appalachian stress study 1 - a detailed description of in situ stress variations in devonian shales...

8/9/2019 Appalachian Stress Study 1 - A Detailed Description of in Situ Stress Variations in Devonian Shales in the Appalachia…

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B6, PAGES 7129-7154, JUNE 10, 1989

ß

Appalachian Stress Study

A Detailed Description of In Situ Stress Variations

in Devonian Shales of the Appalachian Plateau

KEITH F. EVANS

Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York

TERRY ENGELDER

Department of Geosciences, Pennsylvania State University, University Park

RICHARD A. PLUMB

Schlumberger-Doll Research, Ridgefield, Connecticut

We describe an experiment to measure variations in the state of stress within a horizontally bedded

Devonian shale/sandstone/limestone sequence in western New York. A total of 75 stress measure-

ments were made in three wells a kilometer or so apart using a wireline-supported hydraulic-fracturing

system. The stressprofiles indicate that a major drop in horizontal stress evel occurs in the generally

massive shales. This drop occurs principally across he lowermost member of a group of sand beds and

corresponds o an offset in Sh and Sr• of 3.5 and 9 MPa, respectively. Above the sands, "thrust"

regime conditions prevail, although the amount by which Sh exceeds S• is undetermined since

instantaneousshut-in pressures ISIPs) were clipped at the level of S• due to fracture rotation. Below

the sands, the regime is strike slip with both horizontal stresses showing lateral uniformity despite

substantial variations in topography. The magnitude of Sh in the sand beds themselves and a lower

limestone remains at least as great as St, despite the decline in shale stress. Hence stress contrasts

between these beds and neighboringshalesbecome pronouncedwith depth. The contrast n Sh and SH

between the lowermost sand and the immediately underlying shale is at least 6 and 14.5 MPa,

respectively. SH levels in the lower strike-slip regime are about 1.75 times greater than Sh and are less

than the value required to initiate slippage on favourably oriented frictional interfaces. For the upper

thrust regime and the sand and limestone beds, however, the inferred lower bound on S • is close to

the slippage hreshold for a Coulomb friction coefficient of 0.6. The orientation of S• is ENE with a

standard deviation of 20ø Fracture traces were usually splayed, occasionally spanning 30øof well bore.

No systematic correlation between mean orientation and lithology is evident. Significantly different

orientations were obtained for adjacent tests in which almost identical ISIPs were observed,

suggesting hat the fractures quickly reorient themselves to propagate normal to the least principal

stressdirection. Similarly, vertical traces were observed in those tests where ISIPs apparently reflect

S•,, suggesting hat rotation to horizontal was rapid.

INTRODUCTION

Spatial variations in the state of stress of crustal rocks in

even the simplest of geological situations reflect a poorly

understood interplay through time between evolving mate-

rial properties and gravitational or tectonic loading condi-

tions. Numerous other factors may influence the pattern of

stress variations developed at a given locality, notably

structural setting and the presence of structural discontinu-

ities. Stress discontinuities have been observed to coincide

with basement-sediment interfaces [Haimson and Lee,

1980],aults Martha t al., 1983Stephanssonnd i•ngman,

1986], detachments [Becker et al., 1987; Evans, 1989], and

volcanic intrusions [Haimson and Rummel, 1982] as well as

material property variations [Warpinski et al., 1983]. Yet

despite the potential for complexity there can be little doubt

that the pattern of stress variation contains valuable infor-

mation regarding the nature of contemporary loading and

those events in the rocks past which have left some imprint

on an attribute of the stress distribution. If this information

Copyright 1989 by the American Geophysical Union.

Paper number 88JB03448.

0148-0227/89/88JB-03448505.00

is to be sensibly interpreted and the physical processes

governing the contemporary stress distribution better under-

stood, it is necessary to characterize the stress variations to

such a degree that whatever systematic relationships may

exist between the stress distribution, material property vari-

ations, and local structure are clearly established. Through

this process, it may be possible to identify the characteristic

signature of those constituent mechanisms which, super-

posed through time, account for the observed stress distri-

bution.

It is within the field of petroleum reservoir engineering,

where knowledge of formation stresses can significantly

improve the yield of hydrocarbons, that the subject of stress

variations and their origin has received greatest attention

[Warpinski, 1986]. The emphasis, however, has been to

devise methods for predicting attributes of the in situ stress

distribution from inexpensive surface or wireline log data,

rather than to measure the formation stresses themselves

[Frisinger and Cooper, 1985]. Attributes of particular inter-

est are the orientation of maximum stress [Lacy, 1987] and

the contrast in least horizontal stress magnitude between

reservoir rocks and adjacent beds [Kry and Gronseth, 1982;

Warpinski et al., 1982]. Despite this commercial importance,

7129


2/26

7130 EVANS ET AL.' APPALACHIAN STRESS STUDY, 1

N

38 ø --

84 ø

I

42o --

8;> ø

44ø-- • I

• /

;

/

: •---

• CANISTEO ......

ß REA... •?- -

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.

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j•..r",,,.,..½

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{

CONTOURS TO BASE OF DEVONIAN

SECTION OF THE APPALACHIAN

PLATEAU

CONTOUR INTERVALS OF IOOOF• A.M.S.L. DRAWN

ON TOP OF ONONDA6A LIMESTONE OR EQUIVALENT

,• SUBSURFACEAULTSHICHREAKHE NONDA6A

IMESTONE (DRAWN FOR WESTERN NEW YORK

• ANDICINITYNLY.)

....... MAR61N OF SILURIAN SALT DEPOSITS

I I I I 0 I00 200KM

84 ø 82 ø I I I

Fig. 1. Location of the South Canisteosite and nearby Eastern Gas Shale Project wells in western New York. The

structural trend of the basin is defined by the contours drawn on top of the Onondaga imestone whose stratigraphic

position s indicated n Figure 3. Dashed ineations n the study area signifyblind reverse aults underlyinganticlinal

structuresßThe bold dotted line denotes the margin of the Silurian salt deposits.

there are few studies which feature sampling of the stress

field in both the spatial extent and detail necessary o reveal

clearly systematic relationships between stress and lithol-

ogy. In this and companion papers we report such a study in

which 75 hydrofracture stress measurements were con-

ducted in three boreholes a kilometer or so apart which

penetrate a horizontally bedded sandstone/shale/limestone

sequence. This paper describes the stress measurements

themselves and the pattern of stress variations that they

define. Our interpretation of the observed stressvariations s

presented by Evans et al. [this issue] (hereinafter referred to

as paper 2) where we provide more detailed discussionof

tectonic and stratigraphic background to the study area and

the results of material property analyses. The basinwide

counterpart to this study is presented by Evans [1989].

Aspects of the measurements which have implications for

the hydrofracture technique in general are dealt with more

fully by Evans and Engelder [1989].

BACKGROUND

The measurements were made in three boreholes, a ki-

lometer or so apart, which span the side of a 230-m-highhill

to the west of the village of South Canisteo in Steuben

County, New York (Figures 1 and 2). The easternmost well

(Wilkins 1) lies on the floor of a prominent NNW striking

valley proximate to the village. The neighboringwellhead

(Appleton 1) lies some 100 m up the hill a distanceof 1.4 km

to the WSW, and the westernmost (O'Dell) lies another 100

m higher, some I km due west of the Appleton.

Details of the stratigraphy of the study area are presented

in paper 2. Here we mention only those features of immedi-

ate importance to the discussionof the measurements hem-

selves.A stratigraphiccross section along the profile A-A' of

Figure 2 is shown n Figure 3. The "mechanical" base of the

section of interest is formed by the extensive Salina salt

depositswhich serve to decouplemechanically he Devonian


3/26

EVANS ET AL.: APPALACHIAN TRESS TUDY, 1 7131

77035 '

1900

O'DE

1

I i I

77o32'30"

WiLKiNS e

LETC

....... FAULTS DRAWN ON ORISKANY SURFACE

42 ø

12'

30"

42 ø

I0'

Fig. 2. Topographyf thestudy rea howinghe ayout f the hreewells. hedashedines howhe ocationf

blind reverse faults.

section rom the underlying strata [Evans, 1989]. In western

New York the Devonian sedimentsare prodeltaic and con-

sist predominantly f alternatingsequences f black and

graymudstone/siltstoneurbidire iles he colorof which s

thought o reflect subsidence ulses associatedwith the

Acadian orogeny [Ettensohn, 1985]. Below the Sonyea

Group the lithology is dominantlycalcareous,with the

cyclical ecurrence f calcareous iltstones radingupward

into limestones [CliJJ•'Minerals Inc., 1980]. Above the

Sonyea,quartziticclasticsbegin o appearwith quartz-rich

bedsbecoming ncreasingly ommonabove he black shale

base of the Rhinestreet shale. These thin (< 15 m) quartz-rich

beds will be referred to as sands, although in detail they are

composed f intercalated edsof fine-grained andstonend

siltstone. hey are widespread ut rarely individuallyexten-

siveon scales reater han tensof kilometers.An exception

is the K-sand known locally as the Grimes sandstonewhich

is recognizedn neighboring ounties. he section elow he

sands s cut by a family of blind reverse aults which ramp up

from the Salina salt detachment. Only the larger of these

northeast rending aults which showdisplacements f up to

15 m extend sufficiently igh o cut the Tully limestone.They

most ikely formed n response o compression uring he

Alleghanian rogenyand are a common eatureof decolle-

ment thrustingover salt [Davis and Engelder, 1985].

