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Stresses Around a Borehole
Prof. Dr. Eissa Mohamed Shokir
Common Borehole Stability Symbols
s1,s2,s3: Major, intermediate, minor stress
Sv, Sh, SH: Total earth stresses, or Sv, Shmin, SHMAX, or sv, shmin, sHMAX
sr, sq: Radial, tangential, borehole stresses
sr, sq, sv, shmin, sHMAX, etc…: Effective stresses
r, ri: Radial direction, borehole diameter
po, p(r): Initial pressure, p in radial direction
MW, pw: Mudweight, pressure in borehole
E, n: Young’s modulus, Poisson’s ratio
f, r, g: Porosity, density, unit weight
k: Permeability
These are the most common symbols we use
Terminology and Symbols Problems
Often, the terminology and symbols used are confusing and irritating
This complexity arises because: The area of stresses and rock mechanics is
somewhat complex by nature
The terminology came from a discipline other than classical petroleum engineering
There is still some inconsistency in symbology, such as Sh, Sh, Shmin, sh, all for shmin …
We will try to be consistent
Please spend the time to understand
Physical principles are the most important
Other Conundrums How do we express stresses?
As absolute stresses? As stress gradients? As equivalent density of the overburden? As equivalent mud weights?
e.g. PF = 18 ppg means 18 pounds per US Gallon is the fracture pressure at some (unspecified) depth (fracture gradient = (s3/z).
e.g. shmin gradient is 21 kPa/m (or 21 MPa/km)
e.g. The minimum stress is 2.16 density units
e.g. shmin is 66 MPa (at z = 3.14 km depth)
All of these are the same! (or could be)
Which method is used usually depends who you are talking to! (Drillers like MW…)
The Basic Symbols, 2-D Borehole
Far-field stresses are natural earth stresses and pressures, genera-ted by gravity, tectonics…
Borehole stresses are generated by creation of an opening in a natural stress field
Far-field stresses: scale: 100’s of metres
Borehole stresses scale: 20-30 ri (i.e. local- to small-scale)
Far-field stress
r q
s’r
s’q
ri
pw
shmin
sHMAX
po
Borehole stress
Important to Remember…
sq is the tangential stress, also called the hoop stress, you will see it repeatedly referred to in these terms
sq lies parallel (tangential) to the wall trace The magnitude of sq is affected by:
In situ stresses MW and cake efficiency Temperature and rock behavior
It is the most critical aspect of the stress condition around a borehole… High sq values lead to rock failure Lower sq values usually imply stability
Borehole Stability Analysis Concept
First, we need stresses around the borehole… In situ stresses are vital
Δp, ΔT, chemistry affect these stresses
Mud cake efficiency
In some cases, rock properties are also needed
Then, we must compare the maximum shear stress with the rock strength… We need to know the rock strength
We need to know if the rock has been weakened by poor mud chemistry and behavior
If matrix stress exceeds strength, we say the rock has yielded (or “failed”)
Plotting Stresses Around a Borehole
Usually, we plot sq, sr values along one or the other of the principal stress directions
Vertical
borehole
sr
sq
radius
s
pw = 0
smin
smax
Far-field stresses
Vertical borehole
smax
smin
Stresses Around a Borehole One Dimensional Case:
A borehole induces a stress concentration
Two- and three-dimensional cases are more complicated (discussion deferred)
Stress “lost” must be redistributed to the borehole flanks (i.e.: s concentration)
F (F/A =
stress) F F
Initial stress
High sq near
the borehole,
but low sr!
