Module C-2: Stresses Around a Borehole - II Argentina SPE 2005 Course on Earth Stresses and Drilling...
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Transcript of Module C-2: Stresses Around a Borehole - II Argentina SPE 2005 Course on Earth Stresses and Drilling...
Module C-2: Module C-2: Stresses Around a Borehole - IIStresses Around a Borehole - II
Argentina SPE 2005 Course on Earth Stresses and Drilling Rock Mechanics
Maurice B. DusseaultUniversity of Waterloo and Geomec a.s.
Stress TrajectoriesStress Trajectories
circularopening,
pw
stress trajectories arelines which representthe “flow” of stresses
through the solid body
HMAX
v
v
HMAX
shear stresses cannot pass through a fluid,
however, compressive stresses can (i.e. a fluid pressure in a borehole)
on the boundary of the opening, is zero and
r = pw (pressure)
Example of a horizontal well
Stress TrajectoriesStress 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)
Typical Borehole Instability Typical Borehole Instability IssuesIssues 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 BoreholeYield of Rock Around a Borehole
Borehole pressure= pw = MW z
HMAX
hmin
Axial borehole fractures develop during drilling when MW is higher than (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
High
Shear yieldTensile yield
Borehole Stability and Rock Borehole Stability and Rock FailureFailure
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 lossMud support pressure can sustain the hole, even
if the hole is surrounded by yielded (fragmented) rock
Cat-Scan of Hole YieldCat-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
Intact portion
Sheared region
Equal far-field stresses - h
Are Breakouts Serious?Are Breakouts Serious?
MAX
min
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 EffectsGeochemical Effects Swelling or shrinkage can occur because of
geochemical effects in shalesGeochemical changes lead to swelling or
shrinkage!This ΔV changes the tangential stresses (Δσ’θ)
Swelling always leads to problems:Rock yield from high hoop stressesDeterioration of cohesion from chemistry changes
and small volume changesSqueezing 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?What is a Washout? When shale yields (high ), it weakens
and tends to fragment If filter cake is poor, r is low (no support
for the shale fragments) sloughing Washouts develop all around the borehole,
roughly symmetric (made worse by fissility)
gage
gage = ri
hmin
HMAX
Stresses “flow” around borehole
breakouts
Washouts, no strong orientation
yielded shale
Borehole Wall Features & FailureBorehole 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, Sandstone Mudcake, p Supportp Support
borehole
p(r), steady-state, no mud-cake
mudcake
limited solidsinvasion depth
p across mudcake
p(r) with mudcake
pressure
po
pw
distance (r)
sandstone
Excellent supportMW
HMAX
hm
in
Filter Cake in SandstonesFilter 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, Shale Mudcake, p Supportp 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
HMAX
hm
in
Filter Cake in ShalesFilter 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 p support is lost leading
to sloughing, 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 ManagementCake 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:Filtrate penetrates the small fracturesNo p 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 p across the wallBut! Geochemical effects can take place.
Damage Effect on Damage Effect on p Supportp Support
pressure
po
pw
distance (r)
pressure gradient drops with time
low permeability shale, no mudcake
A(intact borehole)
B(damaged borehole)
no p for wall support
shale
transientpressurecurves
mud pressure
formation pressure
p(r) curves with time
High leads to rock damage. This permits pressure penetration, loss of radial mud support. It is time-dependent, and reduces stability.
borehole
Thermal DestabilizationThermal Destabilization
shear stress
normalstress
r
r
T + T
po
+
To
mudsupport
Shear strength criterion for the rock around the borehole
initialconditions
i,j
heating leads to borehole destabilization
When the stress state semicircle “touches” the strength criterion, it is assumed that this is the onset of rock deterioration (not necessarily borehole collapse…)
Y
Thermal Alterations of Thermal Alterations of
Kirsch elastic solutionthermoelastic heating (convection)thermoelastic cooling (convection)
(r) for cooling
radius
max
Tw
Except for heating, mostprocesses reduce the ]max
value at the borehole wall
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 -
(r) for heating
To
Initialh
borehole
What Happens with Hot Mud?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 HoleHeating and Cooling in the Hole
depth
T
casing
geothermaltemperature
bit
cooling
heating
mudtemperature
shoe
+T
-T
muddownpipe
mud upannulus
coolingin tanks
BHA
drillpipe
openhole
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 , often promoting failure and sloughing.
At the bit, cooling, shrinkage, both of which enhance stability.
Commercial software exists to draw these curves
Expansion and Borehole Expansion and Borehole StressesStresses
“lost”
“elastic” rocks resistribute the “lost” stress
D
“elastic” rocks redistribute thermal stresses as well
expanding “rocks”
High near the hole
This is the standard elastic case of borehole stress redistribution
This is the case of rock heating when the mud is hotter than the formation
DSee Module C
Thermal Stresses Around Thermal Stresses Around BoreholesBoreholes Heat transfer: conductive or convective
Conductive: low permeability rock – shale, saltConvective: 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
Effect of Rock Yield on Effect of Rock Yield on
Kirsch elastic solutionYield solution AYield solution B
(r)
radius
max
Except for heating, mostprocesses reduce the ]max
value at the borehole wall
These curves show 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.