FIELD PROCEDURES

Depth Calibration and Logging

Prior to stress esting,a borehole eleviewer og wasrun in

eachwell using he same railer-basedwireline as was used

in the stress measurements.The objective was to identify

intervals free of natural fractures for stress testing and also

to calibrate he wireline depth standardagainstcommercial

naturalgammaand density ogswhichwere used o identify

stratigraphic orizonsof specific nterest or stressmeasure-

ment. Figure4 shows he ultrasonic eflectivity magingof a

stratigraphicallyquivalent 0-m sectionof eachwell which

spanshe K-sand Grimessandstone). he sandcan clearly

be distinguishedrom the mudstones oundingabove and

belowby the higher eflectivity.The capabilityof recogniz-

ing majorbed boundariesrom a log run on the samewire

line as was used to lower the hydrofracturing tool was

crucial n permittingprecisepositioning f the hydrofractur-

ing tool with respect o the sand beds.

Owing o easeof access,he Wilkinswell was selectedor

detailed study, and a complete suite of Schlumbergergeo-

physicalogswas run. Thesedata and their bearingon the

interpretation f the stress ataare discussedy Evansand

Engelder 1987] and Plumb et al. [ 1987].

Stress Measurement Instrumentation

Stress measurementswere conducted using the wire line

hydraulic-fracturingystem llustratedn Figure5. The sys-

tem consistsof a trailer-mountedhydraulic winch supporting

1 km of standard 7-conductor armored cable, a hydraulic

high-pressureumpcapableof delivering 0 L/min of water

at a surface ressure f 50 MPa, and a compressor. luid to

both inflate the downhole straddle packer and fracture the

interval is delivered downhole via a single high-pressure

hose clamped to the wireline every 30 m to prevent entan-

glement.A downhole alve operated y wire tensiondeter-

mineswhether luid s ported o the fracturing nterval or the

packers Rumreel t al., 1983].Fluid pressures monitored

downholewith a transducer 69 MPa maximum pressureand

I part n 103precision) ndrecorded t the surface ogether


4/26

7132 EVANS TAL.' APPALACHIANTRESSTUDY,

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5/26

EVANSET AL.: APPALACHIANTRESSTUDY,1 7133

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Fig. 4. BoreholeeleviewereflectivitymagespanningheK-sandntervaln each f the hreewells.ntervals

stress-testedreshownogetherith heobservedSIP.The ightmostogwasobtainedollowinghestressestingf

the Wilkins well. The traces of the induced fractures are visible in the lower intervals shown.


6/26

7134 EVANS ET AL.' APPALACHIAN STRESSSTUDY, 1

6-METER HIGH

TR

WIRE LINE SHEAVE

HIGH-PRESSUR E HOSE SHEAVE

ROOF TRAILOR

LIC WINCH WITH I KM CABLE

DIESEL COMPRESSOR

HIGH-PRESSURE (50 MPo

AT I0 lit/mln) PUMP

500M HOSE DRUM

WITH ROTARY COUPLING

LINES RUN TO FIELD LAB

DOWNHOLE SIGNAL

HOSE-WIREINECLAMP SYSTEM EMOTE

(EVERY OM) CONTROLINES

DDLE PACKER

REMOTE HYDRAULIC

LINES F OR FLOW CONTROL

Fig. 5. Schematic diagram of the wireline "microfrac" stress measurement system used in the study.

with flow rate on strip chart and analog tape recorders. The

system was developed by F. Rummel and coworkers at both

Ruhr University and MESY Systems, Bochum, Federal

Republic of Germany, and is identical in operation to that

described by Rumreel et al. [1983]. The straddle packer used

is a standard high-pressurewash tool manufacturedby TAM

International of Houston, Texas. A 1.45-m straddle interval

length was used in all stress tests. Packer seal length was

1.04 m. Evans [1987] has reported a laboratory study of the

mechanical behavior of this straddle packer under conditions

which simulate those encountered during the stressmeasure-

ments.

Stress Measurement Technique

Upon selection of an interval for stress testing, the pack-

ers were lowered so as to straddle the interval. Precise depth

control was attained through reference to color-coded marks

emplaced on the wireline every 10 m and originally cali-

brated using a laser-ranging distance-measuringdevice hav-

ing a standard error of 5 cm over a kilometer. A correction

was applied to account for cable stretch under the weight of

the tool. The packers were then inflated so as to apply a

"squeeze" pressure of between 5 and 7 MPa against the

borehole wall. The downhole valve was then actuated to

isolate the packers and effect a hydraulic path from the

high-pressure hose to the straddled interval. The interval

was then ready for testing. A typical test procedure is

illustrated in Figure 6 which shows the time history of

downhole pressure and surface flow rate obtained during the

testing of the Tully limestone interval at a depth of 1009.5 m

in the Wilkins well. A permeability test was first conducted

by raising the pressure in the interval by 2 MPa and

monitoring the stability of the pressure after shut-in. A stable

pressure was taken as confirmation that no permeable natu-

ral fractures intersected the straddled interval. Pressure was

then released at the surface, and the interval pressure was

allowed to fall to hydrostatic in preparation for breakdown.

The pumps were then turned on full (at 10 L/min) until the

attendant steady increase in downhole pressure either

ceased or changed slope, thereby indicating fluid loss into a

presumed induced fracture. Pumping was then abruptly

stopped and the interval shut in until the pressure had

stabilized. The system was then flowed back at the surface.

Care was taken to monitor the volume of fluid injected and

returned during all pump cycles. Flow back was interrupted

periodically to monitor interval repressurization as the frac-

ture drained pressurized fluid back into the interval. The

effect of this can be seen as occasional positive jags in the

pressure record during flow back periods. These jags also

served to confirm that a fracture had been induced. As the

rate of pressure increase during these flow back interrup-

tions is directly proportional to the flow rate of fluid entering

the interval from the draining fracture, the jags provided a

measure of drainage state. Once the shut-in repressurization

rate had become negligible (i.e., the fracture had adequately

drained), the first reopen pump test was conducted. During

this cycle, 10 L of fluid was injected, again at full pump rate,

and the interval subsequently shut in and flowed back,

observing the same procedure as in the breakdown pump.

Once fracture drainage had again diminished to low levels,

further pumping cycles were conducted involving the injec-

tion of progressively larger volumes of fluid.

During the suite of reopening pump tests, instantaneous

shut-in-pressuresISIPs) tended to decline from one cycle to

the next (Figure 6). Successively larger volumes were

pumped in subsequent cycles until both the injection pres-

sure and the subsequent ISIP stabilized. The largest fluid

volume injected during a single pump in any test was 100 L.

It is well known that fracture reopening pressuresPRO

tend to decline with successivepump cycles. Hickman and

Zoback [1983] have suggested, argely on empirical grounds,

that PRO measured on the second reopening cycle is the

more appropriate value for maximum horizontal principal

total stressSu estimation using the method of Bredehoeft et

al. [1976]. Although the physical processes underlying the

decline in PRO are not understood, inadequate drainage of

the fracture pressure from the preceding pump cycle is

certainly a contributing factor. To eliminate this potential


7/26

EVANS ET AL.' APPALACHIANSTRESSSTUDY, I 7135

n- 40

LU 30

LU 20

0

'r I0

z

o o

WILKINS WELL:

g •o

009.5M (DS #W 2)

o o

_

_h_

210

hrs

i i •

4O

TIME (minutes]

• BRK ' '

._. • R•01 02R03 REOC1 i ition

• t I .........leost'S•P

•0 , J./ ,ithostat

I0

o

ca TIME (minutes}

Fig. 6. Pressure nd flow rate recordsobtainedduring he testingof the Tully limestone. he bottom iguresare

detailsof the top and show he pressure ecordwrittenduringall pumping nd subsequenthut-inperiods.

effect from our estimatesof PRO, we reoccupied27 of the

intervals tested some hours to days after performing the

initial fracture sequenceand conducted a "fully-drained"

reopeningpump test. Where an interval was not reoccupied,

we used he value of PROobtained or the second eopening

pump test.

The flow rates and volumes administered during a typical

test are modest in comparison to those used by other

workers. Calculations which assume that the induced frac-

ture can be representedas a penny-shaped quilibriumcrack

in an idealized elastic medium, constrained by reasonable

values of fracture toughnessand elastic modulus, suggest

that fracture radii of the order of 5-10 m can be anticipated

usingour procedures Evans and Engelder, 1987]. Drainage

of the fracture between pump cycles s important in limiting

fracture dimensions and also in minimizing the disturbance

to fluid pressuresand hence otal stresses)n the vicinity of

the test interval. In those cases where downhole injection

pressure ppeared o be limited by our modest njection ate,

slow pump tests were performed to demonstrate he inde-

pendenceof instantaneous hut-in pressure o flow rate.

Induced racturegeometryat the well bore was determined

by conducting ostfracture eleviewer surveysof the tested

intervals. Fracture definition was enhanced by setting an

impression ackeragainst ach ractureat sucha pressurehat

the squeeze i.e., radial stress) xertedby the packer ubberon

the borehole wall was equal to the reopen pressure to ensure

that no fresh fractures were induced). The "squeeze" effi-

ciencyof the packer n questionwas determined rom labora-

tory testsas88% [Evans,1987].The fracture magingechnique

provedsatisfactoryn 70% of casesand has he advantages f

both speedand of revealing in principle) he full extent of the

induced racture even though t may run out of the interval.

This proved o be commonand will be discussedater. Con-

ventional mpressionpacker surveyswould normally reveal

only the fracture race in the interval.

RESULTS

A tabulated descriptionof each measurementconducted

including depth, lithology, pertinent pressure parameters,

percentage f total pumped luid volumereturnedduring est

and estimated stressmagnitudes s presented n Table 1 for

the Wilkins, Appleton, and O'Dell wells. A descriptionof

the trace of all induced fractures successfully maged with

the borehole televiewer is presented n Table 2. The discus-

sion hat follows presentsgraphical epresentations f these

data.