(F/A)
(2F/A) F
F = force, A = Area, F/A = stress
Stress Redistribution
Around the borehole, a “stress arch” is generated to redistribute earth stresses
elastic rocks have rigidity (stiffness)
“lost” s
“elastic” rocks resistribute the “lost” stress
Everyone carries an equal
load (theoretical socialism)
In reality, some carry more
load than others (higher s’q
near the borehole wall)
Far away (~5D): ~no effect
These guys may “yield”
if they are overstressed
D
Stresses “Arch” Around Borehole
The pore pressure in the hole is less than the total stresses
Thus, the excess stress must be carried by rock near the hole
If the stresses now exceed strength, the borehole wall can yield
However, “yield” is not “collapse”! A borehole with yielded rock can still be stable…
shmin
circular
opening,
pw s H
MA
X
Arching of Stresses
arches lintels
load
stress arching
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
Yield of Rock Around a Borehole
Borehole pressure
= pw = MW z
sHMAX
shmin
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
Low sq
High sq
Shear yield
Tensile yield
Shear Stresses
Shear stress is the cause of shear failure
The maximum shear stress at a point is half the difference of s1 and s3
max = (s‛1 - s‛3)/2, or (s‛q - s‛r)/2 in the figure
Vertical
borehole
sr
sq
radius
s
pw = 0
Vertical borehole
smax
smin
Assumptions:
The simplest stress calculation approach is the Linear Elastic rock behavior model
This behavior model is very instructive
It leads to (relatively) simple equations
r
i2
2
i2
2
i4
4
i2
2
i4
4
ri2
2
i4
4
i
= ( + )
2(1-
r
r) +
( - )
2(1-
4 r
r+
3r
r) 2
= ( + )
2(1 +
r
r) -
( - )
2(1 +
3r
r) 2
= -( - )
2(1 +
2 r
r-
3r
r) 2
in all cases, r r , is taken CCW from reference
ss s s s
q
ss s s s
q
s s
q
q
q
q
max min max min
max min max min
max min
cos
cos
sin
.
r
q
sr
sq
ri
Symbols used
smin
smax
Far-field stress
pw = 0 Known as the “Kirsch” Equations
Comments
Note that the equations are written in terms of effective stresses (sq, sr, s’min…), with no pore pressure in the hole
Far-field effective stresses are the earth stresses, and they have fixed directions
sq, sr can be calculated for any specific point (r, q) around the borehole, for r ri
Later, one may introduce more complexity: T, p(r), non-elastic behavior, and so on…
These require software for calculations; various commercial programs are available
Calculations with In Situ Stresses
For a vertical borehole, the least critical condition is when s’hmin = s’HMAX = s’h s’q]max in this case = 2· s’h if pw = po
However, we can still get rock yield!
However, in most cases, especially in tectonic regions and near faults… The stresses are not the same!
This means that the shear stresses are larger around the borehole after it is drilled
This means that rock yield is more likely!
Borehole stability issues are more severe
Lost circulation more critical
What is a Linear Elastic Model?
The simplest rock behavior model we use… Strains are reversible, no yield (failure) occurs
Linear relationship between stress & strain
Rock properties are the same in all directions
σ‛a
σ‛r = σ‛3
σ’a = σ‛1
εa – axial strain
σ’ –
str
ess (σ‛
1 –
σ‛3)
E = Ds/De =
Young’s modulus
Stress-strain plot
From The Elastic Model
Even in an isotropic stress field (e.g. shmin sHMAX for a vertical hole), shear stress concentration exists around the hole This can lead to rock yield. How to counteract?
We can partly counteract with mud weight E.g.: if pw = shmin = sHMAX = sh (i.e.: MW = sh/z)
If the filter cake is perfect (no Dp near hole)
In practice, this is not done: if MW = sh/z, we are at fracture pressure & drilling is slower!
Higher MW reduces the magnitude of the shear stress, which reduces the risk of rock yield, but increases LC risk, slows drlg…
From The Elastic Model
Fracture breakdown pressure is calculated to be Pbreakdown = 3σ’hmin - σ’HMAX + po In practice, this is not used for design
Fracture propagation is Ppropagation = shmin, also taken to be PF (fracture pressure) for planning of MW programs This is often taken to be MW]max
MW is usually maintained to be less than shmin
In practice, it is often possible to use some methods to “strengthen” the borehole
This allows drilling somewhat “overbalanced”, when pw > σhmin, (this must be done carefully!)
