SPESPE 15641

Earth Stress Measurements in the South 13elridge Oil Field,Kern County, Californiaby KS. Hansen and W.R. Purcell, She// Development Co.

SPE Members

Copyright 19SS, Society of Petroleum Engineers

This paper was prepared for preeantslion at the 61 st Annual Technical Crinference and Exhibition of the Societ y of Petroleum Engineers held in NewOrleans, LA October 5-e, 1986.

This paper wasselected for presentation by an SPE Program Committee following review of information contained m an abstract submitted by the●uthor(s). Contents cd the paper, as presented, have not been rwiewad by the Society of Petroleum Engineers and are subject to cofractiin by the●uthor(a).Thematerial, as presented, does not necessarily refleot any position of the Sodefy of Petrofeum Engineers, its offiiers, or members. Papars

presented ●t SPE meetings ●re subject to pufsfiiafkwr rwiew by Editorial Committees of the Socii of petroleum Engineers. Permission to ~ isrestricted to ●n abatrast of not mora than 300 words. Illustratiis may not be copied. The ●bstractshould contain conspicuous acknowledgment ofwhere ●nd bywhomthe paper is prasenwd. Write Pubficatim Manager, SPE, P.O. Sox S3363S, Richardson, TX 7S0S3.3S3S. latex, 7XfeSS SPEDAL.

ASSTRACT

Within Sections 33 and 34 of the SouthSelrldge oil field, where the principal eerthatressam are assumed to lie in vertical orhorizontal plenee, the azimuth of the greaterhorizontal earth etrees as ❑easured or inferred byaerveral df f ferent techniques ie N15°E ~ 15°. Thisagrees with valuee reported in the literature forthe regional etrees orientation, which range fromN-S to NNE-SSW.

At depths of about 850 and 1300 feet, themagnitude of the vertical strese (overburden) liesbetween those of the greater and lesser horizontalstresses determined from open-hole microfracteets. At about 2100 feet, however, the verticaland leeser horizontal stresses are equal, to withinthe limits of accuracy of our measurements. Iftrends of etress versus depth established at thethree measurement points continue downward, thereis the possibility that at depths below 2100 feetthe minimum stress may be vertical. .

The meet reliable methode used for determiningin-situ atress orientation at South Selridge aresurface tiltmetevs and orientation of out-of-round(elliptical) boreholee, Study of natural fracturesin the nearby Chico-Martinez Creek outcrop providedimportant eupportfng evtdence of stress orienta-tion. Irttpreasion packers run during the microfractaste and seismic data recorded during routinehydraulic fracturing procedures yielded much lessdefinitive information concerning fracture andetreae direction.

References and illustrations at the end of paper.

INTRODUCTION

Welle completed in the diatomitelporcelanitereeervoir of the North and South Selrldge oilfielde (Figure 1) are usually hydraulically frac-tured at tha time of their initial completion.Current expectations are that primary recoverytechniques will recover only 5% to 6% of theoriginal oil in place. Supplemental recoverytechnlquea, therefore, may provide a significantportion of the ultimate production from thle majoroil accumulation. The euccees of any supplementalrecovery technique will depend in pert on orienta-tion of the extensive hydraulic fracture eyeteminduced during primary production, the interactionof theee induced fractures with the natural frac-tures present at Selridge, and the interdependencyof both induced and natural fractures on the stateof stress within the reservoir.

Aa a first etep in addressing these problems,we have attempted to determine ae precieely aepossible the atate of atrees in thediatomite/porcelanite reservoir at Selridge. Thethree principal earth stresses may reasonably beassumed to be oriented vertically and horizontallyin this reeervoi.r. A complete description of thestate of atreas therefore requires determination ofthe magnitude of the three etresaes and theazimuth of one of the two horizontal streaaea. Thevarious methods used for detemr,ining state ofstress at Selridge are basad on geologicalobservation and on borehole mechanica.

Orological methods allow orientation endpossibly relative magnitudes of the three principalstreaaee to ba inferred. For example, in araaa ofrecent or active faulting, the following streeareglmee are believed to prevail, For high-anglenormal faulting, Sv > SH > Sh$ with the greater

SPE 15641 K. S. HANSENANDW. R. PURCELL 2

Ior~20r.:*1 stress S oriented along the direction,f surface strike. %or near-vertical strike-clip!aulting, SH > Sv > Sh, with SH oriented at aboutiO 0 to 45° with zespect to the fault. In the case]f low-angle thrust faulting, SH > Sh > Sv, with SHperpendicular to the surface strike of the fault.