Instantaneous Shut-in Pressures

A total of 22 intervals were stress tested in the Appleton

well, 43 in the Wilkins, and 10 in the O'Dell. Instantaneous

shut-inpressurewas selectedas the pressureat which the

post-shut-in ressuredeclinecurve departed rom the linear

drop defined immediately following shut-in (tangent

method). A tabulation and discussion f the numeroussuites

of ISIPs obtained during the Canisteo tests is presented by

Evans and Engelder [1989] and is not repeated here. In

summary we note the following:

1. In almost all tests the tangent method yielded a clearly

defined ISIP estimate for all reopen pump cycles. The

inflection point could be determined to within 0.15 MPa.

Pressure versus log time plots of several selected pressure

decline curves (notably those for the Tully limestone test


8/26

7136 EVANS ET AL.' APPALACHIAN STRESSSTUDY,

TABLE 1. Description of Each Measurement

Data Point Descriptors

Measured Parameter Values

Formation" Lithology

Quartz Interval Breakdown

Fraction, Data Depth, Pressure,

% Set m MPa

Reopen Tensile

Pressure, ' ISIP, Strength,

MPa MPa MPa

Fluid,

Vin/Vout

(Pumps1+2)d

Estimated Parameter

Values

Method 1 Method2

Sn, Sn, T,

MPa MPa MPa

PC

A

R

R

C

C

M

M

WR

WR

PY(b)

PY(b)

L

G

G

T

T

Mo

D

D

H

H

H

PC

A

S.S

(D-sand)

S.S

(D-sand)

Sil.M.S.

S.Sil. S.

(E-sand)

Sil. S.

Sil. S.

M.S.

Sil. S.

(F-sand)

Sil.M.S.

Sil. S.

M.S.

M.S.

Sil. M.S.

S.Sil. S.

(G-sand)

M.S.

M.S.

S.SiI.S.

(HI-sand)

M.S.

SiI.S.

(H2-sand)

SiI.M.S.

S.SiI.S.

S.SiI.S.

(J l-sand)

S.SiI.S

(J2-sand)

SiI.S.

M.S.

S.SiI.S.

(K-sand)

S.SiI.S.

(K-sand)

SiI.M.S.

Sil M.S.

Sil S.

Sil M.S.

Sil M.S.

Sil M.S.

Sil M.S.

Sil S.

Sil M.S.

M.S

L.S

M.S

M.S

L.S

L.S

Sil. S.

Sil. M.S.

M.S.

S.Sil. S.

(B-sand)

Sil. M.S.

S.Sil. S.

(D2-sand)

Sil. M.S.

S.Sil. S.

(E-sand)

25 W43 186.

11.5

35 W42 188.5 18.5

5 W41 194.5 35.7

23 W40 198.5 26.65

15 W1 203.5 21.8

13 W5 207 24.8

0 W20 252.7 22.4

20 W21 257.2 27.4

5 W22 266.0 27.6

16 W4 342.0 19.1

0 W23 386.0 28.3

0 W3 420.0 21.5

10 W2 486.0 18.75

20 W30 501.4 21.2

0 W39 560.5 18.3

0 W14 579. 21.15

24 W10 582.5 23.7

2 W24 592.5 17.4

13 W25 597.4 24.9

8 W38 621.8 21.1

5 W26 652.2 17.35

17 W27 662.5 19.7

13 W28 674.0 27.05

9 W29 680.0 20.5

2 W6 692.0 20.3

20 W31 707.5 21.9

15 WI5 712.5 24.05

6 WI6 724.0 19.55

4 WI7 729.0 19.4

13 WI8 747.0 18.45

I1 W37 778.15 19.4

17 WI9 832.5 20.8

14 W32 840.0 19.1

8 W33 860.5 25.15

20 W9 889.5 27.25

13 W34 951.0 21.9

17 W35 960.5 23.1

10 W36 977.6 22.4

15 W7 985.5 21.7

15 W8 991.15 18.9

0 W12 1009.5 35.05

0 Wll 1013.5 26.8

9 W13 1037.1 23.45

Wilkins Well

A 1 186.8 25.1

A2 230.0 21.5

A3 248.5 29.5

A4 277.5 18.0

A6 294 18.75

A7 305 34.8

A8 312 23.75

8.05 c 6.05 9.1 -+ 4.0

7.4 c 6.25 9.1 _+ 4.0

9.2 • 6.10 9.1 _+ 4.0

7.3 g 6.45 8.8 _+ 3.0

9.7 6.4 8.8 -+ 3.0

8.55 6.7 8.8 -+ 3.0

10.4 7.75 8.8 -+ 3.0

16.75 8.1 8.8 -+ 3.0

13.15 7.75 8.8 -+ 3.0

13.85 10.5 8.8 -+ 3.0

13.45 g 11.45 8.8 -+ 3.0

13.1 g 13.35 8.8 -+ 3.0

17.05 14.4 8.8 -+ 3.0

18.1 15.15 8.8-+ 3.0

15.7 16.05 8 .8 -+ 3.0

15.65 e 17.2 8.8 -+ 3.0

21.3 18.2 8.8 -+ 8.0

15.2 • 16.5 8.8 -+ 3.0

19.5 • 18.3 8.8 -+ 3.0

18.3 • 18.35 8.8-+ 3.0

15.7 g 17.0 8.8 -+ 3.0

18.0 g 20.6 8.8 -+ 3.0

22.4 • 20.6 8.8 _+ 3.0

17.3 g 19.1 8.8 -+ 3.0

16.95 18.5 8.8 -+ 3.0

22.7 • 21.95 8.8 -+ 3.0

20.9 21.85 8.8-+ 3.0

15.7 g 15.6 8.8 -+ 3.0

15.8 15.4 8.8 + 3.0

17.0 15.95 6.6 -+ 1.2

16.9 16.05 6.6 -+ 1.2

18.35 17.0 6.6 -+ 1.2

17.0 16.25 6.6 -+ 1.2

17.9 17.05 7.8 -+ 1.2

20.45 18.5 7.8 + 1.2

20.65 20.2 7.8 -+ 1.2

20.45 20.0 7.8 -+ 1.2

21.6 20.85 7.8-+ 1.2

20.9 19.9 7.8 -+ 1.2

19.65 19.7 7.8 -+ 1.2

33.3 c 30.6 5.2 -+ 3.

25.5 24.45 5.2 -+ 3.

21.5 • 21.8 7.8 -+ 1.2

Appleton Well

6.4 •' 5.38 9.1 -+ 4.0

7.5 g 6.5 9.1 -+ 4.0

9.25 x 7.5 9.1 -+ 4.0

10.5 '• 8.0 9.1 -+ 4.0

14.7 •' 9.0 9.1 -+ 4.0

10.9 • 10.5 9.1 + 4.0

11.8 •' 10.4 8.8 -+ 3.1

0.56

0.45

0.65

0.77

0.75

0.75

0.10

0.70

0.52

0.50

0.72

0.2

0.58

0.4

0.3

0.3

0.71

0.62

0.25

0.36

0.42

0.42

0.31

0.38

0.65

0.6

0.6

0.23

0.24

0.20

0.25

0.19

0.30

0.42

0.30

0.30

0.45

0.27

0.33

0.28

0.40

0.39

0.30

0.66

0.69

0.57

0.60

0.26

0.24

0.76

13.8 8.1 3.45

7.3 9.3 11.1

-9.05 r 7.0 26.5

-0.(:½ 9.9 19.35

4.0 c 7.3 12.1

1.9 9.35 16.25

6.95 10.15 12.0

2.9 r 4.75 10.65

1.6c 7.25 14.45

17.5 14.0 5.25

10.7 16.7 14.85

22.8 22.4 8.4

28.0 20.9 1.7

27.1 21.95 3.1

32.65 26.45 2.6

33.0 29.7 5.5

33.4 27.0 2.4

34.5 27.95 2.2

32.35 28.9 4.4

37.55 31.55 2.8

35.4 28.25 1.65

43.8 36.7 1.7

36.3 32.15 4.65

38.3 32.7 3.2

36.5 31.1 3.35

45.1 35.5 -0.8

42.6 37.05 3.15

28.2 23.3 3.85

27.75 22.55 3.6

27.95 22.8 1.45

26.95 22.85 2.5

27.8 23.65 2.45

27.2 22.7 2.1

24.5 23.95 7.25

26.45 25.45 6.8

36.25 29.7 1.25

34.35 29.2 2.65

37.4 30.4 0.8

35.2 28.2 0.8

37.3 28.75 -0.75

40.65 47.6 1.75

30.45 36.95 1.3

38.55 32.7 1.95

-1.9 r 7.65 18.7

4.6 9.5 14.0

-0.5 c 10.55 20.25

12.1 10.5 6.5

14.2 9.15 4.05

2.5 17.3 23.9

12.9 16.05 11.95


9/26

EVANS ET AL.: APPALACHIAN STRESSSTUDY, 7137

TABLE 1. (continued)

Data Point Descriptors

Measured Parameter Values

Formation" Lithology

Quartz Interval Breakdown Reopen

Fraction, Data Depth, Pressure, Pressure,' ISIP,

% Set m MPa MPa MPa

Tensile Fluid,

Strength, V,n/Vou

MPa (Pumps1+2) a

Estimated Parameter

Values

Method

Method 1

SH, Su, T,

MPa MPA MPa

Appleton Well (continued)

A Sil. S. A9 356.5 31.5

A S.S. A10 366.0 17.85

(F-sand)

R M.S. All 374.25 29.2

R M.S. A12 440.7 27.55

R M.S. A13 527.5 21.7

R Sil. S. A14 677.5 20.8

R Sil. S. A15 701.0 17.8

R S.Sil. S. A16 704.5 19.8

(H2-sand)

R Sil. M.S. A17 748.5 18.6

R S.Sil. S. A18 771 25.35

(Jl-sand)