Here, we plot the tangential stress, s’q
Higher stress difference is serious! It gives rise to higher s’q values. Rupture??
Borehole Stresses if shmin sHMAX
pw
2·σhmin
σ HMAX σ hmin
= 1.0) (
σhmin
σHMAX
= 1.4) (
1.6·σ hmin
3.2·σ hmin
σ HMAX σ hmin
σHMAX
Calculated from Kirsch equations,
along principal stress directions
2σhmin
σhmin
Far-field stresses, shmin, sHMAX, are: shmin – po, sHMAX – po
wellbore pressure pw assumed to be equal to po
pw
sq ~ sq ~
It gets worse in tectonic cases!
When shmin - sHMAX is large, the borehole wall in the sHMAX direction is in tension! Induced fractures can be generated during pw surges
High sHMAX - shmin Cases (Tectonic)
σ hmin
pw
σ hmin
σ HMAX
sq ~ 5σ hmin
σ hmin sq ~ 8σ
hmin
= 2.0) ( σ HMAX σ hmin
= 3.0) ( σ HMAX σ hmin
σ HMAX
*Note: here, borehole pressure, pw, is assumed = po
pw
θ rw
0
+90°
-90°
Plot of the Tangential Stresses
σHMAX
σHMAX
Refer to paper by Grandi for details
Here, σθ stresses at the wall (ri) are plotted as a function of θ
Note the symmetry
σθ(ri)
Borehole Wall Stresses (@r = ri)
Now, introduce effective stresses: e.g. symbols s for total, s for effective
Maximum stress at the borehole wall: σq]max = 3·σHMAX - σhmin – po (total stresses)
sq]max = 3·σ’HMAX - σ’hmin (effective stresses)
Minimum stress at the borehole wall:
σq]min = 3·σhmin - σHMAX - po (total stresses)
s’q]min = 3·σ’hmin - σ’HMAX (effective stresses)
For a general 3-D solution for inclined wellbores: use a software solution (big equations!)
Preliminary Comments…
Creation of a borehole: high tangential stresses (sq), low radial stresses (sr)
The larger sHMAX - shmin, the higher sq is (in the direction of shmin), the lower sq is (in the direction of sHMAX)
Radial effective stress (sr) is low near the borehole wall, zero right at the wall
pw = 0
sr
sq
radius
s
More Preliminary Comments…
If both stresses are equal (sh) and MW = po: at borehole wall: sq = 2sh, and sr = 0
If sHMAX – shmin is large, sq is increased, and sr doesn’t change too much
This greatly increases the shear stresses
These shear stresses are responsible for failure of the rock, breakouts, sloughing…
How do we control this? High effective mud weights reduce this
Mud cooling shrinks rock, reduces stresses
Avoid shale swelling, promote shale shrinkage
Mud Weight Effect (equal s case)
pw = 0.3s
sr
sq
pw = 0.8s
sr
sq
Here, we assume for simplicity that we
have “perfect” mud cake, and that the
pore pressure in the rock is zero
radius
s
radius
s
pw = 0
sr
sq Assume sHMAX = shmin = s
radius
s
Let’s Include Pore Pressures…
pw = 0.6s
sr
sq
Assume sHMAX = shmin = s
radius
s
Pore pressure - po
Positive support force = pw – po is applied in the case of a perfect mud cake:
this is a strong stabilizing force because it increases confining stress, this
will be discussed later, when we introduce rock strength
Mud
pressure -
pw
Much of what we do in mud chemistry and MW management is to try and
keep a positive support force right at the wall. This acts like a liner in a
tunnel, keeping the rock from deteriorating and reducing the shear stresses.
