Topics in Energy and Environmental Geomechanics – Enhanced ...
Transcript of Topics in Energy and Environmental Geomechanics – Enhanced ...
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Topics in Energy and Environmental Geomechanics –Enhanced Geothermal Systems (EGS)
Lecture 3. Borehole Geomechanics(16, 23 Sept 2013)
Ki-Bok Min, PhD, Associate ProfessorDepartment of Energy Resources Engineering
Seoul National University
Imaage log and borehole breakout at Pohang H-5 hole
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IntroductionContents of the course
– Week 1 (2 Sept):Introduction to the course/Climate change & Emerging Subsurface Eng Application
– Week 2 (9 Sept): Fundamentals of Geomechanics– Week 3 (16 Sept):Borehole Stability– Week 4 (23 Sept): Borehole Stability– Week 5 (30 Sept): No lecture (business trip)– Week 6 (7 Oct) : Hydraulic Stimulation (focus on Hydraulic fracturing)– Week 7 (14 Oct): Hydraulic Stimulation– Week 8 (21 Oct): Induced seismicity– Week 9 (28 Oct): Induced seismicity
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IntroductionContents of the course
– Week 10 (4 Nov): Drilling Engineering (invited lecture)– Week 11 (11 Nov): Well logging (invited lecture)– Week 12 (18 Nov): EGS Case studies– Week 13 (25 Nov): EGS Case studies– Week 14 (2 Dec): Student Conference– Week 15 (9 Dec): Final Exam (closed book or take-home exam)
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outline
• Motivation for borehole geomechanics– Importance
• Theoretical background – Stress/strain, constitutive relation, stress equilibrium equation,
governing equation– Strength and Failure criteria– Stresses around a borehole
In situ stress, injection pressure, pore pressure, thermal stress
Elastic analysis, Elastoplastic analysis
Stresses in hollow cylinder
Stresses in deviated borehole
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outline
• Borehole stability Analysis– Borehole breakout– Stress estimation from borehole observation– Drilling induced tensile failure– Borehole stability problems
Tight hole/stuck pipe
Lost circulation/mud losses
– Stability during drillingMud weight window
– Other topics
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Borehole Stability ProblemsDrilling operation
Drilling equipment and mud circulation (Vincent, 2006)
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Borehole Stability ProblemsDrilling operation
(Vincent, 2006)Well Plan in Pohang EGS site
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Borehole stability problem
• Importance?– Borehole instability cause substantial problems – can cause 5-10%
of drilling cost. Mainly mechanical collapse– Instability usually occur in shale or mudstone.– Demand ↑ for more sophisticated well trajectories – highly
deviated, horizontal, deep wells. – Environmental impact due to lost circulation– Safety issue too from kick/borehole blow out in petroleum industry
Various well profiles (Vincent, 2006)
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Borehole stability problem
• Factors– In situ stress– Injection pressure or mud weight– Reservoir rock mechanics properties– Anisotropy of rock properties– Thermal effect– Chemistry (especially shale/mudstone)
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Borehole Stability problem
• Borehole problem is problematic because (Fjaer et al., 2008);– Drill bit may be > 1,000s m away, visual observation not possible
(cf. tunneling excavation)– Large variation in reservoir in stress (depleted reservoir/non-
depleted shale, drilling through faults). In situ stress measurement difficult
– Large variation/uncertainty in formation properties. Coring is costly.– Many mechanisms contribute; mud chemistry, stress, temperature,
pore pressure…– Operational condition is complex
• Sound understanding on theory and insightful engineering judgment should be combined.
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Borehole Stability Problemsnature of the problem
Rock cutting from Pohang EGS site. ~few mm
One of the biggest rock core in the world at AECL URL in Canada (2002). ~ 1m
REALITY
DREAM
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Borehole stability problemWork Flow chart
Fjaer et al., 2008, Petroleum related rock mechanics, 2nd ed., Elsevier
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Borehole stability problemExample
• Extended Reach Drilling (ERD) hasbeen employed for increasing oilrecovery.
• Total Depth = 9,327 m
• Since 1979, total depth for wells hasincreased steadily.