R S.Sil. S. A19 778.5 21.4

(J2-sand)

R Sil. S. A20 788.0 23.3

R Sil. M.S. A21 834.0 17.35

M Sil.M.S. A22 927.0 18.7

WR Sil. S. A23 1000.0 27.35

D S.S OD1 246.8 26.7

D Sil.M.S. OD3 342.0 21.5

PC Sil. M.S. OD2 428.3 42.2

R Sil. M.S. OD10 864.5 31.6

R Sil. S. OD9 896.5 52.5

(J2-sand)

R Sil. M.S. OD8 909.5 30.4

R Sil.M.S. OD7 922.5 29.2

R Sil. S. OD4 931.0 30.9

(K-sand)

C M.S. OD6 942.0 44.5

C M.S. OD5 950.2 33.4

10.0 •'

15.5 e

12.85 •

14.9

15.65 •

16.9 •

15.3

16.8 •

16.8 •

21.3 e

16.15 c

19.3 e

15.3 •

16.7 •

19.6 •

O'Dell Well

19.6

10.9

12.5

10.0 8.8 _+ 3.1 0.57

10.65 8.8 _+ 3.1 0.65

10.6 8.8 _+ 3.1 0.75

12.5 8.8 _+ 3.1 0.45

14.65 8.8 _+ 3.1 0.75

18.3 8.8 _+ 3.1 0.50

18.3 8.8 _+ 3.1 0.49

19.2 8.8 _+ 3.1 0.53

19.2 8.8 _+ 3.1 0.50

22.05 8.8 _+ 3.1 0.50

22.05 8.8 _+ 3.1 0.55

20.1 8.8 _+ 3.1 0.32

15.9 8.8 _+ 3.1 0.55

16.4 6.6 _+ 0.9 0.25

19.6 7.8 _+ 1.1 0.20

3.5 14.4 19.7

18.75 12.5 2.35

7.35 14.9 16.35

14.0 17.85 12.65

25.4 22.6 6.05

35.6 3O.7 3.9

38.35 32.05 2.5

39.0 33.0 3.0

39.75 32.75 1.8

41.3 36.55 4.05

45.15 41.65 5.25

37.3 32.5 4.0

30.15 23.4 2.05

27.1 22.4 2.0

28.5 28.45 7.75

7.5 9.1 + 4.0 0.2 2.25 r

9.3 9.1 + 4.0 0.75 11.8

11.4 9.1 + 4.0 0.2 -3.5/

20.6 8.8 + 3.1 27.9

24.5 8.8 + 3.1 0.45 20.15

19.5 8.8 _+ 3.1 0.3 27.1

20.0 8.8 _+ 3.1 0.4 29.65

24.2 8.8 + 3.1 40.47

18.2 6.6 _+ 1.2 0.2 6.5/

16.2 6.6 _+ 1.2 11.55

0.25 r 7.1

13.5 10.6

17.1 29.7

aD, Dunkirk; H, Hanover; PC, Pipe Creek; A, Angola; R, Rhinestreet; C, Cashaqua; M, Middlesex; WR, West River; PY(b), Pen Yan

(black shale section); L, Lodi limestone; G, Geneseo black shale; T, Tully limestone; Mo, Moscow.

bLog nterpretationsf lithology enerally ssumehathigh adioactivity ndboundwatercontent mplyhighclayand ow quartzcontent

and fine particle size. The percentage volume of quartz as estimated from the GLOBAL TMmark of Sclumberger) computer-processing og

is indicated. S.S., quartzitic sandstone,definition of lithology in terms of log response s low y (80 API), neutron 4> < density 4>;S.Sil. S.,

sandy siltstone; Sil. S., siltstone, moderate y (100-120 API), neutron 4> 15-20%, p • 2.7; Sil. M.S., silty mudstone; M.S., mudstone, high

(>20%, p • 2.7.

eReopen pressure listed is for first reoccupation after initial fracturing pump.

aAverage ractionof fluid returnedduring irst and second eopening umps.

eReopenpump 2 value: greater than 3% decline in reopening pressure on subsequentpumps >0.4 MPa.

fEstimated alueof S, < Shand s hence ot allowable.

gReopen pump 2 value: less than 3% decline in reopening pressure on subsequentpumps


10/26

7138 EVANS ET AL.' APPALACHIAN TRESS TUDY, 1

TABLE 2. Summary of Trace Data

DS

Depth,

m

Formation

Member a

Strike of Trace, øE of N

Image

East Limb West Limb Quality'

Extent of Trace Out of

Interval, m

Upward Downward

Direct

Evidence

of

Bypass?c

Downhole

Packer

Pressure,

MPa

W43 a

W42 a

W41

W40

Wl a

W5

W20

W21

W22

W4 a

W23

W3 a

W2

W30

W39 a

W14

W10

W24 a

W25

W38

W26

W27 a

W28

W29

W6

W31

W15

W16 a

W17 a

Wi8 a

W37 a

W19 a

W32 a

W33 a

W9 a

W34 a

W35 a

W36 a

W7 a

W8 a

W12

Wll a

W13 a

A1

A2

A3

A4

A6

A7 a

A8

A9 a

A10

All

A12

A13 a

A14

A15

A16

A17

A18 a

A19

A20 a

A21

A22

A23

186.0

188.5

194.5

198.5

203.05

207.0

252.7

257.2

266.0

342.0

386.0

420.0

486.0

501.4

560.5

579.0

582.5

592.5

597.4

621.8

652.2

662.5

674.0

680.0

692.0

707.5

712.5

724.0

729.0

747.0

778.15

832.5

840.0

860.5

889.5

951.0

960.5

977.6

985.5

991.15

1009.5

1013.5

1037.14

186.8

230.0

248.3

277.29

293.78

304.78

311.85

356.33

365.83

374.25

440.49

527.25

677.15

700.17

704.17

748.14

770.64

778.14

787.63

833.58

926.56

999.53

Wilkins Well

H (D-sand) 5

H (D-sand) 5

PC 229 3

A(E-sand) 64? 4

A 88 3

A 194 2

A 5

A (F-sand) 1797 4

R 5

R 198 2

R 40 272 2

R 88 241 2

R 268? 4

R (G-sand) 29 2

R 94 274 1

R 83 272 1

R (H 1-sand) 5

R 66 275 2

R (H2-sand) 230 2

R 5

R 26 222 2-3

R (Jl-sand) 43 ñ 35 243 2

R (J2-sand) 5

R 55 249 1

R 59 239 2

R (K-sand) 280 2

R (K-sand) 32 249 2

R 96 275 1

R 93 251 1

C 64? 2517 4

C 5

M 38 281 1

M 79 190 1

WR 81 261 1

WR 60 2

PY 63 248 2

PY 74 254 2

L 49 266 2

G 72 250? 3

G 82 249 1

T 62? 4

T 57? 4

Mo 75 2

D

D

H (B-sand)

H

H (D-sand)

PC

A (E-sand)

A

A (F-sand)

R

R

R

R

R

R (H-sand)

R

R (J-sand)

R (J-sand)

R

R

M

WR

Appleton Well

5

5

85 3

85? 4

77? 4

67 226 ñ 39 2

5

72 264 2

78 3

5

92 272 3

99 295? 3

90? 270? 4

77 256 2

83 291 4

85 249 2

9 328 4

25 3

1787 4

284 2

233/297 1

244 1

9 9

9 9

0.0 0.0

0.7(?) -0.7

0.96 e -0.75

0.0 0.0

9 9

-0.4 0.2(?)

9 9

0.62 0.0

0.55 0.53

0.56 1.0 e

0.54(?) 0.79(?)

0.44 0.34

0.15 0.97 e

0.20 0.46

0.25 -0.20?

0.41 0.0

-0.30 0.35

9 -0.20

0.15 0.20?

-0.2 1.21 e

9 9

0.65 0.51

0.0 0.0

0.4 0.0

0.0 0.51

0.71 1.07 e

0.72 1.0 e

0.85 e 0.0

9 9

0.5 1.6 e

1.0 e 0.33

0.41 1.41 e

0.78 1.0 e

0.0 0.30

0.15 1.4 e

0.64 1.2 e

1.21 e 1.1 e

1.2 e 1.2 e

0.0 1.1(?)

9 9

1.1 e -0.7

9 9

9 9

-0.5 0.0

0.07 -0.6?

0.0 0.0

0.9 e 0.52

horiz. horiz.

0.53 1.25 e

0.0 0.0

9 9

0.0 0.4

1.36 e 0.75

0.0 0.4?

0.35 0.5

0.0 0.5?

