topics in energy and environmental geomechanics – enhanced

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

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

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)

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

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

Borehole Stability ProblemsDrilling operation

Drilling equipment and mud circulation (Vincent, 2006)

Borehole Stability ProblemsDrilling operation

(Vincent, 2006)Well Plan in Pohang EGS site

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)

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)

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.

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

Borehole stability problemWork Flow chart

Fjaer et al., 2008, Petroleum related rock mechanics, 2nd ed., Elsevier

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

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

Some Mathematical Equations that capture the important physics

Borehole stability problemEffect of internal pressure

• Increase of internal mud/hydraulic pressure

2

2r wRPr

2

2wRPr

pw

Tangential stress접선응력

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


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


• The area of stress disturbance is within 2~3 times of borehole radius

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

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

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


Stress distribution in a hollow cylinder with b = 2a (J,C & Z, 2007)


NTNU & SINTEF short course (2012)

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

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

Stress around a circular holeAnisotropy

• Some formulae considering anisotropy (internal pressure, uniaxial stress)

Lekhnitskii, 1963


• 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


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



• 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


Some Important Phenemona that are encountered around boreholes

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

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

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

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)

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


– 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)


• Drill pipe wash out (Pohang EGS site, 2013)

Min (2013)

PMC (2013)


Min (2013)


• 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

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)

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


• 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

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

Borehole Stability ProblemsMud Weight Window

• Effect of Increased of mud weight

Dusseault, 2012

Borehole Stability ProblemsMud Weight Window

Itasca Short Course, 2011


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


• 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

Borehole Stability ProblemsTime-delayed failure (Fjaer et al., 2008)

• Poroelastic time dependent effect

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)

Borehole Stability ProblemsTime-delayed failure

• Creep can be also important

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

Borehole stability problemThermal stress

• Downhole : Cooling stabilize• Uphole: Heating can induce failure

Dusseault, 2012

Borehole stability problemBorehole Trajectory

• Drilling trajectory (최적시추궤적결정)

– Determine the trajectory that can induce the least stress concentration

Dusseault, 2012

Presentation

• Thermal effect• Pore pressure effect• Chemical effect• Borehole breakout (width, depth, issues?)• Borehole stability and stress estimation• Deviated borehole• Core discing

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

topics in energy and environmental geomechanics – enhanced

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