Oseberg in North Sea (Norway)
Okland & Cook, SPE, 1998
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Borehole stability problemExample
Loaded and unloadedat approximately 20 MPa
a) Draupne-similar Shale b) J2 (Outcrop shale)External Pressure= 13.6 MPa
External Pressure= 32.2 MPa
Severely Damaged!!
Undamaged
Okland & Cook, SPE, 1998
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Some Mathematical Equations that capture the important physics
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Borehole stability problemEffect of internal pressure
• Increase of internal mud/hydraulic pressure
2
2r wRPr
2
2wRPr
pw
Tangential stress접선응력
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Borehole stability problemElastic Stress distribution
R: 보어홀의반경r: 보어홀중심에서반경방향의거리
θ: SH,max으로부터반시계방향으로측정
SH,max , SH,max: 최대및최소수평응력
2 2 4max min max min
2 2 44 31 1 cos 2
2 2H h H h
rS S S SR R R
r r r
2 4max min max min
2 431 1 cos 2
2 2H h H hS S S SR R
r r
2 4max min
2 42 31 sin 2
2H h
rS S R R
r r
σθ
σr
τrθ
Sh,min
SH,maxR
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Borehole stability problemElastic Stress distribution
σy
σy
-σy
3σy σx
3σx
-σx σx+ =σx
-σy+3σx
3σy-σx
σx
σy
σy
Under uniaxial stress condition
Maximum Stress concentration 3
Minimum Stress concentration -1
Biaxial Stress condition:
By superposition
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Borehole stability problemElastic Stress distribution
• The area of stress disturbance is within 2~3 times of borehole radius
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Borehole stability problemGeneralized Kirsch’s solution
• Boundary stress + internal pressure + temperature;– 응력경계 (Principal in situ stress boundary)– 내부주입압력 (Internal pore pressure (mud/water pressure))– 온도변화 (Temperature change)
2 2 4max min max min
2 2 4
2
24 31 1 cos 2
2 2H h H h
wrS S S SR R R
r rRP
r r
2 4
max min max min2 4 0
2
231 1 co
1s 2
2 2H h
wH h
wS S S SR R
r rRP Tr
E T
2 4max min
2 42 31 sin 2
2H h
rS S R R
r r
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Borehole stability problemGeneralized Kirsch’s solution
– At the borehole wall (r = R), maximum and minimum hoop stresses are;
– Without considering temperature change,
,min min m 0ax 13 wwh H P E TS S T
,max max m 0in 13 wwh h P E TS S T
,min min max3 wh HS S P
,max max min3 wh hS S P maxHS
minhS
maxHS
minhS
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Borehole stability problemSolution in hollow cylinder
PiPo
a b
2 2 2 2
2 2 2 2 2( ) ( )
( ) ( )o i i o
rb P a P a b P P
b a b a r
2 2 2 2
2 2 2 2 2( ) ( )
( ) ( )o i i ob P a P a b P P
b a b a r
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Borehole stability problemSolution in hollow cylinder
Stress distribution in a hollow cylinder with b = 2a (J,C & Z, 2007)
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Borehole stability problemSolution in hollow cylinder
NTNU & SINTEF short course (2012)
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Borehole stability problemSolution in hollow cylinder - elastoplastic
0 ln / 1r a
0 ln /r r a
a r r b
20 1 2ln / /2r a r
20 1 2ln / /2
a r
2 0
0
21 2ln / / Pa b
0Failure criterion: r
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Borehole stability problemSolution in hollow cylinder - elastoplastic
1 0 3 0 31 sin
Failure criterion: 2 tan1 sin
C q S
20 0/ 2 / / 2r iP C r a C
20 03 / 2 / / 2iP C r a C
20 0 0
1 / 2 /2r P P C r a
20 0 0
1 / 2 /2
P P C r a
r b a r
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Stress around a circular holeAnisotropy
• Some formulae considering anisotropy (internal pressure, uniaxial stress)
Lekhnitskii, 1963
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Stress around a circular holeAnisotropy
• 탄성계수의이방성에따라응력집중의정도가달라짐.