0.0 0.9

9 9

0.0 0.0

0.0 1.6 e

0.7 0.5

0.56 0.0

0.75 0.0

•/(lower)

,/(lower)

X

X

? (upper)

X

X

X

X

•/(lower)

X

•/(lower)

X

X

X

X

X

•/(lower)

X

X

X

? (lower)

X

X

X

X

X

X

X

•/(upper)

((lower)

•/(lower)

X

X

•/(lower)

X

•/(upper)

•/(lower)

•/(lower)

? (upper)

X

•/(lower)

X

X

X

(lower)

(lower)

(upper)

(upper)

(lower)

8.8

9.3

9.15

9.45

7.0

6.7

8.4

8.6

8.6

8.4

10.9

9.6

9.8

12.5

12.4

13.4

11.5

13.0

13.55

13.15

14.65

14.55

15.2

15.05

14.8

14.65

14.65

15.4

15.45

15.4

15.9

16.45

16.9

17.1

14.7

18.0

18.6

18.5

16.15

16.2

16.2

16.7

17.2

11.8

11.3

11.7

12.3

12.6

12.5

13.0

13.0

13.5

13.8

13.8

14.8

16.7

16.7

16.9

17.8

18.2

18.1

18.3

18.1

19.15

19.9


11/26

EVANS ET AL.' APPALACHIAN STRESS STUDY, 1 7139

• 60

u3 40

i,u 20

0

z

• o

o

.-- 4o

O'DELL WELL

/

--

896.5M (DS#OD9

i r i

o o

1

TIME (minutes)

4•0 50

I0 LU

0 a:'•

-20 u_

n• 0 0 0 0 0 0

rn n n n n,- n n

LI.I

:::) 30

i.u 20

o

z

o

r• 10

0 2 4 6 8 I0 12 14 16 18

TIME (minutes)

Fig. 7. Pressure and flow rate records obtained during the testing of the J2-sand in the O'Dell well. The erratic

variations in pressure during pumping are most likely due to a tool malfunction. However, the shut-in pressure decline

curves obtained following each pump appear to be unaffected.

more, the ISIPs showed no correlation with packer setting

pressure (Figure 8). The worst affected test, DS OD4 (931 m)

which was conducted immediately after pull was applied to

the wireline, yielded a poorly defined SIP of 23.7 MPa. Two

subsequent eoccupations of this interval after 1 and 2 days

(still with the malfunctioning tool) yielded ISIP suites of

26.5/25.0/25.0 and 24.8/24.2 MPa, respectively. In view of

the seemingly well-behaved nature of the post-shut-in pres-

sure decline and the conformance of the ISIPs to anticipated

reproducible trends we accept them as measures of the least

principal stress.

Depth profiles of the resultingstable SIP values are plotted

in Figure 8 for each well. The depth axes are shifted such hat

common stratigraphichorizons are aligned. The diagonal ine

represents the overburden load as estimated from a Schlum-

berger ensityogmeanvalueof 2.71g/cm . This s possibly n

overestimate, as direct density measurements on core from a

neighboringwell (Eastern Gas Shale Project (EGSP) well NY 1)

suggestn average alueof 2.65 g/cm [Kalyoncu t al., 1979].

The comparatively high density of these shallow sedimentary

rocks reflectsa high clay content (Figure 7 and paper 2) and the

presence of calcite and pyrite. Several features of these data

are noteworthy.

First, with the exception of the section between the H- and

K-sands, the ISIPs define remarkably consistent linear

trends showing very little scatter. Wherever closely spaced

tests were performed, essentially the same ISIP values were

observed. We take this as a measure of data quality.

Second, we recognize two distinct stress regimes, sepa-

rated by a transition zone between the H- and K-sands. The

aD, Dunkirk; H, Hanover; PC, Pipe Creek; A, Angola; R, Rhinestreet; C, Cashaqua; M, Middlesex; WR, West River; PY, Pen Yan; L,

Lodi limestone; G, Geneseo black shale; T, Tully limestone; Mo, Moscow.

•'Image uality: , veryclear race,no vagueness;, clear raceof one imb,vagueraceof another; , smallsegment f one raceclear,

speculative nfill required to estimate an azimuth; 4, trace identification somewhat speculative; 5, no trace identified.

CEvidenceof bypass:v (lower), indicateswell bore below packersbecame pressurizedduring stress esting; v (upper), indicates low out

of wellhead was observed during pumping; X, indicates well bore was not pressurizedat end of testing and no fluid flow was observed from

well bore during tests. This observationcould only be made for data sets subsequent o (i.e., numerically greater than) W13 for the Wilkins

and A17 for the Appleton when the static fluid head had reached the surface.

dTheres evidenceromeither racture race engthor well borepressurizationhat luidbypass f packersealsmayhaveoccurred.

ePotentialbreach of seal: fracture trace appears o extend more than 0.85 m along the 1.04-m packer seal length.


12/26

7140 EVANS ET AL.' APPALACHIANSTRESS TUDY, 1

ß ß eeß

_ _-

0 • 0 0 0

0 0 0

o• • •O 00 0

0 0 0 0

oE o o

e

W

" -: -= Z

' ---• 0

- _

_ _

o

o o o

o o o

_ _

_ _

: =

: =

: =

_ _

_ _

_ _

i I

O0 0 0 0 00 0 0

o• • •O 00 0

(w) 3D•d•flS •0338 hid3•

I

iI

iI

w

N_ • Z

'•z•>

z •7> '1-

N

I-- 0 r•

Z

_• ..


13/26

EVANSET AL.: APPALACHIAN TRESS TUDY, 1 7141

pc •---

A

Mi

WR

PY

O.

MPa

I0. 20. :50. 40. 50.

I I I I I

ß \

ß

I SlPH

x WILKINS

• ß = APPLETON

, A O' DELL

**

..-..... ..........._._._.••.,,•_,••_.- - .-,;•:

v,•, • •'• ......................................

'•'?•'•"•"-'"'X• ' '•' ............................... r•=;_-;•.;.--.•,••

• "'"-""....."'"'-'"'"*m•• ,m,

............... ..... • .-.-.-.---.--•---------.-•...•.... ........ . ...

__

200 -

F 400

n

x

x... x

MEAN STRIKE (Eøof N)

0 45 90 135

I I i

x

x

x

x

IIIIllllllllllllll11111111,1111llllll

IIIIIIIIIIIIIIIIIII ........ IIIIIIIIIIIIIIIIIIIIIIIIIIIIII

x

Fig.9. Superpositionf SIPsrom ll hree ells na commontratigraphicectionßlso hownre stimatesf

maximumorizontaltressalculatedsing ethodsubjecto heassumptionhat SIP sequalo he east orizontal

stress. he errorbar reflectsheuncertaintyn tensile trength.he diagonalinescorrespondo the near-lithostat

trends in each well which we believe denote the vertical stress.

upperregime s characterizedy ISIPs which ie on or

marginally bove he overburdenrend or eachwell. Spe-

cifically, or the O'Dell, Appleton, nd Wilkinswells the

ISIPs increaseinearlywith depthat a rate which exceeds

the densityog derived verburdenradient y a factorof

1.0, 1.07, and 1.16, respectively.Henceforthwe refer to

these trends as the "near-lithostat trends." As the wellhead

heights ifferby up to 213 m and beddings essentially

horizontal, the ISIPs measuredat a given stratigraphic

horizon n the upper egime ary significantlyromwell to

well. In contrast, he lower regime, extendingbelow the

K-sand, is characterized y ISIP values substantiallyess

than the overburden. Moreover, at a given stratigraphic

horizon, the ISIPs are essentially he same in each well,

irrespectivef the different verburdenoads.This s evi-

dent n Figure9, wherewe showa superpositionf all data

plotted ccordingo stratigraphicepth, nd n the detail o

Figure presentedn Figure10 note hat he diagonalines

in Figure9 represent verburden, hereashose n Figures

9 and10representhenear-lithostatrends). hisconstancy

suggests subsurfaceorizontal tress egimewhich s

laterallyuniformon a scaleof at leastseveral ilometers.

Third, we recognize transition one between he upper

and ower stress egimeswhichextends rom the H- to the


14/26

7142 EVANS ET AL.: APPALACHIAN STRESSSTUDY, 1

100m

ISIP (MPa)

14 16 18 20 22 24 26

, , I , , I , i I ] • I , I I , •

o \ ",... \

I• \ '.,.

Wilk App O'D

Wilkins • shale ß sand

Appleton o shale ß sand

o

• O'Dell [] shale ß sand

FJg. 10. Detail to Figure 10 showing the stratigraphicvariation in

ISIPs observed in the vicinity of the transition zone.

K-sands (Figure 10). Within this zone, ISIPs measured in the

quartz-rich siltstone beds remain precisely on the near-

lithostat trends established n the upper regime of each well,

whereas ISIPs measured in the "shales" fall significantly

below the appropriate trend by an amount which generally

increases with depth. The pressure and flow rate (injection

only) records obtained during the testing of the K-sand and

the immediately underlying shale in the Wilkins well are

shown in Figure 11. The location of both these intervals (DS

W15 and DS W16) is shown in Figure 4. The difference in

pumping pressures (-6 MPa) is remarkable considering the

intervals are only 10 m apart. Similarly, in the O'Dell well,

systematic contrasts in ISIPs between both K- and J-sands

and the intervening shale attain a value of 5 MPa. At greater

depth we find one ISIP measured in the Tully limestone also

lies close to the near-lithostat trends (Figure 8; note diagonal

lines represent overburden) and contrasts strongly with

those measured in the adjacent shales.

Fracture Trace Geometry

Owing to time constraints, postfracturing televiewer sur-

veys were conducted in only the Appleton and Wilkins

wells. In 70% of cases t was possible o identify a new trace

of sufficient extent and clarity to determine sensibly the

strike and vertical extent of the induced fracture(s).

Sketchesof the identified fracture traces as they appeared n

the postfracturing televiewer survey are presented n Figure

12. A dashed trace indicates that recognition was uncertain.

Table 2 gives a summary of the image data for the Wilkins

and Appleton wells. Here we note the following:

The vast majority of the fractures were determined to be

within 10øof vertical. The only clear exceptionwas the fracture

induced at the H-sand horizon (311.85 m) in the Appleton well

which was determined to be horizontal. It is generally difficult

to recognize horizontal fracture traces in strongly bedded

formations, and it is not possible o discount heir presenceon

the strength of the televiewer images alone. The principal

evidence hat they were not common arose rom our practice of

settingan impressionpacker in each nterval for 30 min prior to

the televiewer survey. Upon returning he packer to the surface

after having "pressed" the uppermost 11 induced fractures in

the Appleton well (except the interval at 311.85 m), the rubber

was found to be decorated with vertical fracture traces with no

horizontal traces evident.