If it is lost by poor cake…, deterioration can be expected, especially in shale.
perfect cake
Filter Cake Efficiency
The better the filter cake, the better the support pressure on the borehole wall Support pressure = pw - pi
If there is poor filter cake, support pressure on a shale may be almost zero!
This support pressure is a true effective stress that is acting in a radial outward direction, holding rock in place!
In WBM in shales, the support pressure tends to decay with time! Soon after increase in MW – good stability After some time (days, weeks), sloughing can
start again because support p decays
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
What is a Washout? When shale yields (high sq), it weakens and
tends to fragment
If filter cake is poor, sr is low (no support for the shale fragments) sloughing
Washouts develop all around the borehole, roughly symmetric (made worse by fissility)
gage
gage = ri
shmin
sHMAX
Stresses
“flow”
around
borehole
breakouts
Washouts,
no strong
orientation
yielded shale
Borehole Wall Features & Failure
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
0 90 180 270 360
washout
breakouts
axial fractures
Natural fracture traces
Sandstone Mudcake, Dp Support
borehole
p(r), steady-state,
no mud-cake
mudcake
limited solids invasion depth
Dp across mudcake
p(r) with mudcake
pressure
po
pw
distance (r)
sandstone
Excellent support MW
sHMAX
shm
in
Filter Cake in Sandstones
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
pw
The positive support pressure in a
sandstone is usually close to pw – po
because permeability is high
po
Damaged rock held
in place by +ve
mud support
Filter cake
Shale Mudcake, Dp Support
borehole
p(r), steady-state, @ t = ∞
now, no more mud-cake effect!
mudcake?
p(r) initially, @ t = 0. This is an
excellent support condition
pressure
po
pw
distance (r)
shale
MW
shale
Because no mudcake can form on a shale, slow
pressure penetration takes place, and the support
pressure effect is slowly destroyed
This is a time-dependent process
sHMAX
shm
in
Filter Cake in Shales
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 and breakouts…
A time-dependent process!
pw
The support pressure in
shale is a function of time
po
Damaged rock is not
held in place by mud
pressure and high k
Support lost with time
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.
Gilsonite: natural asphalt
Glycol is any of a class of organic compounds belonging to the alcohol family; in the molecule of a glycol, two hydroxyl (OH) groups are attached to different carbon atoms. The term is often applied to the simplest member of the class, ethylene glycol.
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
Heating and Cooling in the Hole
depth
T
casing
geothermal temperature
bit
cooling
heating
mud temperature
shoe
+T
-T
mud down pipe
mud up annulus
cooling in tanks
BHA
drill
pipe
open
hole
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.
Heating is most serious at the last
shoe. The shale expands, and this
increases sq, often promoting
failure and sloughing.
At the bit, cooling, shrinkage, both
of which enhance stability.
Commercial software exists to draw
these curves
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 σθ, 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
Horizontal vs. Vertical Wellbore?