Borehole-mechanice methods examine state ofItress at the borehole well under conditions of:enaile failure (hydraulic fracturing) orcompressive shear failure (borehole sloughing).

Nhen a fracture is induced at a borehole wallmd then extended into the formation it tends toLign iteelf in a plane perpendicular to the least

1 If, as is assumed here,wincipal earth stress.:he principal earth atresaes are orientedvertically and horizontally, and if one of the twoHorizontal principal etreeses is the least of the;hree, then a hydraulically induced fracture will.eave a vertical well bore in a vertical plane andTill maintain that orientation as the fracture iamopagated outward into the formation. The induced!racture will align with the direction of SH* ThUS

I measure of fracture azimuth gives the direction~f S and vice versa. on the other hand, if the

1!~ert cal stress is the leaat of the threa, principalItreszes, then the fracture may2still leave the3s11 tn a vertical ortentatton, but as It entera aregion in which the earth’s stresses are]aaentially undisturbed by the presence of the~orehole the fracture will tend to turn ov”er and>ecome horizontal.

By the time a fracture has been extended ftveto ten borehole diameters into the formation it]hould be responding to earth stresses which, at:his distance from the borehole, are distorted byless than one percent.3 The closure pressure of:he fracture should then very nearly equal theleaat principal earth stress, which will be either:he vertical stress Sv or the lesser horizontalJtress S , The instantaneous shut-in pressure[ISIP) t~at fs recorded following fractureinitiation and propagation is aometimea ateaaonably good measure of the fracture closurestress, and in thla study at Belridge is consideredto be a direct measure of the leaet earth stress,

If there is sufficient imbalance between thetwo horizontal stresses, compre s ve shear failure~f the borehole wall may occur, t,! This cauaeasloughing and hole enlargement along a preferredcross-sectional axia. In a vertical hole, enlarge-ment will generally be greater along an axisaligned wfth the direction of ~ (see Appendix).If there are known instances of out-of-round bore-holes that can reasonably be attributed to streaa-related shear failure, then oriented four-armcaliper surveya or b~rehole televiewer logs can bsused to d t rmine azimutha of the horizontalstre8se#

STRESS MAGNITUDES

During October and November 1980, elevensmall-volume hydraulic fractures (“microfracs”)were induced to determine magnitude of the

principal horizontal atresses. The fractures mreformed between straddle packere set in open hole attwo or three different elevations in each of fourclosely spaced wells. The packer and wira lineservices were coordinated by J. M. Gronaeth of theTerra Tek Company, Salt Lake City, Utah, which alsoprovided technical advice, pumping equipment, andsurface-pressure and flow-rate meaaurementa.

Location and Procedure

The four wells selected for this studypenetrate the contact between the BelridgeDfatomite and the overlying Tulare Sandatone nearthe crest of the structure (Figure 2). Threeintervals were fractured in Wells A, B, and C andtwo intervals in Well D. Correlation of electriclogs assured that corresponding intervals werefractuzed in all four wells. The eight-foot (2.4-m) intervals between straddle packers$ in which thesmall fracturea were induced, fell within largerintervals that were later hydraulically fracturedthrough perforated casing during routine completionoperation of the four wells,

Preparatory to microfracturfng in open hole,the straddle packer assembly was run In on drillpipe with the hole and PIPS filled with the“standard” mud ueed field-wide for drilling thediatomite. Mud density wee nominally 8.9 ppg (1.1kg/L). After the peckers were eet, fresh water waspumped In on top of the mud at a constant rate of 3gal/rein (11 L/tin). Preesure was recorded aa afunction of tfme at both the surface and In theinterval between the pecbra by a surface recordingAmerada preesure gauge. A digital memory raedout(DHR) preesure recorder was hung below the bottompacker to detect any pressure communication thatmight occur during the fracturing operation.

Breakdown of the formation was followed by asuccession of five or more periods wherein mud watipumped into the induced fracture for one to fiveminutes followlng which the ey.etem was shut in fortwo to ten minutes, The three pressure ve. timerecords obtained in Well B are shown in Figure 3.In this figure, the continuous solid lines arerecordings of the downhole Amerada pressure gauge,and tha dotted lj.nes are the DMRrecords adjubtedto the elevation of the Amerada gauge.