The inferred mean strikes of the vertical fracture limbs

from the Wilkins and Appleton well surveys are shown in

Figures 13 and 14, respectively. The data are segregated nto

successive 200 m depth groupings. Fractures induced in the

quartz-rich beds are indicated by dashed lines. In several

cases only one fracture limb could be identified. This should

not be taken to imply uniaxial fracture propagation from the

well bore as it may be due to limited televiewer resolution.

Bilateral fractures were rarely straight and 180ø opposed.

Rather, there was a strong tendency for bifurcation, the

splays occasionally spanning40ø of well bore. Where splay-

ing was observed, the orientation of the limbs shown in

Figures 13 and 14 represents the mean value.

A large degree of scatter in limb orientation is evident.

Some of this can be ascribed to instrumental error. Our

experience in running three different televiewer tools during

the surveys, often with multiple passes of the same interval,

led us to conclude that image orientation varied in a seem-

ingly nonsystematic manner at the level of about ___7from

magnetic north. This was found even for the same tool

imaging the same interval on different days. Even allowing

for this, however, the scatter in orientation is surprisingly

large, given the smooth variation in ISIPs. Using the circular

statistical methods of Mardia [1972], we find a mean orien-

tation of N68.5øE (true) for the Wilkins and N80øE for the

Appleton, the standard deviations being 24ø and 23ø respec-

tively. In Figure 9 we have plotted mean strike alongside the

corresponding stress estimates (see also Figures 17 and 18).

For bilateral fractures it was assumed that the two traces

represent the expression of a single planar fracture that

intersects the well bore, not necessarily diametrically. Un-

like the stress magnitudes, no systematic first-order correla-

tion with lithology is evident in the stress transition zone.

Furthermore, outside of this zone, the scatter in orientation

is not reflected in the inferred stress magnitudes which define

smooth consistent trends. From this we conclude that the

scatter largely reflects the distribution of flaws which serve

as fracture initiation points and that the fractures tend to

align themselves with the maximum stress direction after

propagating some short distance from the well bore. Similar

scatter in well bore fracture orientation has been reported by

Overbey and Rough [1968] for induced fractures in sand-

stones equivalent to our B-sand in a neighboring county.

DATA ANALYSIS

Interpretation of ISIPs

As the televiewer imaging indicates that almost all induced

fractureswere vertical at the well bore, it might seem ustifiable

to follow convention and interpret the ISIP values as direct

measuresof the least horizontal principal stressSh. This is


15/26

EVANS ET AL.' APPALACHIAN STRESSSTUDY, 1 7143

WILKINS: SAND 712.5M

r•

(/3

(/3

• 150

Q.

J

O

Z

O

BREAKDOWN

RE OPENI

50 l [ I

5 io 15

j•O0 • • • I • • •

u_ ViN= .5.0. V,N I0•

You = 3.7J You = 6.5

I I ,

WILKINS: SHALE

BREAKDOWN

200

724 M

REOPEN I

RE OPEN 2

V•N= 15.0.

VOUT 5.5 •.

REOPEN 3

REOPEN 2 REOPEN 3

Vm = 40 •

J I00

0

Z

0

I i i

0 5 I0 15 20 2b

.

- =4 V,N 0.0. V,N 15• VlN=25•

Vou = 2 6 J) Vou =2.8 J) Vou = 2.6 0

Fig. 11. Pressureand injection rate records obtained during two tests; one conducted n the K-sand and the other

in the immediately nderlying hale.Note the differencen pumping ressures ven hough he intervalsare only 10 m

apart (see Figure 4).

certainly reasonable or the lower regime where ISIPs are

significantlyess han the anticipated ertical stress.However,

in situationswhere the measured SIPs correspond losely o

the overburden, as in the upper regime, convention must be

treated with caution. For then there exists he possibility hat

the least horizontalstressmay exceed he vertical stressby an

arbitrary amountand thereby have induced he fracture to turn

from vertical to horizontalas it propagated rom the well bore

[Warren and Smith, 1985],resulting n an ISIP that reflects he

vertical stress Zoback et al., 1977]. In such circumstances he

ISIP will provideonly a lower bound o the true value of Sh

which will remainotherwiseundetermined. his posesa ques-

tion for the interpretationof those SIPs which fall close o the

near-lithostat rends. Do they reflect least horizontal stress

magnitudes or do they reflect vertical stress? n situations

where significant opographyexists and stressmeasurements

from several neighboringboreholeswith wellheadsat different

elevations re available, t is possible, n principle, o decide

which is the case hroughexaminationof the spatialvariation

of the three-dimensional ISIP distribution. Since the wellheads

at the SouthCanisteositediffer n elevationby up to 200 m, the

data are well suited to this form of analysis. A detailed

discussion f the modeling s presentedby Evans and Engelder

[1989]. In summary of the results and their bearing on the

interpretationof the near-lithostat SIPs we note the following:

Below a few hundred meters depth, the influence of topog-

raphy on the lateral variation of vertical stress s much more

pronounced than on the lateral variation of the horizontal

stresses.This is illustrated in Figure 15 which shows contours

of valley-normalhorizontal stressand vertical stresspredicted

to result from the erosion of a symmetric valley into a laterally

confinedhalf-space.The model s from Savage et al. [ 1985]and

assumesplane strain conditions and a Poisson's ratio of 0.33.

All stressmagnitudesare normalized by pgb, where b is the

depth of the valley floor below the plateau top, here equal to

213 m. The upper and lower pair of figures correspond to

different theoretical topographicprofiles defined by the bold

line at the surfaceof the eroded half-space.A two-dimensional

approximationof the mean topographicslope n the immediate

vicinity of the Wilkins and Appleton wells is shown for com-

parison. Further details are given by Evans and Engelder

[1989] and Savage et al. [1985]. The important point is that the


16/26


/ '\ : s

NOøE N øE

NOøE • •E

NOøE

NOøE I-

NOeE I" INOeE I- N E O

I • ,• I•E

OøE INOøE I-

NOOEøE

Fig. 12. Sketchesof induced racture trace geometry. A dashed race indicates hat identification s uncertain. For

each diagram he sectionof boreholepressurized ies between he two bold lines bounding he "data set number." The

outer solid ines indicate the upper and lower extent of the packer seals.The orientation of the left margin of each trace

is given with respect to magnetic north.


17/26

EVANS ET AL.: APPALACHIAN STRESSSTUDY, 1 7145

- I

PROBABLYREEXISTING

/N -

NOOE E ('•o

- N40øE / 9o

N6Z E

_

E

N l N e• N44øE /N6•E

N0 E I N6eE F N 0 E -

N 90øE

Fig. 12. (continued)

vertical stresscontoursare stronglydeflecteddownward under

the valley, mimicking he surface opography although ess so

at depth),whereas he valley-normalhorizontalstress ontours

below 200 m are only slightly perturbed. The valley-parallel

horizontal stress magnitudesare given by the plane strain

relationry= •' trx+ tr0. Hence hese ontoursnotshown) re

also deflecteddownward but by a small fraction of the deflec-

tion to the vertical stress. For more complicated three-

dimensional opography, such as applies to the study area

(Figure 2), it is reasonable o infer on the basis of the plane

strain model results hat topographically nduced ateral varia-

tions in horizontal stress are a fraction of the corresponding

variation in vertical stress.Thus, if the measured SIPs indeed

representhorizontal stresses nd all other stress ield "compo-

nents" which augment he modeled gravitational stresses re

laterally uniform, then ISIPs measured n differentwells at the

samestratigraphicevel shouldbe similaror, at most, differ by

only a small raction of the overburdendifference n the wells.

By "overburden" we mean simply pgd, where p is the bulk

density of the rock and d is the depth below the wellhead.

Referring to Figures 9 and t0, we find that below the K-sand,

where there is no reason to doubt that the ISIPs reflect least

horizontal stress, he ISIPs measuredat common stratigraphic

levels are indeed identical. This lends strong support for the

inferenceswe draw from applying the Savage et al. [1985]

model and implies hat nongravitational orizontal stresscom-

ponentspresent n the sectionbelow the K-sand are, indeed,

laterally uniform. In contrast, ISIPs which lie on the near-

lithostat trends differ at common stratigraphic evels by an

amountwhich is slightlygreater han the overburdendifference

and hence conform more closely to the spatial variation of

vertical stress. We next examine how closely.

The difference between the vertical stress and overburden

gradients n the Wilkins well and, to a lesser extent, the

Appleton well is significant.Physically, this reflects the fact

that in a borehole located on a valley floor (i.e., the Wilkins

well), the true vertical stress, such as would be sampledby a

pressurizedhorizontal racture, will increasewith depth at a

rate greater than the overburden due to "loading" by the

surroundinghigher topography. In Figures t6a and t6b we

show a comparisonbetween vertical profiles of the overbur-

den, the vertical stresspredicted for the two theoretical topo-

graphicprofiles eatured n Figure 15, and the observednear-

lithostat ISIPs, for the Wilkins and Appleton wells,

respectively.The correspondence etween the predictedver-

tical stress and the ISIPs for both wells is reasonable consid-

ering he limitationsof the two-dimensionalmodel n represent-

ing he three-dimensionalopographyof the studyarea. For the

O'Dell well the predicted vertical stress, near-lithostat SIPs

and overburden are essentially coincident. Thus the near-

lithostat trends correspondclosely to the predicted vertical

stress rofiles n eachwell. Consequently, e take SIPs which

fall on these trends to be direct measures of vertical stress and

to constituteonly lower bounds to the true magnitude of the

least horizontal stress. This changes the earlier provisional

interpretationof Evans and Engelder [1986] n which the ISIPs

lying on the near-lithostat rends were taken as exact measures

Sh

Least Horizontal Stress in the Transition Zone

The outstanding eature of ISIPs in the transition zone is

that values measured in shales are systematically smaller

than those in the sands (Figure t0). In the preceding discus-

sion we have shown that ISIPs in the sands are best


18/26


DEPTH

RANGE

(m)

I

iooo

8OO

6OO

4( •0

WILKINS WELL

SANDSTONœ BED MEAN ORIENT OF INDIVIDUAL

MUDSTONE/SILTSTONE FRACTURE LIMBS AT WELL BORE.