3P-P
Isotropic rock(E/Eʹ= 1)
Transversely isotropic rock
(E/E ʹ = 2)
P P
Transversely isotropic rock
(E/E ʹ = 3)
P3.3P-0.7P
3.6P-0.6P
0 30 60 90-1
0
1
2
3
4y
σθθ (E/E'=1) σθθ (E/E'=2) σθθ (E/E'=3)
angle (o)
S
y
Kim H, 2012, MS thesis SNU
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Stress around a circular holeAnisotropy
zy
x
P
P
1.8
2.0
2.20
30
60
90
120
150
180
210
240
270
300
330
1.8
2.0
2.2
o
o
o
o
o
o
o
o
o
o
o
o
Nor
mal
ized
tang
entia
l stre
ss,
P
E/E'=1 E/E'=2 E/E'=3
Kim H, 2012, MS thesis SNU
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Stress around a circular holeAnisotropy
• 어떤파괴조건식을적용하느냐에따라파괴범위가다르게나타남
0
30
60
90
120
150
180
210
240
270
300
330
o
o
o
o
o
o
o
o
o
o
o
o
o
등방성파괴조건식
이방성파괴조건식
Kim H, 2012, MS thesis SNU
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Some Important Phenemona that are encountered around boreholes
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Borehole stability problemborehole breakout vs. hydraulic fracturing
– Required internal pressure to induce tensile stress (neglect T effect);
– …. To induce hydraulic fracturing
– Required uniaxial compressive strength not to have borehole breakout (if pw=0)
min max3w h HP S S
max min3c h hS S
maxHS
minhS
maxHS
minhS
Tensile stress/hydraulic fracturing
Borehole breakout
Borehole breakout
Tensile stress/hydraulic fracturing
min ax 0m3 h HwP S TS
Assumption: Impermeable reservoir & impermeable borehole wall
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Borehole stability problemborehole breakout
– wellbore enlargements caused by stress-induced failure of a well occurring 180 degree apart
– Induced by compressive (shear) failure
– Occur in the direction of minimum horizontal stress
Zoback, 2007
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Borehole stability problemborehole breakout (Pohang EGS site)
Borehole breakout (670 – 900 m) by borehole acoustic scanner
Drilling induced fractures (DIFS) (770 – 810 m)
Borehole breakout
DIFS
Hydraulic Fracturing
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Borehole stability problemborehole breakout – rock spalling
• Similar observation can be found in underground construction
V notched failure due to high in situ stress(400 m, Winnipeg, Canada, Chandler, 2004)
Winnipeg, Canada (Min, 2002)
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Borehole Stability ProblemsTight hole/stuck pipe
• Tight hole/stuck pipe– Hole collapse
Increased borehole size due to brittle failure and caving of the wellbore. Stuck because of accumulated cavings (“sloughing shale”)
Reduced borehole size by plastic deformation – gumbo shale. Require reaming and result in a lost drillpipe
– Inappropriate hole cleaningWhen drilling cuttings (could be also from formation failure) were not fully
removed
www.wellideas.com
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Borehole Stability ProblemsTight hole/stuck pipe
– Differential stickingConditin in which Drilling string cannot
be moved along the axis of the borehole
Caused by overpressure in the hole
Usually in the permeable formation
– Deviation from ideal trajectoryDogleg and keyseat problem
Differential sticking and key seat (Schlumberger oilfield glossary)
Dogleg: a particularly crooked place in a wellbore where the trajectory of the wellbore in three-dimensional space changes rapidly (schlumberger)
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Borehole Stability ProblemsTight hole/stuck pipe
• Drill pipe wash out (Pohang EGS site, 2013)
Min (2013)
PMC (2013)
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Borehole Stability ProblemsTight hole/stuck pipe
Min (2013)
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Borehole Stability ProblemsTight hole/stuck pipe
• Consequencese of tight hole/stuck pipe– Loss of time ( reaming or sidetracking needed)
– Difficulty in wireline logging
• Good well design necessary– Mudweight and composition– Casing setting depth– Well trajectory– Diagnostic analysis needed
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Borehole Stability ProblemsLost circulation/mud loss
• Lost circulation/mud loss– Significant drilling fluid is lost into the formation – e.g., through
(new or existing) fractures. – Can cause temperary pressure drop flow into the well from
permeable layers higher up + gas lead to kick, and blow-out
– Problem: mud is expensive, mud is limited at the site, safety issue.– Solution: keep the mud weight sufficiently low, use prevention
additive (Lost Circulation Material, LCM)
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Borehole Stability ProblemsMud Weight Window (Fjaer et al., 2008)
• Static mud pressure
– Mud pressure in the well is usually higher than the static pressure above in the order of 5-10%.
