Reservoir Geomechanics

Reservoir Geomechanics  In situ stress and rock mechanics mechanics applied applied to reservoir reservoir processes processes

Mark D. Zoback Professor of Geophysics

Week 1 – Lecture 2 The Tectonic Stress Field

Outline Section 1 • Basic Definitions •  Anderson s Stress Classification Scheme • Stress Orientations Near Salt Domes ’

Section 2 • Stress Magnitudes at Depth • HW -1 Calculating S v Section 3 • Measuring In-Situ Stress • Regional Stress Patterns • Local Stress Perturbations 2

Outline Section 1 • Basic Definitions •  Anderson s Stress Classification Scheme • Stress Orientations Near Salt Domes ’

Section 2 • Stress Magnitudes at Depth • HW -1 Calculating S v Section 3 • Measuring In-Situ Stress • Regional Stress Patterns • Local Stress Perturbations 2

Figure 1.1 – pg.6 

3

Components of a Geomechanical Model Principal Stresses at Depth Sv – Overburden SHmax – Maximum horizontal principal stress Shmin – Minimum horizontal horizontal principal stress

Sv

 Additional Components Components of a Geomechanical Model UCS Pp

Shmin

SHmax

Pp – Pore Pressure UCS – UCS – Rock Strength Strength (from logs) Fractures and Faults (from Image Logs, Seismic, etc.) 4

Zoback and Zoback (1980, 1989) 5

Generalized World Stress Map 180

270

0

90

180

70

70

35

35

0

0

SHmax  in compressional domain

-35

-35

SHmax  and Shmin in strike-slip domain Shmin in extensional domain

180

270

0

90

180 9-2

6

Complex Stress State Surrounding Salt Domes

7

Salt Bodies in the Gulf of Mexico

Figure 1.10a – pg.25 8

Schematic Stress Contours

Figure 1.10b – pg.25 9

 Anderson Classification of Relative Stress Magnitudes Sv

Normal

Shmin SHmax a.

Strike-Slip

Sv > SHmax > Shmin Sv

SHmax Shmin SHmax > Sv > Shmin Sv Reverse

b.

Tectonic regimes are defined in terms of the relationship between the vertical stress (S v) and two mutually perpendicular horizontal stresses (SHmax and Shmin)

SHmax Shmin c.

SHmax > Shmin > Sv 10

Relative Stress Magnitudes and Faulting Regimes

Regime /Stress

S1

S2

S3

Normal

Sv

SHmax

Shmin

Strike-Slip

SHmax

Sv

Shmin

Reverse

SHmax

Shmin

Sv

Table 1.1 – pg.8

11

 Anderson 1 – Faulting Styles

Sv

Normal

SHmax

Shmin

"

Shmin

! hmin

SHmax a.

Strike-Slip

Sv > SHmax > Shmin Sv

Normal

! v

SHmax

Shmin

X

SHmax Shmin Strike-slip

SHmax > Sv > Shmin Sv Reverse

b.

SHmax

Shmin

! Hmax

SHmax Shmin c.

SHmax > Shmin > Sv

Reverse Map View

! v

Cross-section

Stereonet 12

 Anderson 1 – Faulting Styles

Normal

Sv

SHmax

Shmin

"

Shmin

Geologic Structures Reflect Either Normal Sv > SHmax > Shmin a. Past Sor Current SStressS Fields (or Both) Strike-Slip but X S In This Class We are Almost Always S Strike-slip b. Going SHmax v > Shmin to > Sbe Interested in the Current Stress State ! hmin

SHmax

! v

v

hmin

Hmax

Shmin

SHmax

Hmax

hmin

Reverse

Sv

! Hmax

SHmax Shmin c.

SHmax > Shmin > Sv

Reverse Map View

! v

Cross-section

Stereonet 13

Figures 5.1 a-d – pg.141

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Limiting cases Radial Extension Sv  >> Shmax = Shmin

Anderson’s Classification Normal faulting (NF) Sv  > Shmax  > Shmin

Intermediate cases Normal/Strike-Slip faulting Sv  Shmax  > Shmin =

Strike-slip faulting (SS) Shmax  > Sv  > Shmax 

Strike-Slip/Reverse faulting Shmax  > Sv = Shmin

Reverse faulting (RF) Shmax  > Shmin > Sv  Radial Compression Shmax  =Shmin >> Sv 

15

Variations of Regional Stress Magnitudes

Hurd and Zoback (in press)

16

Stress Orientations in North America

17

 Anderson 2 – Classification of Relative Stress 180

270

0

90

180

70

70

35

35

0

0

SHmax  in compressional domain

-35

-35

SHmax  and Shmin in strike-slip domain Shmin in extensional domain

180

270

0

90

180 9-2

18

Outline Section 1 • Basic Definitions • Anderson s Stress Classification Scheme • Stress Orientations Near Salt Domes ’