Fig. 13. Mean orientation of induced fracture traces in the

Wilkins well in successive depth intervals. The letters identify the

specific sand bed. Orientation is specifiedwith respect o true north.

interpreted as measures of vertical stress. Since vertical

stressmust be a monotonically increasing unction of depth,

the ISIPs in the shalescannot also be S•,measures.Thus, do

they reflect least horizontal stress? We have noted that

below the K-sand, ISIPs at common stratigraphic horizons

are essentially the same, a result which both supports their

interpretation as exact measuresof St, and also implies that

nongravitational stress "components" present n the section

are essentially laterally uniform on the scale spanned by the

wells. Referring to Figure 10, we find that ISIPs recorded in

the shale between the J- and K-sands are also fairly well

clusteredand hence can be taken as reliable measuresof St,.

An implication is that least horizontal stressmagnitude n the

shale undergoes a sudden laterally uniform step change of

about 3.5 MPa across the K-sand. Above the J-sands, the

shale ISIPs are still significantly less than the level of the

vertical stress mplied by our analysis. However, they show

considerable lateral variation. If they are interpreted as

direct measures of Sh, the implication is that for some

reason, least horizontal stress (or, rather, its nongravity

constituents) does not display the same pattern of lateral

uniformity above the J-sand as it does below. Above the

G-sand, ISIPs generally fall close to the near-lithostat trends

(Figure 9). Thus, if shale ISIPs in the vicinity of the H-sand

reflect Sh, then least horizontal stressattains the level of the

vertical stress some short distance above the H-sand.

It is worth briefly examining the possible effects of the

known bedding-planeweakness of the shale [Blanton et al.,

1981] n promotinghorizontal racture-controlledSIPs (i.e., S•,

measures). Near the H-sand in the Wilkins well, shale SIPs are

systematically 1 MPa lower than those of the sands which

define he evel of S•, Figure 10). fthe shale SIPs measure h,

then we must conclude that the presence of bedding-plane

weaknessdid not influence he observed SIP even thoughS•,

and St,differedby only 1 MPa. Furthermore, since t is unlikely

that the sands exhibit a greater strength anisotropy than the

shales, we might also infer that if the induced fractures turned

horizontal n the sandsduringpropagation, hey did so because

Sh n these beds exceededS•, rather than through exploitation

of bedding-planeweakness. A corollary is that ISIPs which fall

on the near-lithostat rends can be taken to represent ower

bounds to the true magnitude of Sh. The variation in S•

between the sands and shale of the transition zone and between

the Tully Limestone and surroundingshalesmay thus be even

greater than the ISIP values plotted in Figures 8-10 suggest.

Maximum Horizontal Principal Stress

Two methods are commonly used to estimate S t from

open-hole hydraulic fracturing data. Both are based upon

Hubbert and Willis's [1957] discussion of the mechanics of

vertical borehole rupture in the presence of arbitrary far-field

horizontal stresses. The first considers the well bore fluid

pressurerequired to induce an axial fracture in a previously

unfractured vertical borehole penetrating a uniform, linearly

elastic porous medium. Using (implicitly) the fact that for such

a medium, the Kirsch solution for the stress concentration

about the borehole due to the far-field horizontal stresses is

independent of the poroelastic properties of the medium,

Haimson [1968]derived an expression or calculatingSu from

the well bore breakdownpressurePb which we write given by

(] 2,) }

o=3Sh-Pb+T-pwaU+ (p•an_pp)

(1)

Here T is the ensile trengthT > 0), Pp s the ambient ore

pressure >walls the porepressurewithin he boreholewall

A p

at breakdown and q is Biot's constant. The expression s

DEPTH

RANGE

(m)

I

8OO

/

/

APPLETON WELL

SANDSTONEBED MEAN ORIENT OF INDIVIDUAL

MUDSTONE/SILTSTONE FRACTURE LIMBS AT WELL BORE.

Fig. 14. Mean orientation of induced fractures traces in the Ap-

pleton well in successivedepth intervals.


19/26

EVANS ET AL.' APPALACHIAN STRESS STUDY, 1 7147

CASE I:

STEEP SLOPE; HALF HEIGHT (107M) ATTAINED IN 320M.

½ 200

o

o o

uJ -200

• -400

o

uJ -600

T S21•D

a_ -800

HORIZONTALTRESS, Fx/Pcjb

VERTICALTRESS,CFv/pgb

APR

WlLK. PROFILb. 1-OPOG i

PROFILEORO.•.•_••"'•'...-O'OF"•-

I --

---0.5 Coniours

2-_ Denore

e- holfspoce_-e

stresses crxtOp/pgb

, _1.2_ -

- - -I .0

3 --

-I.8

_

-I.5

I I I

o I 2 3 4

X/b

WILKINS

J

LL

LIJ

J

J

0

d

APPLETON

-4.2

I

0 I 2 3 4

CASE 2:

LESS STEEP SLOPE; HALF HEIGHT ATTAINED IN 533M.

PLATEAU TOP

)"•=13m

I

2OO

-200 -0.6

-i.0

-i.2

1.8

-4OO

400 800

-600 -I.5

0 400 800 0

-80O

DISTANCE ALONG PROFILE (m)

COMPRESSION NEGATIVE

Fig. It;. Contours of (left) horizontal stress and (right) vertical stress predicted to result from the erosion of a

symmetric valley into a laterally restrained half-spaceof Poisson's atio 0.33. Two topographic profiles are considered.

Approximate topographic slopes n the vicinity of the Wilkins and Appleton wells are shown by the dashed and dotted

lines, respectively, for comparison with the model slope. All stress magnitudes are normalized to pgb and must be

multiplied by 5.66 to obtain values in megapascals.Horizontal stressmagnitudes (left) are essentially laterally uniform

at stratigraphic horizons deeper than 300 rn below the valley floor. Vertical stress right) increases with depth below the

valley floor at a rate greater than the "overburden" which is delineated for each well by the graduations along the well

profile.

exact for two end-member situations. The first is where well

bore fluid does not infiltrate the borehole wall during the time

taken to raise interval pressure to breakdown levels, and in

this case,p•/all __pp. The seconds wheresignificant

infiltration occurs such that interval pressure at breakdown

extends some considerable distance into the wall. In this

case, ;all__Pb' n viewof hesubmicrodarcyermeability

of the majority of rocks in question, we assume infiltration

was egligiblend ake •/all _ p.Equation1) hus educes

to

Siq= 3Sh Pb + T- Pt, (2)

It is of note that had infiltration been significant, we would

expect to observe evidence of horizontal fracture initiation

given the inferred stress regime above the H-sand [Evans et

al., 1988] yet this was not found. Also Kalyoncu et al. [1979]

examined core specimens taken from a well 20 km distant

from the Canisteo site (EGSP NY1) which sample the same

lithologies encountered in the Canisteo wells and reported

no detectable permeability. A further demonstration of the

remarkably low permeability of these rocks is given by the

observation that after a cement plug had been placed in the

Appleton and O'Dell wells at the level of the Onondaga


20/26


a

o

2oo

4OO

Depth

(m)

600

8OO

IOO0

1200

MPa

0 10 20 30 40

, • I , I , I ,

.J '• Super-lithostatSlPs

I • and pred. Sv profiles

,,.,•/Sv:ase

' " Sv: Case 2

ßSlPMPa)

- Overburden

b o

o

2OO

4OO

Depth

(m)

600

8OO

IOO0

1200

MPa

10 20 30 40

I , I , I ,

APPLETON WELL

Super-lithostat ISlPs

and pred. Sv profiles

ß

Sv: Case 1 &

Overburden

K-san• Sv: Case 2

ß ISIP (MPa)

3OO Topographicrofile

Wilkins

Height Cas

(m) 0 e•

0 •ase 2

0.0 1.0 2.0 3.0

Distance from valley bottom (km)

300 Topographic profile

Appleton

Height Case

.

. .

0.0 1.0 2.0 3.0

Distance from valley bottom (km)

Fig. 16. (a) Profiles of vertical stress along the vertical trace of the Wilkins well shown in Figure 15 for the two

theoretical topographic profiles considered shown n the bottom figure). Also shown s the overburden and those SIPs

which define the near-lithostat trends. Evidently, the near-lithostat trends correspond closely with the predicted vertical

stress profile. (b) Same as Figure 16a but for the Appleton well.

limestone (Figure 3) and the water level baled back to 600 m

below surface, the level remained stable for 2 years until the

wells were refilled with brine several months prior to stress

testing. The Wilkins well was loaded with brine some 2 years

prior to stress testing.

A drawback with the first method is the necessity to

estimate the "appropriate" in situ tensile strength of the

rock in question, a requirement which at best can only be

fulfilled in a statistical sense [Ratigan, 1981]. To overcome

this, a second method was suggestedby Bredehoeft et al.

[1976] which considers the well bore pressure required to

reopen the fracture induced during the breakdown pump. If

it is assumed hat the fluid pressure within the fracture near

the well bore remains at the level of the formation pore

pressure during the pumping period that leads to reopening,

the mechanical formulation follows that underlying equation

(2) with the exception that the tensile strength term is zero.

That is,

SH-- 3Sh - PRO - Pp (3)

where PRO is the well bore pressure at which the fracture

begins to open at the well bore.

