stress characterization - petroleum geomechanics...nov 30, 2017 · alberta. he has a basc in civil...

© petroleum geomechanics inc.

Stress Characterization Mr. Patrick M. Collins, BASc, MSc, PEng

Petroleum Geomechanics Inc., CalgaryAbstract:

Rock stresses control how our reservoirs will respond to stimulation and production, whether they be in tight formations or oilsands and their surrounding formations. The accurate determination of rock stresses can be essential to a successful recovery process; their inaccurate determination can be catastrophic. The intent of this presentation is that attendees will become cognizant of the geomechanics that constrain stresses in their own reservoirs to within credible physical limits that are consistent with the geology and our operations, such that the rock stresses are accurate and recovery processes successful.

All rock is stressed. These stresses result from the rock’s depositional history, geomorphology, chemical and physical alterations such as tectonics, and lastly, by the changes imposed by us as we stimulate and produce the reservoir. Of these, the pre-existing rock stresses before any human intervention are the dominant stresses that affect the performance of the reservoir, whether it be hydraulic-facture stimulation or high-pressure injection of steam or solvents.

This presentation will review how rock stresses are developed, how they are altered by natural and man-made changes, and how they are determined. Examples from western Canada will be provided as examples of what one should expect in typical reservoirs.

Presenter’s Biography

Patrick Collins, P.Eng. is the president of Petroleum Geomechanics Inc., based in Calgary, Alberta. He has a BASc in Civil Engineering from the University of Toronto and an MSc in Geotechnical Engineering from the University of Alberta.

After working on the seminal AOSTRA Underground Test Facility’s SAGD pilots, Patrick consulted in reservoir engineering and geomechanics in Italy and the UK for several years before returning to Canada. He has over 35 years’ experience in heavy oil, oilsands, SAGD, CSS, and CHOPS; and in geomechanics related to drilling and completions, wellbore stability, caprock integrity, minifrac tests, rock stress analysis, formation overpressures, hydraulic fracturing, and sanding.

Patrick consults internationally in heavy-oil recovery and in geomechanics, and is an expert witness in heavy oil, thermal recovery, and geomechanics. He is a member of SPE, AAPG, CWLS, CSPG, CHOA, and APEGA.

20171130 SPE Calgary’s Geomechanics Group - Breakfast Lecture Series 1

Patrick M. Collins, MSc, PEngPetroleum Geomechanics Inc.

Calgary, Alberta

Stress CharacterizationStress Characterization

2© petroleum geomechanics inc.

Outline

• how rock stresses are developed• rock’s depositional history,

geomorphology– chemical – physical alterations such as tectonics

• stress alteration– natural– man-made changes

• stress determination• examples from western Canada


Why are Rock Stresses Important?• they control how our reservoirs will respond

– drilling– stimulation– production

• accurate determination of rock stresses can be essential to a successful recovery process

• inaccurate determination can be catastrophic• Objective: you become cognizant of the geomechanics

– constrain stresses in their own reservoirs to within credible physical limits

– consistent with the geology and our operations

Rock stresses are accurate Recovery processes successful


Basic Mechanics: Scalar vs. VectorScalar quantity has only

magnitude, e.g.:• length, area, volume• speed• mass, density• pressure• temperature• energy, entropy• work, power

Vector quantity has both magnitude

and direction:

• displacement, direction• velocity• acceleration• momentum• force• lift , drag , thrust• weight• velocityvolume

veloci

ty


Basic Mechanics: Force and Stress

AreaForceStress

AF

""sigma

=

Force

Areax

y

z

3D: Normal Stress

Shear Stress in X-direction

Force

Area =

2D: Normal Stress

Shear Stress

Shear Stress in Y-direction


Basic Mechanics: Principal StressesIn 3-D, there are more surfaces and more shear & normal stresses acting on each surface:

By rotating the solid there is a unique orientation in which ALL shear stresses disappear, leaving only Normal Stresses

These are called “Principal Stresses”: 1 > 2 > 3

Usually, one principal stress is in the vertical axis, resulting in the principal stresses being:

-

vertical stress, v-

maximum horizontal stress, H-

minimum horizontal stress, h

v

H

h

1

2

3


Total Stresses vs. Effective Stresses

v

H

h

Total Stress = Effective Stress + Fluid Pressure

borne by rock matrix

′v

′H

′h

borne by fluids in the pore space

= + pf

Generally, as fluid pressure increase, effective stresses decrease-

hydraulic fracture stimulation, CSS, oilsand dilation, etc.