σv = 0.9 psi/ft, σh = 0.6 psi/ft, p = 0.4 psi/ft
In non-tectonic systems (shmin ~
sHMAX) vertical holes are subjected
to lower shear stresses; they are
generally more stable than
horizontal holes
sq = 1.3 psi/ft, sides
Horizontal Hole
Vertical Hole
sq = 0.1 psi/ft,
top, bottom
sv = 0.5 psi/ft
sh = 0.2 psi/ft
sh = 0.2 psi/ft
Stress State 0.5
0.2
0.2
0.2
sq = 0.4 psi/ft
Tectonic Stress Conditions
Vertical effective stress = 0.5 psi/ft Min. horizontal effective stress = 0.3 psi/ft Max. horizontal effective stress = 1.0 psi/ft
Vertical well
0.1
2.7
0.1
2.7
This orientation is the
best one for this case,
showing the importance
of knowing the in situ
stresses
1.2
0.4 0.4
1.2
sv = 0.5 psi/ft
shmin = 0.3 psi/ft
sHMAX = 1.0 psi/ft
2.5
0.5
Horizontal well aligned with
minimum stress, shmin
0.5
2.5 Horizontal well aligned with
minimum stress, sHMAX
Which aligned of Horizontal
well is the best
sq =
sq =
sq
TABLE 1
Maximum Stress Minimum Stress (σθ]min)
No. Hole
Configuration Gradient ( psi/ft)
Magnitude ( psi)
Gradient ( psi/ft)
Magnitude ( psi)
1 Vertical 2.7 13,500 -0.1 -500
2 Parallel to minimum
horizontal stress 2.5 12,500 0.5 2,500
3 Parallel to maximum
horizontal stress 1.2 6,000 0.45 2,000
Stress at borehole wall (σ’θ) in a tectonically active area (Compressive stresses are +ve; Tensile stresses are -ve)
Depth of investigation is 5,000 ft
(σθ]MAX)
3-Dimensional Borehole Stresses
Effective stresses:
s1 = s1 - po s2 = s2 - po s3 = s3 - po
z
x
y
F Y
s1
s2
s3
po
F, Y are dip and dip direction (wrt x) of the borehole axis x, y, z are coordinates oriented parallel to s1, s2, s3
s1, s2, s3 are the principal total stress magnitudes po is the pore pressure
Borehole radial,
axial & tangential
stresses, sr, sa, sq
Almost always, principle stresses can be
taken as and to the earth’s surface
What About the Axial Stress??
Axial stress, sa, acts parallel to the hole wall, to sr, sq
Usually ignored in borehole stability
However, if sa is very large compared to sr & sq, it can also cause yield
More sophisticated analysis req’d
Almost always, using the hole angle and azimuth, we do the following: Determine maximum and minimum
stresses in the plane of the hole
Carry out a 2-D stability analysis
sr, sa, sq
The Best Well Orientation
In a relaxed (non-tectonic) basin, sv > shmin ~ sHMAX, vertical wells are the most stable
In a tectonic basin, an estimate of the stresses is essential; for example: If sHMAX > sv > shmin, we still have to know the
specific values to decide the best trajectory
If sHMAX = 0.7, sv = 0.5, shmin = 0.4 psi/ft, a horizontal well parallel to sHMAX is the best
If sHMAX = 0.7, sv = 0.6, shmin = 0.4 psi/ft, a well parallel to shmin is likely the best
Careful Rock Mechanics analysis is best
+0ther factors: fissility, fractures…
Stresses and Drilling
sv >> sHMAX > shmin
shmin
sHMAX
sv
sHMAX >> sv > shmin
sHMAX ~ sv
>> shmin
sv
shmin
sHMAX
sv
shmin
sHMAX
To increase hole stability, the best orientation is that which minimizes the principal stress difference normal to the axis
60-90° cone
Drill within a 60°cone (±30°) from the most favored direction
Favored hole orientation
“Showing” the Best Trajectory
This is a polar plot of “ease of drilling”
Related to magnitude of shear stress on wall
This is based in situ stress knowledge
In this example, a horizontal well, W to E, seems to be “easiest”
A horizontal well N to S is the worst (all other factors being equal)
shmin
sHMAX
sv
Typical Troublesome Hole (GoM)
8.00
9.00
10.00
11.00
12.00
13.00
14.00
15.00
16.00
3000’ 4000’ 5000’ 6000’ 7000’ 8000’ 9000’
PP
Sh
Sv
Planned Casing
Actual Casing
Drill MW
MW to Keep Hole Open
Increase MW to
get out of hole
Pore pressure MWmin Lade shmin
sv
Planned Csg Actual Csg
Drill MW
MW to keep
hole open
4960 Stuck Pipe: no
rotation, no circulation
Hole tight with pumps off
Losing 300 bbl.hr (ballooning?)
