Borehole Stability Presentation
description
Transcript of Borehole Stability Presentation
-
Borehole Stability during Drilling
-
Borehole Stability Problems
Tight hole / Stuck pipe incidents Responsible for 5-10% of drilling time Most frequently occurring in shale Often high pore pressure, and in presence of
swelling clay minerals (e.g. smectite) Often in deviated wells
Lost circulation / Mud losses May lead to kick / blow-out Caused by fluid lost into natural fractures or by
new fractures generated
-
Tight hole / Stuck pipe
Causes: Mechanical borehole collapse (often by shear
failure) Increased hole size by brittle failure; stuck because of
accumulated cavings (sloughing shale) Reduced hole size by large (plastic) hole deformations
(gumbo shale) Inappropriate hole cleaning Differential sticking (only in permeable zones with
mud cake) Difficult hole trajectory: Key-seat, dog-legs
-
Tight hole / Stuck pipe
Consequences: Lost time, reaming, side-track (or $) Problems in further well operations (logging,
cementing; continued drilling) Solutions:
Overall well design i.e. Casing programme Mud weight Mud composition Drill somewhere else..
Note: The solution depends on the cause Need for diagnostics
-
Tight hole / Stuck pipe Diagnosis Hole
collapse Inappropriate hole cleaning
Differential sticking
Drilling environment Shale * * [ (Permeable) reservoir rock * *
Observations during drilling Rotating before stuck * 0 Moving up / down before stuck * 0 Rotating after stuck [ Circulating after stuck * Excessive cuttings & cavings *
Observations after drilling Non-gauge hole diameter from caliper * [ Low density / high porosity / low wave velocities
*
-
Lost circulation / Mud losses
Consequences: Dangerous situation, a major safety issue Risk of life and equipment
Solutions: Overall well design i.e.
Casing programme Mud weight Lost circulation material (LCM)
-
Borehole Stress AnalysisStresses at vertical impermeable borehole wall (based onlinearly elastic rock and isotropic horizontal stresses):
0 1 2 3 4 5 6 7 8 9 10
r/R
S
t
r
e
s
s
e
s
z=v
Borehole wall
pw
r
h
pf
2r w
h w
z v
pp
== =
Impermeable wall:
Perfect mud cake
During drillout in shale
Case a: z r > >
-
Borehole Stress Analysis
Case b: z r > >
0 1 2 3 4 5 6 7 8 9 10
r/R
S
t
r
e
s
s
e
s
r
z=v
hpw
pf
-
Borehole FailureAnalysis Borehole stresses + Mohr-Coulomb
failure criterion Minimum permitted well pressure to
prevent shear failure at borehole wall(hole collpase)
20( )
,min 2
2 (tan 1)tan 1
h faw
C pp
+ = +2
0( ),min 2
(tan 1)tan
v fbw
C pp
+ =or min minw wp gD=
' ' 21 0 3 tanC = +
Minimum mud weight
-
Borehole Failure Analysis
Tensile failure at the borehole wall mayoccur at high well pressure:
Tensile failure may also occur at lowwell pressure (in underbalance):
'0T = ,max 02fracw h fp p T= +
,,min 0
rad tensw fp p T= ' 0r T =
-
The Mud Weight Window
Minimum mud weight Hole collapse in shale (shear failure case a or b) Radial tensile failure in shale Pore pressure (in case underbalanced drilling is
prohibited) Maximum mud weight h (minimum horizontal stress) in case of pre-
existing natural fracturesFracturing of borehole wall
-
Vertical borehole in anisotropic horizontal stress field
2( )cos 22 ( )cos 2
r w
H h H h w
z v H h
pp
== + =
h HH
h
-
Boreholes in anisotropic stress fields If hole axis is parallel to a principal stress, then
we can use the borehole stresses for a verticalhole also for horizontal holes, but we need to rotate the coordinate system first.
In general: Holes are most stable towards shearfailure initiation when drilled along a directionwith low stress anisotropy and with low stress level in the plane perpendicular to it.