Stress vs. Stress GradientStress Gradient is the stress

divided by the true vertical depth (not measured depth)

e.g: 7596 kPa at a depth of 360m is 21.1 kPa/m

• be aware that most depths use the kelly

bushing

(block) as datum, mKB, instead of ground level, mGL– difference becomes

significant at shallow depths

Kelly Bushing

Ground Level~ 1.2m –

3.4m


In Situ Stresses

1.

Vertical Stress (sigV

or v

)

2.

Maximum Horizontal Stress (sigHmax

or H

)• orientation

3.

Minimum Horizontal Stress (sigHmin

or h

)4.

Formation Pressure, Pf• less important than Pinj

except for “thief zones”


Vertical StressGenerally: integrate bulk density log

dzz

Bv 0 density varies with • mineralogy• degree of compaction/consolidation: oilsands vs. deeper sandstones • porosity, e.g.: vuggy

carbonates

• offshore: depth of water, salinity, high-porosity sea-floor sediments• onshore: depth of lakes, ponds


Stress Orientations


Calliper Logs and Inferred

Wellbore Geometry

1

2

3

4 Hole In-Gauge

Breakout

Keyseat

Cal 1-3Cal 2-4

Washout

4-arm Bit Diameter

Dep

th

Undergauge


Borehole Breakout

Maximum Horizontal

Stress

Vertical Wellbore

Minimum Horizontal

Stress

H

H

h

Borehole Breakout

h Breakout Limit


Niagara Falls Spillway Tunnel Niagara Falls Spillway Tunnel --

20092009

Borehole Elongations

0

50

100

150

2000

10 2030

4050

60

70

80

90

100

110

120

130140

150160170

180190200

210220

230

240

250

260

270

280

290

300

310320

330340 350

Well 5333

Breakout Limit = 5 mm

No. of Events = 395

Well Breakouts

Azimuth (degrees)

125

150

175

200

225

250

275

300

325

350

375

400

425

450

0 90 180 270 360

Well 5333Calliper (mm)

125

150

175

200

225

250

275

300

325

350

375

400

425

450

175 200 225 250

Dept

h (m

MD)

Well 5333 Well 5333

125

150

175

200

225

250

275

300

325

350

375

400

425

450

0 50 100 150Gamma Ray

Breakout

Dep

thGR / ResistivityAzimuthCalliper

(6-arm)

Well

Res GR

Bit


0

50

100

150

200

010

2030

40

50

60

70

80

90

100

110

120

130

140150

160170

180190

200210

220

230

240

250

260

270

280

290

300

310

320330

340350

Maximum Horizontal Stress Orientation

Hmax

Borehole Breakout


Tectonic Stress in North America

www.world-stress-map.org1

Hmax


Tectonic Stress in Alberta

www.world-stress-map.org

Hmaxperpendicular to Rocky Mountains

ref. Heidbach, Oliver; Rajabi, Mojtaba; Reiter, Karsten; Ziegler, Moritz; WSM Team (2016): World Stress Map Database Release 2016. GFZ Data Services. http://doi.org/10.5880/WSM.2016.001


Tectonic Stress in Alberta

Hmax

from Borehole Breakouts

PeaceRiver Athabasca

Cold Lake

CalgaryHmax

mechanisms

Thrust fault focal


20© petroleum geomechanics inc.www.world-stress-map.org

Maximum Horizontal Stress

East Coast, Canada



Maximum Horizontal Stress

Gulf of Mexico

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwi7vITIuebXAhViwFQKHZwGAs8QjRwIBw&url=http%3A%2F%2Ffluid-venting-system.weebly.com%2Fpresentation-with-illustrations.html&psig=AOvVaw3L6Hw5QiqmF--0KcHPGQvJ&ust=1512136462887678