17 ½” x 20 ” 17 ½” x 20 ” 16 ” Liner 16 ” Liner 13 3/8 ” 13 3/8 ” 14 ¾” x 17 ½” 14 ¾” x 17 ½”
Pack-off
Str
ess, p
ressu
re in
pp
g
Depth in feet
The Plan … The Reality
Hole planned from offset wells (sv, shmin, log correlations to strength data, po…)
Jagged line is a prediction of MW to sustain reasonable borehole stability
Brown line: chosen MW program from stability calculation (using “Lade” criterion)
Red line was the actual mud weight needed to cope with a series of problems
The casings were set higher than expected and an extra string was eventually needed
Typical Troublesome Hole (GoM)
8.00
9.00
10.00
11.00
12.00
13.00
14.00
15.00
16.00
3000’ 4000’ 5000’ 6000’ 7000’ 8000’ 9000’
PP
Sh
Sv
Planned Casing
Actual Casing
Drill MW
MW to Keep Hole Open
Increase MW to
get out of hole
Pore pressure MWmin Lade shmin
sv
Planned Csg Actual Csg
Drill MW
MW to keep
hole open
4960 Stuck Pipe: no
rotation, no circulation
Hole tight with pumps off
Losing 300 bbl.hr (ballooning?)
17 ½” x 20 ” 17 ½” x 20 ” 16 ” Liner 16 ” Liner 13 3/8 ” 13 3/8 ” 14 ¾” x 17 ½” 14 ¾” x 17 ½”
Pack-off
Str
ess, p
ressu
re in
pp
g
Depth in feet
How do We Sustain Stability?
MW control (up or down)
Mud properties control (reduce ECD)
Trip and connection policy (speed, surge…)
Inhibitive WBM: minimize chemical effects
OBM: eliminate chemical effects
Air or foam UB drilling (shallow, strong rx)
Use fn-gr LCM, gilsonite in fractured shale
Cool the drilling mud to reduce sq, reducing the chances of rock failure
When all else fails, sidetrack, set casing
Well Design and Cost Optimization
High risks are mainly related to low MW, rapid drilling, increased well blowout risks… Low cost if successful.
Low risks are mainly associated with slow drilling and high MW, but drillings time is long… Generally costly…
In between, there is a level of acceptable risks with a lower cost factor
Well Design Costs
Ac
tual
(Lik
ely
) W
ell
Co
sts
High Risk Low Risk
Borehole Cost Optimization
Affected by drilling speed, casing string costs, cleaning problems, cost of drilling mud, risks, trip problems…
Optimizing this in “real time” is the challenging task of the Drilling Engineer
Mud Weight
Str
ess to S
trength
ratio
1.0
0.8
0.6
Flu
id influx
She
ar
failu
re
slo
ugh
ing
Safe Lost
circulation
“Ballo
onin
g”
The shape of the
cost curve changes,
depending on the
stresses and where
we are in the hole!
Borehole Stability and Hydraulics
Borehole management is not only stresses, rock strength, MW and mud properties!
It is also dependent on hydraulics: Pumping strategy and cleaning capabilities
Gel strength, viscosity, mud density
BHA design, ECD, even tripping policy
How do We Predict RM Stability?
We need to know the rock stresses in situ Vertical, horizontal usually, sv, shmin
Pore pressures (especially overpressure cases)
We need to know the rock strength Lab testing of core
Correlations to geophysical log data bases
Testing of drill chips (penetrometers, sonic…)
Then, we make predictions of stability MW
This is an indicator only! Careful monitoring on the active well
Improvement of our “calibrations”, ECD…
Lessons Learned
Stress concentrations arise naturally when a hole is drilled
The tangential stress sq is critical Affected by stress, tectonics, rock behavior…
Borehole cake and mud support are critical
We can calculate stresses, but rock parameters are (E, n, Y, Co, To…) needed
We can reduce the effects of high sq
MW, lower T, better cake, OBM…
We can use log data and correlations to predict the MW for stability
Recommended