Deviated holes are usually less stable because ofshear stresses at the borehole wall.
-
Borehole Stability
so far elastic behaviour + brittle failure
But: Note the following field observations:
Boreholes are often stronger than predictedby elastic+brittle theory
Hole collapse is often time-delayed ( days) with respect to drill-out
Oil-based mud gives better stability thanwater-based mud
Addition of salt (in particular K+) mayimprove stability
-
Borehole Stability: Plasticity
Stress-strain curves for
( Elastic-brittleElasto-plastic )
material behaviour
Elastic-brittle: No load-bearing capacity after
failure initiation Elasto-plastic: Can still sustain load after failure
initiation Borehole failure criterion can be specified by a critical amount of plastic
strain, or by a critical extent of a plastic zone Using the elastic-brittle equations with an upscaled strength gives
acceptable results
-
Borehole Stability: Time-delayed borehole failure
Mechanisms:CreepConsolidationCoolingChemistry
time
s
t
r
a
i
nCreepThermally activited process onatomic / molecular level. Rocks may creep to failure.
-
Time-delayed borehole failure:Consolidation Establishment of pore pressure equilibrium takes
time Time constant is given by shale permeability
nanoDarcy week Borehole stresses from mechanical + fluid flow
equilibrium After steady state has been reached (vertical hole;
isotropic horizontal stresses):
( )( )
0
0
1 21
1 21
2 w f
w f
f
w
h w
z v
w
r
p p
p p
p
p
p
p
+ ==
=
+ =
-
Time-delayed borehole failure:Consolidation
0 1 2 3 4 5 6 7 8 9 10
Distance from Borehole Centre r/R
B
o
r
e
h
o
l
e
S
t
r
e
s
s
e
s
[
M
P
a
]
Tangential stress
Radial stress
Axial stress
Pore pressure (permeable wall)
Pore pressure (impermeable wall)
Permeable vs. Impermeable wall
Well pressure
0
5
10
15
20
25
0 10 20 30 40 50
Normal Stress [MPa]
S
h
e
a
r
S
t
r
e
s
s
[
M
P
a
]
Mohr-Coulomb Shear Failure criterion
Impermeable borehole wall (t=0)Permeable
borehole wall (t= )
-
Time-delayed borehole failure:Cooling The drilling fluid is usually (at t=0) colder than the
formation to be drilled. Thermal stresses at borehole wall:
Leads to improved stability wrt collapse, but enhancesrisk of fracturing (and mud losses).
The thermal stress contribution may be very significant! Temperature equlibrates with surroundings over time, so
improved stability is temporary.
( )( )
21
1
T
r w
h w f
z wv T
w
f
E T T
E T
p
T
p
==
+
=
+
-
Chemistry: Mud composition and Time effects on borehole stability
Water-based drilling fluids: Osmosis: Shale is thought of as a semipermeable
membrane, only permitting water movement Ions do move through shales and interact through
e.g. ionic exchange Oil-based drilling fluids:
Capillary support Osmosis; oil phase acting as a membrane?
-
Mud Chemistry: Osmosis
Shale
Ionic exchange
ions
water
High salinity drilling fluid
Shale
Ionic exchange
ions
water
High salinity drilling fluid
Osmotic potential acts like an excess pore pressure:
is reduced by membraneefficiency
-
Mud Chemistry: Osmosis Borehole stresses in presence of an osmotic
potential:
This is a temporary effect: As time goes, equilibrium will be established, and thechemical effect removed.
One conclusion from this: The more salt, thebetter!