Stress Magnitudes


In Situ Stress State

z

Hmax

Hmin


Eaton Equation

fpfpVH

1

pfH H

ref. Eaton (1969) JPT

V


In Situ Stress: North Sea

In-Situ Stresses

Pf

Horz. Stress

Vertical

Sim. Horz. Stress

NE1

ND

NE2

RANNOCH 1,2,3

ETIVE 1 & 2

BROOM

DUNLIN

NE3NF1

T1

HEATHER

10700

10800

10900

11000

11100

11200

6750 7000 7250 7500 7750 8000 8250 8500 8750 9000 9250 9500 9750 10000 10250 10500 10750 11000 11250 11500

Stress (psi)

Dept

h (ft

TVD)

Pf

Avg. Pf

Horizontal Stress

Vertical Stress

Avg. Horz.Avg. Vertical

RFT

Horz. Stress from Lab v

Simulation Horz. Stress

Stress (psi)

Dep

th (f

tTVD

SS) V

pf

Ho


PLANNING

• Location of the Microfrac Well(s)– Geology– Stress state

• Geophysical logs– dipole sonic

• Well completion– Cased vs. uncased

• Perforating• Fracturing fluid

– salinity


Wireline

Unit & Pumper


Tests to Determine Rock Stresses

• Step-rate too coarse• Pump-in/shut-in• Pump-in/flow-back


Hydraulic Fractures1.

Hydraulic fracture stimulations, (“HF”), propped or unpropped, can be massive and are intended to enhance a well’s deliverability by:– creating flowpaths from the wellbore into the reservoir, – transecting zones of lower permeability connecting zones– intersecting natural fractures – reducing flow concentrations near the wellbore (gas)

2.

Minifracture (“minifrac”) is loosely defined – miniature non-propped hydraulic fracture, i.e. a scaled-down

treatment – “data frac”: the small, non-propped fracture performed

immediately preceding a hydraulic fracture stimulation – inject 20 -

150 m3– calibration

3.

Microfrac– no proppant– 0.1 to 100 l/min, with injected volumes of 1 to 1000 litres. – “minifrac”


Kirsch distribution of radial and tangential stresses before fracturing

(assuming uniform stress)

Tangential

Radial

Pw

Wellbore

Radius

Pres

sure h

ar

Pw

- h

Pw

- hAt the wellbore, it is the Tangential stress that falls to zero fracture

initiation should be axial (along the wellbore)away from the wellbore, it will rotate to be

perpendicular to the minimum in situ stress

r

t


Pump-in/Shut-in

Time

Formation breakdown

Fracture extension

ISIP

Fracture re-opening

Fracture closure

Pumping Rate

Pressure

Injection ~6h ~20min

~1h


Pump-in/Flow-back

FCP FCP FCPFRP FRP

Flow-back Rate

FCP = Fracture Closure PressureFRP = Fracture Re-opening Pressure

Time

Time

Pres

sure

-Rat

e +

Pump-in Rate

* Flow-back rate must

be constant!


Variable Post-Closure with Flow-back

Is it fracture closure?

Flow-back RateTime

Pres

sure

-Rat

e +

Pump-in Rate

* Flow-back introduces uncertainty but saves time in tight zones

TimeTime

?high rate

low rate


Microfracture Interval Selection

2 zones in oilsands1.

stress calibration• Comparable to mudstones

2.

transmissibility2 zones in caprock• plus additional caprock formations?