Section 2 • Stress Magnitudes at Depth • HW -1 Calculating S v Section 3 • Measuring In-Situ Stress • Regional Stress Patterns • Local Stress Perturbations 19

 Anderson 3 – Critically Stressed Faults  As much of the crust is in a state of failure equilibrium, stress magnitudes can be estimated (Chapter 4)

Earthquakes Triggered by Reservoir Impoundment 20

Limiting cases Radial Extension Sv  >> Shmax = Shmin

Anderson’s Classification Normal faulting (NF) Sv  > Shmax  > Shmin

Intermediate cases Normal/Strike-Slip faulting Sv  Shmax  > Shmin =

Strike-slip faulting (SS) Shmax  > Sv  > Shmax 

Strike-Slip/Reverse faulting Shmax  > Sv = Shmin

Reverse faulting (RF) Shmax  > Shmin > Sv  Radial Compression Shmax  =Shmin >> Sv 

21

Range of Stress Magnitudes at Depth Hydrostatic Pp

Figure 1.4 a,b,c – pg.13 22

Range of Stress Magnitudes at Depth Overpressure at Depth

Figure 1.4 d,e,f – pg.13 23

Calculating the Vertical Stress, S v z

SV(z) =

! !(z)gdz ~ !- gz 0

Considering water depth in offshore areas

z

Sv(z) = !wgzw +

! !(z)gdz ~ ! gz  + !g(z-z ) 0

-

w

w

w

 Equation (1.6) – pg. 9

24

Visund Field, Northern North Sea

25

 Anderson Classification of Relative Stress Magnitudes Sv

Normal

Shmin SHmax a.

Strike-Slip

Sv > SHmax > Shmin Sv

SHmax Shmin SHmax > Sv > Shmin Sv Reverse

b.

SHmax

Hydraulic Fractures  Always Propagate Perpendicular to the Least Principal Stress, S3 In 1948, HF Orientations Were Hotly Debated, Were They Horizontal, Vertical, Radial?

Shmin c.

SHmax > Shmin > Sv 26

 Anderson Classification of Relative Stress Magnitudes Sv

Normal

Shmin SHmax a.

Strike-Slip

Sv > SHmax > Shmin Sv

SHmax Shmin

Hydraulic Fractures  Always Propagate Perpendicular to the Least Principal Stress, S3 What Happens when Shmin ~ Sv?

SHmax > Sv > Shmin Sv Reverse

b.

(SS/RF Stress Field) SHmax Shmin c.

SHmax > Shmin > Sv 27

Visund Field, Northern North Sea

28

Determining Overburden Stress from Density

Figure 1.3 – pg.11

29

Outline Section 1 • Basic Definitions • Anderson s Stress Classification Scheme • Stress Orientations Near Salt Domes ’

Section 2 • Stress Magnitudes at Depth • HW -1 Calculating S v Section 3 • Measuring In-Situ Stress • Regional Stress Patterns • Local Stress Perturbations 30

Horizontal Principal Stress Measurement Stress Orientation Stress-induced wellbore breakouts (Ch. 6) Stress-induced tensile wall fractures (Ch. 6) Hydraulic fracture orientations (Ch. 6) Earthquake focal plane mechanisms (Ch. 5) Shear velocity anisotropy (Ch. 8) Relative Stress Magnitude Earthquake focal plane mechanisms (Ch. 5) Absolute Stress Magnitude Hydraulic fracturing/Leak-off tests (Ch. 7) Modeling stress-induced wellbore breakouts (Ch. 7, 8) Modeling stress-induced tensile wall fractures (Ch. 7, 8) Modeling breakout rotations due to slip on faults (Ch. 7)

Table 1-2 p. 15

31

Visund Field Orientations

32

Stress Map of Northern North Sea

Figure 1.8 – pg.23 33

Ice Retreat Model

34

Modeled Shmin/Sv Compared to Observations

Figure 9.3 – pg. 272 35

Visund Field Orientations

36

Modeled SHmax Directions Compared to Observations

37

Seismotectonics of Northern S. America

38

Stress State in Northern S. America

*Light blue arrow indicates relative motion of the Costa Rica-Panama block with respect to the central North Andean  block

 Fig. 1.9 – p. 24

39

Regional Stress in Western California

 Fig. 6.8a,b p. 182

40

Stress Map of Southern San Joaquin

Figure 1.6 – pg.21 41

Southern San Joaquin Valley

42

Complex Stress Field in the Elk Hills Field

43

Stress Orientation Rotation Near Fault at 3100 meters

44

Modeling Fault-Induced Stress Perturbation at the Wellbore Wall

45

Multi-Scale Stress Perturbations

 Figure 11.10 – pg. 359

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