Unfortunately,n situations hereS/_/>2Sh - Pp,method

2 (equation (3)) must be applied with great caution. Where

this condition is met, the elastic hoop stress across the

mouth of the fracture (in the immediate vicinity of the well

bore) is less than Sh. Hence although he fracture may begin

to open at the well bore when the true reopening pressure

(required by equation (3)) is reached, the "opening" will not

propagate beyond the immediate vicinity of the well bore

until the pressure eaches he value of Sh, which is the stress

normal to the crack face beyond the well bore stress con-

centration. Unless hydraulically stiff interval pressuring sys-

tems are used, the corresponding fluid loss from the well

bore may become sufficiently great for detection only when

the pressure has reached the value of Sh. In such circum-

stances, a reopening pressure equal to the ISIP will be

recorded, whereas the true reopening pressure that features

in equation (3) is less than this. Equation (3) will then yield

an underestimate or S/_/. n the majority of tests conducted,

we observed reopening pressures which were disturbingly

similar to the ISIP. The reopen pressures were estimated

using the method described by Hickman and Zoback [1983].

The hydraulic stiffnessof our system is 8.5 MPa/L, a value

which is not unusually compliant. However, in view of the

possible mplications of the correspondencebetween reopen

pressures and ISIPs we have analyzed the data using both

methods. Estimates obtained using equation (2) are pre-

ferred.

Estimation of pore pressure. For the purpose of calcu-

lating S/_/ estimates from equations (2) and (3) we have

assumedhat the relevantporepressure p is givenby the

hydrostatic pressure of the fluid in the well bore (density 1.1

x 103 kg/m3). Although ormationpore pressuresn the

"Devonian shales" are generally uncertain owing to the

difficulty of measurement n rocks of such ow permeability,

it is the pore pressure n the vicinity of the well bore, rather

than the remote formation pressure, that is relevant for


21/26


TOTAL STRESS (MPa)

0.0 I0.0 20.0 30.0 40.0 50.0 57'.0

WILKINS WELL

MEAN STRIKE (øE of N)

o 45 90 135

I I I ,

DUNKIRK

2OO

Ii x I

•

ß x x I,

600 • xx ".

ß x

,.. I. •


22/26


0.0 I0.0

0

2O0

400

TOTAL STRESS (MPa)

20.0 30.0 40.0 50.0

6OO

800 -

I000-

APPLETON WELL

57.0

I

MEAN STRIKE (øE of N)

0 45 90 135 180

e , ß

X X F ' ß

--X ß

_

G -

x i i

- 5 ', ß

•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII1•11111111111111111111111111IIIIIIII1•1•

''..'...............'..............''''...............'..''............`...........`...`...'.';.`;.........'.'.'.'.'.........'.......

• Sh •overburden(p= 2 71 O*kg/m)

x SH ' Method2 (reopen)

I SH ' •thod I (breokdown)error denotes RMS sootier in tensile strength meosurements

Quortz-rich beds in mud/siltstones

....."'"'"'"'"'" Limestone beds in mud/siltstones

Fig. 18. Summary of total stress estimates or the Appleton well. See Figure 17 for explanation.

B

•H

K

provide only lower bounds to the true value of Sh. Where

this is the case, the plotted values of S • must also be taken

as lower bounds since from equations 1 and 2 it is clear that

the underestimate n S• will be 3 times that in Sh.

It is evident that estimates derived using method 2 (re-

opening pressures) are generally less than those derived

from method 1 for the reasons discussed previously. A few

exceptions are evident which we ascribe to exceptionally

high tensile strength of the rock in the straddled interval.

PhysicallyunacceptableS• estimateswhich are less han Sh

are listed in the tables but omitted from the figures. Where

such anomalies occur, the lower bound estimate on S• given

by method 2 remains valid. The majority of the high strength

intervals were encountered in the shallow sections of the

well, and breakdown pressuresas much as 30 MPa in excess

of reopening pressures were observed. We note that Haim-

son and Stahl [1970] also report similarly high breakdown

pressure excesses or tests conducted in neighboring Alleg-

any County at a stratigraphic level equivalent to our B-sand

and ascribe them to mud lining the borehole wall. This was

not the case here, as mud had never been loaded into any of

the holes, all of which were drilled by rotary percussion.

Rather, we believe the inferred high tensile strengths are real

and reflect locally intact rock of unusually small microcrack

size. That the material is capable of such strength s demon-

strated by the results of the Brazilian tests where tensile

strengths as high as 23 MPa were inferred for some speci-

mens [Cliffs Minerals Inc., 1981].

The S,,•,profiles show that maximum horizontal stress

generally follows the same pattern of variation as the mini-

mum. This is most clear in the stratigraphic superposition

shown n Figure 9. Like Sh, the magnitudeof S• in all shales

below the J-sand s the same at common stratigraphic evels

and undergoesa drop across the K-sand. This drop in S•

between shales above and below the K-sand is 9 MPa, which

compareswith 3.5 MPa for Sh. The contrast in S• between

the K-sand and the immediately underlying shale is at least

14.5 MPa.

Proximity of Inferred Shear Stress Levels

to Coulomb Failure Conditions

In the lower regime, where the horizontal stress estimates

are reliable, the ratio between the least and greatest principal

stresses s approximately 1.75 to 1. In the upper regime and

the transition zone sands, where Sh (and hence S•) may be

underestimated, the ratio is at least 1.75 to 1. Such high

levels of shear stress suggest hat the shales may be close to

failure. To evaluate this possibility, we have calculated the


23/26


w

o.o IO.O

o

_

200--

400--

_

600 -

800-

_

IOOO-

TOTAL STRESS (MPo)

20.0 $0.0 40.0

I I I

i i l

I ,

O'DELL WELL

50.0

I

57.0

I

B

ß

•overburden (p =2.71 103kg/m)

Fig. 19. Summaryof stressestimates or the O'Dell well. The estimatesassume hat each ISIP is equal to the least

horizontal stress at that depth.

profile of S, required for incipient shear failure of a cohe-

sionlessCoulomb material supporting the observed Sh and

Sv evels and permeatedby a hydrostaticallypressuredpore

fluid. We take cohesion as zero for convenience. This

assumptionwill promote underestimation of shear strength,

particularly at shallow depths, but it is admissible for our

purpose of determining whether the inferred shear stress

levels approach plausible failure threshold levels. The criti-

cal values of S, are derived from the equation [e.g., Jaeger

and Cook, 1976, p. 96],

S1= S3+ 2/.6{(/.6+ 1) /2 /a,}(S -Pp) (4)

where S] and S3 are the greatest and least principal total

stresses, p is the formationpore pressure, nd /x is the

coefficient of internal friction. Predicted failure thresholds

for both/x - 0.85 and 0.60 are indicated n Figures 17 and 18.

For the section above the K-sand (or more precisely, where

the ISIP lies on the appropriate near-lithostat trend), the

least principal stress s vertical and, neglecting topographic

effects, is approximately equal to the overburden (Sovcr

pgd). Thus we take S3 - 26.6d MPa where d is depth in

kilometers. For the lower regime the least principal stress s

horizontal. Here we use S3 - 20.5d MPa and S3 = 19.0d MPa

to approximate the Sh trends in the Wilkins and Appleton

wells, respectively. For shales in the lower regime the

thresholdS, levels ie above the observedvalues or both/x

= 0.60 and 0.85. Thus shales in the lower stress regime are

not at the point of failure. Consideration of nonzero cohesion

would further strengthen this conclusion. For the upper

regime the plotted S, profiles in both the Appleton and

Wilkins wells lie close to the failure threshold provided that

/x = 0.6. For larger coefficientsof internal friction, or the

adoption or nonzero cohesion, the predicted threshold

would be shifted to values above the plotted SH estimates.

However, as these S, values can only be taken as lower

bounds to the true maximum horizontal stress level, failure

conditions might also be met for /x greater than 0.6 and

nonzero cohesion. Thus the state of stress in the upper

regime, in the transition zone sands, and in the Tully

limestone is compatible with view that it is governed by a

yield-type behavior which maintains stresses t their limiting

value. Since the least principal stress s vertical, the senseof

failure is "thrust." The state of stress in the shales of the

lower regime, however, is below the limiting value.

Effects of Fracture Propagation Around Packer Seats

Fracturing aroundpackers s a common occurrenceduring

open-hole fracturing operations in shales [Daneshy et al.,

1986; Whitehead et al., 1987]. We observed direct evidence

of fluid bypass of the packer seals in 18 tests (Table 2).

Lower packer bypass was recognized by rushes of fluid

uphole during packer depressurization, ndicative of a pres-

surized well bore below. Fluid effusion from the well bore

during pumping indicated upper packer bypass. Inspection

of televiewer images showed that in all but two of these

cases, fracture extension in excess of 0.85 m along the

1.04-m packer seals had occurred (Table 2 and Figure 12).

Furthermore, in the Wilkins well a further eight intervals


24/26


showed fracture extensions which were sufficient to consti-

tute a potential breach of seal. That no direct evidence was

observed in these cases can be partially ascribed to the

uncertainty in interval location on the televiewer images

(which we suggest s less than 40 cm at 1 km depth).

The section below the K-sand interval was particularly

prone to fracture propagation around the packer seal. In

fact, all but two Wilkins well tests in this section showed

either direct (unequivocal) or televiewer (equivocal) evi-

dence of seal breach (Table 2). As this section corresponds

to the lowermost stress regime, it is important to determine

whether the low ISIP values which characterize this zone are

merely a consequenceof seal breach. The following summa-

rized observations testify that the observed ISIP values are

indeed valid indications of Sh.

1. The three lowermost tests in the Appleton well

showed no evidence whatsoever of bypass and are thus

considered valid. These serve to define the existence of the

stress transition in the vicinity of the Appleton well at least.

The ISIPs for these three Appleton tests correspond closely

with values measured at equivalent stratigraphic

appalachian stress study 1 - a detailed description of in situ stress variations in devonian shales...

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