Analytical Methods1.

fracture re-opening

2. ISIP (instantaneous shut-in pressure)

3. pressure vs. square root of shut-in time

4. tandem square root or linear flow plot

5. log(pressure) vs. shut-in time

6. log(dp) vs. log (dt)

7. Horner plot

8. G-function plot

9. step-rate test

10.

pressure derivatives (detect highest frac closure)11.

compliance plot of pressure vs. flowback time


Typical Oilsand Microfrac

P vs Time

Downhole Gauge (kPaa)290m

10061

3000

4000

5000

6000

7000

8000

9000

10000

11000

-100 0 100 200 300 400

Time (minutes)

BH

P (k

Paa)


Pressure vs. Sqrt(Tshutin)

P vs TimeDownhole Gauge (kPaa)

290m

10061

3000

4000

5000

6000

7000

8000

9000

10000

11000

-100 0 100 200 300 400

Time (minutes)

BH

P (k

Paa)

P vs. Sqrt(shut-in)

7600

6380

5100

3000

4000

5000

6000

7000

8000

9000

0 5 10 15 20

Sqrt(Time)

6390 kPaa 21.7 kPag/m5100 kPaa 17.2 kPag/mMultiple frac sets are closing


Minifrac Test Cycle

P vs T imeDow nhole Gauge (kPaa)

1.144000

6000

8000

10000

12000

14000

16000

-100 0 100 200 300 400 500 600

Tim e (m inute s)

P vs. Sqrt(shut-in)

6360

79708580

5000

6000

7000

8000

9000

10000

11000

12000

0 5 10 15 20

Sqrt(Time)

G function

6270

79208840

5000

6000

7000

8000

9000

10000

11000

12000

0.0 0.4 0.8 1.2 1.6 2.0

P vs Log(Shut-in Time)

6000

81008660

5000

6000

7000

8000

9000

10000

11000

12000

-2.0 -1.0 0.0 1.0 2.0 3.0

Log-Time (minutes)

P vs. Sqrt(total time-0.5*Tshutin)

6450

79308500

5000

6000

7000

8000

9000

10000

11000

12000

3 5 7 9 11 13Sqrt(minutes)

Log(P) vs. Log(shut-in)

P= 8027P= 9639

P= 5744

100

1000

10000

0 1 2 3 4 5 6

log(shut-in time), log(seconds)

m=0.5 LinearFlow

m=1 WBstorage

(caprock)


Flow Characteristics for a Well with a Single Finite Conductivity Fracture

bi-linear flow formationlinear flow

wellbore storage

slope m=1

m=0.5

m=0.25m=0.5

m=constantWellbore

Fracture

fracture linear flow

pseudo-radial flow

Log

(pre

ssur

e dr

op)

Log (shut-in time)

fracture closes

pseudo-plateau for microfracs


wellbore storage

slope m=1

m=0.5

m=0.25m=0.5

m=constantWellbore

Fracture


pseudo-radial flow

Log

(pre

ssur

e dr

op)

Log (shut-in time)

fracture closes



wellbore storage

slope m=1

m=0.5

m=0.25m=0.5

m=constantWellbore

Fracture


pseudo-radial flow

Log

(pre

ssur

e dr

op)

Log (shut-in time)

fracture closes



Apparent Tensile Strength

Test Interval #2: 408 - 410 mKB in oilsand

cycle 2

cycle 1

cycle 3

4

6

8

10

12

0 2 4 6 8 10 12 14 16

Time (hours)

Pres

sure

at D

ownh

ole

Gau

ge, M

Paa

0

10

20

30

Tem

pera

ture

at G

auge

, °C


Thermally Induced PressuresCycle 1, Wabiskaw mudstone

350

400

450

500

550

600

2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00

Time (hours)

Wel

l Hea

d Pr

essu

re (k

Pag)

18

20

22

24

26

28

Wel

l Hea

d Te

mpe

ratu

re (d

egC

)

WHP

°C


Common Minifrac Pitfalls

• Insufficient pressure range (terminated early)

• Non-constant flow-back• No real data: measured data replaced

by smoothened data• Misidentifying fracture closure • Extraneous thermal effects


Example of Test Ended Prematurely

Overburden Stress

Injection

Pressure

14h shut-in

What can be said about the stresses below the end pressure?NOTHING!