1 21
1
2
21
r w
h w
z v
p
p
+ +
=
=
=
-
Mud Chemistry: Ionic exchange
Overbalance and exposure
-2.0
-1.0
0.0
1.0
2.0
3.0
0 20 40 60 80 100 120 140 160 180 200 220Overbalance and exposure time [h]
A
x
i
a
l
s
t
r
a
i
n
[
m
S
t
r
a
i
n
]
Deionised water
20wt% NaCl
20wt% KCl
16wt% CaCl 2
5wt% KCl
3.5wt% NaCl5wt% CaCl 2
Seawater
We observe: Ion type dependent shrinkage (K>Na>Ca); not explained by osmosis
Expectedosmoticshrinkage:
~ 0.5 milliStrain for allstrongelectrolytes
Tertiary fieldshale
Overbalance and exposure
-2.0
-1.0
0.0
1.0
2.0
3.0
0 20 40 60 80 100 120 140 160 180 200 220Overbalance and exposure time [h]
A
x
i
a
l
s
t
r
a
i
n
[
m
S
t
r
a
i
n
]
Deionised water
20wt% NaCl
20wt% KCl
16wt% CaCl 2
5wt% KCl
3.5wt% NaCl5wt% CaCl 2
Seawater
We observe: Ion type dependent shrinkage (K>Na>Ca); not explained by osmosis
Expectedosmoticshrinkage:
~ 0.5 milliStrain for allstrongelectrolytes
Tertiary fieldshale
From SINTEFs Shale research project
-
Mud Chemistry: Ionic exchange Experiments indicate that shales when exposed to
KCl: shrink because of ionic exchangeKCl promotes hole
stability up to a limit ( mud cooling) become more plastic KCl promotes hole stability possibly get reduced strength KCl reduces hole
stability become more permeable KCl reduces hole stability
Ionic exchange requires ionic transport into shale; thus a time-delayed effect.
All in all: An optimum salt concentration required.
-
Mud Support Capillary support by non-wetting fluid (e.g. oil):
: surface tension; oil-water = 5010-3 N/m r: pore size; rshale 10 nm pc ~ 10 MPa
So: An overbalance of 10 MPa is required for oil to penetrate into an intact shale. Since there is always an impermeable membrane, the t=0 stability will prevail.
Oil-based muds are not pure oils (so chemical effectscould play a role; cf. osmosis)!
2cp r
=
-
Mud Support
Mud additives that could lead to impermeable borehole wall:
Sodium silicate! Long-chained molecules (glycol; polymers
etc) that are thought to build molecuylarfilter cakes on shale surface.
-
Borehole stress fieldBorehole wall boundaryconditions
Mud cake efficiencyOsmotic membrane efficiency
Mud vs rock temperature
Time dependence
PermeabilityIonic diffusivity & exchange
Thermal diffusivity
Poro-chemo-thermo-elasticity (+plasticity)
Borehole failure criterionShear failure
(/plastic strain limit)/ Tensile failure
Minimum permitted mud weightGuidelines on mud composition & temperature
Maximum permitted mud weightGuidelines on mud composition & temperature
Rock propertiesElastic (& plastic) parameters
Strength parameters
Earth stressesVertical & horizontal stresses
Pore pressure
Well trajectoryHole deviation angle & azimuth
Borehole Stability Analysis
-
Borehole Stability Challenges
Drilling in tectonic areas (ex: SouthAmerica)
Fractured and faulted rock Depleted reservoirs Deep water
Borehole Stability during DrillingBorehole Stability ProblemsTight hole / Stuck pipeTight hole / Stuck pipeTight hole / Stuck pipe DiagnosisLost circulation / Mud lossesBorehole Stress AnalysisBorehole Stress AnalysisBorehole Failure AnalysisBorehole Failure AnalysisThe Mud Weight WindowVertical borehole in anisotropic horizontal stress fieldBoreholes in anisotropic stress fieldsBorehole StabilityBorehole Stability: PlasticityBorehole Stability: Time-delayed borehole failureTime-delayed borehole failure:ConsolidationTime-delayed borehole failure:ConsolidationTime-delayed borehole failure:CoolingMud Chemistry: OsmosisMud Chemistry: OsmosisMud Chemistry: Ionic exchangeMud Chemistry: Ionic exchangeMud SupportMud SupportBorehole Stability Challenges