Initial h
tangential stress -
Rock Yield and Borehole Rock Yield and Borehole StressesStresses 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 FracturesDrilling-Induced Fractures
r
po
radius
stress
reduction in ]min
damaged zone
borehole,pw
limited depth fractures
, intact
, damaged
shift of peak stress site
fractures are propagatedduring drilling and trips
when effective mudpressures exceed
σhmin
σHMAX
Induced Axial FracturesInduced Axial Fractures
Near the borehole, yield causes a reduction in the hoop stress,
The MW may exceed near the wall When this happens, a short hydraulic
fracture opens up, but it terminates against the zone of higher
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 DisplacementBorehole Shear Displacement
Vincent Maury (1987, Elf-Aquitaine) High angle faults, fractures can slip and
cause pipe pinchingNear-slip earth stresses conditionHigh MW causes pw charging
Reduction in n leads to slipBHA 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…
npw
Lessons LearnedLessons 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 coolingRock damage effects, breakouts
Axial fractures are related to stresses Even slip of old fault planes or joints
Additional Material Relevant to Additional Material Relevant to Stresses Around a BoreholeStresses Around a Borehole
Review of Stresses and Boreholes Review of Stresses and Boreholes
In situ stresses:σv (Vertical/overburden stress) (or Sv)
σh (Two horizontal stresses),, hmin and HMAX
(sometimes you will see Sh, Shmin, SHMAX
(h - po) = K·(v - po) In other words… h = K·v K ƒ [/(1- )] if no tectonics…But, is not constant; it varies with (depth)
Fracture gradients (shale vs. sand) Eaton’s curve Ballooning/fracturing (clean sand
fractures first in most stress regimes!)
MORE REVIEWMORE REVIEW
Depleted sandsFracture gradient is lower than expectedA “hesitation squeeze” can increase PFLCM injection, drilling with LCM + solids
Stress concentration around a wellboreGravity dominated stress system - GoMTectonic system – high compression or
extension (Rocky Mtn. Foreland, North Sea Central Graben)
Borehole breakouts are evidence of large differences in stresses – is large
Breakouts vs. hole washouts: not the same These issues should be well understood
In RM, We Can Calculate In RM, We Can Calculate StrengthStrength Rock Strength (next Modules)
Failure in shear Failure in tension
Borehole stability calculations (example…)Minimum pressure for hole collapse:Pw=[(3.hmax-hmin)/2](1 - sin) + Pres·sin
- So.cos Co = 2·So·tan (45+ /2) (shear strength)
We want to calculate stability, and use logs, etc. to make assessments, predictions
Borehole Stability PhilosophyBorehole 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 p: less yield We do the best we can, but there is
much uncertainty.
E Q U A T I O N SE Q U A T I O N S
Effective () vs. Total stress (S or ) = (S - po) or ( - po) Pore press.
= po
Gravity dominated basin:Sv or v Overburden weight (known)
h = v·[/(1- )] (estimate)
[Sh - po] = [/(1- )]·[Sv - po]Here, is Poisson’s ratio, see next sectionRemember that this is just an estimate;
measurements are always preferred…
E Q U A T I O N S (Contd.)E Q U A T I O N S (Contd.)
Eaton & Pilkington’s Correlation to estimate stresses, developed for the GoM[Sh - po] = K[Sv - po]
K-> Stress Factor, empirically derivedSv-> Overburden total stress = v
Sh-> Minimum horizontal total stress = hmin
(Also called fracture gradient, PF)
SHMAX = HMAX ~ Shmin in “relaxed” basins
Different in tectonically stressed cases
E Q U A T I O N S (Contd.)E Q U A T I O N S (Contd.)
The General Stress System v = (Sv - po) or (v - po)
HMAX = (SHMAX - po) or (HMAX - po)
hmin = (Shmin - po) or (hmin - po)
Tangential stress at the borehole wall: Vertical well case (best direction for drlg
in a relaxed basin or offshore continental margin case where HMAX ~ hmin < v)Parallel to vertical wellbore (assuming pw = po)
]max = 3HMAX - hmin
]min = 3hmin - HMAX
E Q U A T I O N S (Contd.)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: ]max = 3v - hmin
]min = 3hmin - v
Well parallel to minimum horizontal direction:
]max = 3HMAX - v
]min = 3v - HMAX
E Q U A T I O N S (Contd.)E Q U A T I O N S (Contd.)
Borehole Stability (Contd.):Pressure for vertical borehole fracture
breakdown:
pw = (3hmin) - HMAX - 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…