Solution: reduce injected volume, lengthen shut-in period

End of Test


Conclusions

• microfracture tests measure rock stresses

• proper design & execution• correct analysis is critical• fractures initiate axially Pc• avoid pitfalls


Formation Pressures


Fluid Pressure• regional drainage is lateral • transmissible formations are acting as lateral drains

Clearwater

Aquifer

Aquitard

Aquiclude

Result: McMurray/Wabiskaw and other formations are underpressured

ref: Bachu & Underschultz(AAPG 1993, fig. 12)


UTF Pore Pressure Profile

Oilsand ChannelMudstone

Devonian

Oilsand Channel

Oilsand X-Trough

U.McM

Mudstone

Sand & Mudstone

P A Y

Depth = 122mGL x 9.81 kPa/m = 1200 kPag (hydrostatic)Measured Pressure = 130 kPag

1070 kPa (underpressured)

ref: Chalaturnyk (1996) © petroleum geomechanics inc.

Wabiskaw SandsClearwater Shale


Rock Stresses & Fluid Pressures

Importance:• the existing stresses in the rock are

the major loads on the rock, even after injection begins

• the pore pressure determines the minimum effective stress, which controls the rock strength & stiffness


… Rock Stresses & Fluid Pressures

• changing stresses & pressures in the rock control each formation’s mechanical response to the injection process

• when the new stresses exceed the strength of the rock, failure will occur

• if rock failure develops and progresses within the shale caprock, mechanical integrity could be compromisedwe need the rock stresses & pressures


TECTONICS


Stress Alteration

Fractures

Salt

FOLDING

FAULTING

HALOKINETICSDissolution


y

Lateral Strain under Semi-Confined Conditions

0

0

z

y

x increases

z

=0

x xxz

x

xy

Ex

1

)1( 2

y

Constraints:•x strains laterally•y constrained•z free to displace

ref. Jaeger & Cook“Fundamentals of Rock Mechanics” pg.113

yx

z

Stresses due to

a Uniform

Lateral

Strain

v

Pf

h

0 50 100 150 200 250Stress (MPa)

x = 0.003y

= 0E

E

.0 .1 .2 .3 .4Poisson's Ratio

Dep

th (m

)

0 20 40 60E (GPa)

shale

sandstone

shale

0 20 40 60 80Stress Gradient (kPa/m)

HG

FE

D

I

AB

C

Unit:

(mod. after Gretener , 1969)

v

is minimum

0

1000

2000

4000

5000

6000x

x

y

y

12


Implications of Tectonics for Induced Fractures

• orientation of fracture propagation• increase in fracture closure stress• fracture containment


Implications of Tectonic Strains

• horizontal stresses can be very different from Eaton’s equation

• tectonic stresses vary from formation to formation (stiffer strata being much more affected)

• stress in the

direction transverse to the tectonic strain will also change


Typical Oilsand Stresses


Typical Athabasca

Pressure Profile

RFT100

0 m

200

500 10000 kPa

Oilsand

Surface Till

Limestone

Wabiskaw

Siltstone

Hydrostatic

Pay

LateralDrainage

Oilsand or

“Aquitard”

ClearwaterClearwater


Typical Stresses

sig Hmin

sig Hmax

Pf

Stress or PressureD

epth

(mK

B)

Vertica StresslPfsig h with tectonicssig H with tectonics

Hydrostatic Gradient

sig Vert•log-derived stresses•corrected for tectonic strain

•sigV

from RHOB profile•Pf from RFTs, etc.•sigHmin

from minifrac•sigHmax

estimated from minifrac and logs

Note the absence of a strong stress contrast above the reservoir! containment?

reservoir


Athabasca Stresses

V

Underpressured 850 kPa

Ho

DEP

TH (m

)STRESS AND PRESSURE (MPa)

10.07.5 15.02.5 12.55.00

400

200

100

0

300

500

pf

y

= 0E

= 800MPa

= 0.3

Inapplicable at shallow depths

MI

N

MAX

Hm

ax

Hmin

Hangingstone

UTF%x

0.0 0.1 0.2 0.3 0.4 0.5 0.6

(Cold Lake)

