractures are propagated
during drilling and trips
when effective mud
pressures exceed sq
radius
limited depth fractures
σhmin
Induced Axial Fractures
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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
Borehole Shear Displacement
Vincent Maury (1987, Elf-Aquitaine)
 High angle faults, fractures can slip and
cause pipe pinching
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 Near-slip
earth stresses condition
 High MW causes pw charging
 Reduction in sn leads to slip
 BHA gets stuck on trip out
sn
pw
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…
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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:
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 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
Review of Stresses and Boreholes
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In situ stresses:
σv (Vertical/overburden stress) (or Sv)
 σh (Two horizontal stresses),, shmin and sHMAX
(sometimes you will see Sh, Shmin, SHMAX
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(sh - po) = K·(sv - po)
In other words… sh = K·sv
 K = ƒ [n/(1- n)] if no tectonics…
 But, n is not constant; it varies with f (depth)
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Fracture gradients (shale vs. sand)
 Eaton’s curve
 Ballooning/fracturing (clean sand fractures
first in most stress regimes!)
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MORE REVIEW
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Depleted sands
 Fracture
gradient is lower than expected
 A “hesitation squeeze” can increase PF
 LCM injection, drilling with LCM + solids
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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 – s is large
 Breakouts vs. hole washouts: not the same
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These issues should be well understood
In RM, We Can Calculate Strength
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Rock Strength (next Modules)
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Failure in shear
Failure in tension
Borehole stability calculations (example…)
Minimum pressure for hole collapse:
Pw=[(3.shmax-shmin)/2](1 - sinf) + Pres·sinf
- So.cos f
Co = 2·So·tan (45+ f/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
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 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 p: less yield
 We do the best we can, but there is much
uncertainty.
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EQUATIONS
Effective (s) vs. Total stress (S or s)
s = (S - po) or (s - po) Pore press. = po
 Gravity dominated basin:
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 Sv
or sv  Overburden weight (known)
 sh = sv·[n/(1- n)] (estimate)
 [Sh - po] = [n/(1- n)]·[Sv - po]
 Here, n is Poisson’s ratio, see next section
 Remember that this is just an estimate;
measurements are always preferred…
E Q U A T I O N S (Contd.)
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Eaton & Pilkington’s Correlation to estimate
stresses, developed for the GoM
[Sh - po] = K[Sv - po]
K-> Stress Factor, empirically derived
Sv-> Overburden total stress = sv
Sh-> Minimum horizontal total stress = shmin
(Also called fracture gradient, PF)
SHMAX = sHMAX ~ Shmin in “relaxed” basins
Different in tectonically stressed cases
E Q U A T I O N S (Contd.)
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The General Stress System
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sv = (Sv - po) or (sv - po)
sHMAX = (SHMAX - po) or (sHMAX - po)
s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 < sv)
Parallel to vertical wellbore (assuming pw = po)

sq]max = 3sHMAX - shmin
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sq]min = 3shmin - sHMAX
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E Q U A T I O N S (Contd.)
Stress at the borehole wall (Contd.):
 Horizontal well cases
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Well parallel to maximum horizontal direction:
sq]max = 3sv - shmin
sq]min = 3shmin - sv
Well parallel to minimum horizontal direction:
sq]max = 3sHMAX - sv
sq]min = 3sv - sHMAX
E Q U A T I O N S (Contd.)
Borehole Stability (Contd.):
Pressure for vertical borehole fracture
breakdown:
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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…