• Equivalent Circulating Density (ECD): static + pressure diffence effect
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Borehole Stability ProblemsMud Weight Window (Fjaer et al., 2008)
• Minimum mud weight– Shear failure
– Radial tensile failure
– Pore pressure (when underbalanced drilling is prohibited)
• Maximum mud weight– Minimum horizontal stress (with preexisting natural fractures)– Fracturing of borehole wall
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Borehole stability problemMud Weight Window (Fjaer et al., 2008)
Condition for shear failure in vertical boreholes (Fjaer et al., 2008)
Condition for tensile failure (Fjaer et al., 2008)Assumption: Permeable reservoir & impermeable borehole wall
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Borehole Stability ProblemsMud Weight Window
• Effect of Increased of mud weight
Dusseault, 2012
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Borehole Stability ProblemsMud Weight Window
Itasca Short Course, 2011
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Borehole Stability ProblemsMud Weight Window (Fjaer et al., 2008)
pore pressure
collapse gradient
Mud weight
Vertical stresshorizontal stress
Fracturing gradient
Fracturing gradient: stress required to induced fractures with the increase of depth, usually psi/ft, Schulumberger dictionary
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Borehole Stability ProblemsMud Weight Window (Fjaer et al., 2008)
• ppg: usually used for unit of mud density. Abbreviation for density, pounds-per-gallon. Ex) density of water is ~8.33 ppg.
Australian Drilliing Industry Training Committee Limited, 1997, Drilling –The manual of methods, applications, and management, CRC Press
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Borehole Stability ProblemsTime-delayed failure (Fjaer et al., 2008)
• Poroelastic time dependent effect
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Borehole Stability ProblemsTime-delayed failure (Fjaer et al., 2008)
• Pore pressure near the wellbore will increase resulting in reduced hole stability with time. Time to reach pore pressure equilibrium (lD: ~10 cm, k 1 nD, ~5-10 days)
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Borehole Stability ProblemsTime-delayed failure
• Creep can be also important
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Borehole stability problemThermal stress
• Thermal stress can be significant
• Cold borehole fluid strengthen the borehole & higher possibility of tensile failure
• Cooling reduces pore pressure too– Why?– Thermal expansion coef. Fluid > rock
• Cooling mud weight increase
,
,
0
0
1
1
w
w
T
z T
E T T
E T T
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Borehole stability problemThermal stress
• Downhole : Cooling stabilize• Uphole: Heating can induce failure
Dusseault, 2012
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Borehole stability problemBorehole Trajectory
• Drilling trajectory (최적시추궤적결정)
– Determine the trajectory that can induce the least stress concentration
Dusseault, 2012
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Presentation
• Thermal effect• Pore pressure effect• Chemical effect• Borehole breakout (width, depth, issues?)• Borehole stability and stress estimation• Deviated borehole• Core discing
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References
• Main references: – Fundamentals of Rock Mechanics (Jaeger, Cook & Zimmerman, 2007), ch.8,
ch.9.1 – 9.4
– Petroleum related Rock Mechanics (Fjaer et al., 2008), Ch.4, ch.9
• Secondary references:– Reservoir Geomechanics (Zoback, 2007), ch.6, ch.8, ch.10
– Geomechanics for Geophysicists, short course materials by SINTEF & NTNU (2012)
– Unconventional Geomechanics, short course by Itasca Consulting Group (2011)
– Vincent M, Stress and borehole stability, Revue Européenne de Génie Civil 2006 10(6-7): 763-801