10


Change in Stress with

Continuous Injection


Change in Horizontal Stress resulting from Increased Pressure

• slow injection will increase back-pressurebut• increased fluid pressure reduces rock stresses (Poisson’s effect)• horizontal stress will increase increased frac pressure• frac pressure

is limited by the overburden stress

yxEE

fpfpVx

22 111

= Biot

coefficient; <


Stress Reorientation due to Sequential Hydraulic Fracture Stages

• microseismic• 4 stage

hydraulic fracturing

• Barnett shale gas reservoir


Athabasca River Outcrop of the McMurray Formation Oilsands

Q1:

What is the formation fluid pressure?Q2:

What is the minimum horizontal stress?Q3:

Several km away (walking distance), would Pf be below/typical/above average?

Q4:

Several km away, would the fracture gradient be below/typical/above average?


Summaryrock stresses: • control how our reservoirs will respond

– drilling– stimulation– production

• accurate determination of rock stresses can be essential to a successful recovery process or hydraulic fracture stimulation

• must be within credible physical limits • have to be consistent with the geology and our operations

20171130 SPE Calgary’s Geomechanics Group - Breakfast Lecture Series 6565

Contact Information________________________________________________ Mr. Patrick M. Collins, P.Eng.Petroleum Geomechanics Inc. Calgary, Alberta, CanadaCanadamobile phone: +1 (403) 874-7066 GMT-7h telephone & fax: +1 (403) 277-2946 GMT-7h email: [email protected]

[email protected](> 10 Mb) [email protected]://www.PetroleumGeomechanics.com/ ________________________________________________

Stress CharacterizationOutlineWhy are Rock Stresses Important?Basic Mechanics: Scalar vs. VectorBasic Mechanics: Force and StressBasic Mechanics: Principal StressesTotal Stresses vs. Effective StressesStress vs. Stress GradientIn Situ StressesVertical StressStress OrientationsCalliper Logs and Inferred Wellbore GeometryBorehole BreakoutNiagara Falls Spillway Tunnel - 2009Well BreakoutsMaximum Horizontal Stress OrientationTectonic Stress in North AmericaTectonic Stress in AlbertaTectonic Stress in AlbertaMaximum Horizontal Stress �East Coast,�CanadaMaximum Horizontal Stress �Gulf of MexicoStress MagnitudesIn Situ Stress StateEaton EquationIn Situ Stress: North SeaPLANNINGWireline Unit & PumperTests to Determine Rock StressesHydraulic FracturesKirsch distribution of radial and tangential stresses before fracturing�(assuming uniform stress) Pump-in/Shut-inPump-in/Flow-backVariable Post-Closure with Flow-backMicrofracture Interval SelectionAnalytical MethodsTypical Oilsand MicrofracPressure vs. Sqrt(Tshutin)Minifrac Test CycleFlow Characteristics for a Well with a Single Finite Conductivity Fracture Apparent Tensile StrengthThermally Induced PressuresCommon Minifrac PitfallsExample of Test Ended PrematurelyConclusionsFormation PressuresFluid PressureUTF Pore Pressure ProfileRock Stresses & Fluid Pressures… Rock Stresses & Fluid PressuresTECTONICSStress Alteration Lateral Strain under �Semi-Confined ConditionsStresses due to a Uniform Lateral StrainImplications of Tectonics for�Induced FracturesImplications of �Tectonic StrainsTypical Oilsand StressesTypical Athabasca�Pressure ProfileTypical StressesAthabasca �StressesChange in Stress�with�Continuous InjectionChange in Horizontal Stress resulting from Increased PressureStress Reorientation due to �Sequential Hydraulic Fracture StagesAthabasca River Outcrop of the McMurray Formation OilsandsSummaryContact InformationCollins - Stress Characterization 20171130 - abstract landscape.pdfStress Characterization�Mr. Patrick M. Collins, BASc, MSc, PEng�Petroleum Geomechanics Inc., Calgary

stress characterization - petroleum geomechanics...nov 30, 2017 · alberta. he has a basc in civil...

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