Vertical Stress, SV

As etated above the principal earth etreesesat Belridge are aasumed to be oriented verticallyand horizontally, The vertical etress Sv wasdetermined from Formation Density Compensated (FDC)logs run in Wells B and D. Differences of leasthan 20 psi (140 kPa) were observed over the inter-vals of interest, and so averages of measurementsfrom these two wells were used to provide Sv aa afunctfon of depth for all four wlla.

Laaat Stress, Sh or SV

Several ISXP’a were meaaured for each intervalfractured. Usually, after one or two pump-inperiods following formation breakdown the ISIPreached a value that then ramained constant to

3 EARTNSTRZSS NSASUSSNENTSIN TNE SOUTHBELRI~ OIL FIELD, KERN COUNTY,CALIFORNIA EM 15641

within *2O psi (i140 kPa) throughout subsequentpwnp-fn periods. h average of three or more

water table at 400 feet (122 m) below g?oundlevel.

ISIP’S wae taken aa a measure of the leastprincipal stress associated wfth each fractured Summary of Stress Nagnitudeainterval. In the two upper Intervala of the threetested, valuee of ISIP were alwaya significantlyless then the corresponding vertical (overburden)

Table 1 gives valuea of Sv, ~, and SH, thelatter calculated for both the caae of complete

strese, indicating that the least of tha threa fluid penetration and for no flow into theprincipal stresses lies in a horizontal plane. In formation. AL1 valuea are baaed on downholethe uppermoet zone the ISIP’a ranged from 71% to79% of the overburden; in the middle zone this

maaaurements of the breakdown preaSUre pb, eXCept

range was 78% to 86%. Nowever, In the deepestIn Test No. 1 where the downhole gauge failed to

interval tested the ISIP’a were nearly tha same asoperate. in this one instance, the preeaurerecorded at the surface waa used after adjustments

the overburden (to within N%) suggesting thepossibility that the vertical stress might be the

were made for the weight of the mud column (1010

leaat of the three principal stresses at thispsi = 6964 kPa) and for friction loss (about 160pst = 1103 kPa) in the tubing and surface lines.

depth. In the calculation of SH it is assumed that the

Greater Horizontal Streaa, SHISIP’a are a direct meaaure of ~. As pointed outabove this is appropriate for the top and middle

The pressura required to initiate a fractureintsrvals, but could conceivably be incorrect forthe bottom interval where it is possible the ISIP

at the borehola wall, pb} can under ideal circ~- is meaauring Sv rather than . If this be the

istances prov de a measure of SH If the masnitude of ?0~ls known* This deduction is baaad on several

case$ then the valuea of SH a wn in Table 1 aretoo low by an amount equal to three times thb true

assumptions, namely, that: (1) the borehole is difference between ~ and Sv.round, smooth, and vertical; (2) the rock ISia, t~plc and responds by linearly elaatic plane In Figure 4, average values of ~, ~, and S ,strain to the changea induced in the original as taken from Table 1, are plotted as functions o1!stress atate of the undisturbed earth by drllltngthe hole; (3) the fracture of the rock is a

the depth corresponding to the midpoints betweenstraddle packer settings. The scatter of meaeure-

tenslonal failure that occurs when the effectivetangential stress equals the tensile strength Tof

ments about the averagea indicates an uncertainty

the rock; and (4) the presence of the straddleof leas than *7O psi (&80 kPa) in Sh and $140 psi(i970 kpa) In SH. Mat of this uncertainty

packere doea not affect the state of stress at thepoint where the fracture firet forms. With these

probably results from experimental error ratherthan from reel differences in the earth stressee

aaswnptions, the equation used to calculate the among the four wells. If the trende shown ingreater horizontal atrasa SH from8the Pressura pb Figure 4 continue downward, it appears that atrequtred to induce a fracture fa: deptha below 2100 feet (640 m), Sv might be the

least of the three principal stresses.sH=3sh-Pb-P+T+A(P -Po) (1)

Comparlaon of ISIP’a frca M.icrofracture andwhere A, the poroelastic constant, hae a value of Conventional Fractureabout 0.77 for diatomite (see Appendix).

In estimating the greater horizontal strassFollowing the microfrac teata in open hole,

from the above equation, two limiting cases ofeach of the four wells was completed in the usual

fluid flow have been considered, namely (1) wheremanner by cementing caalng to bottom and then

there is no restriction to flow ao that the poresequentially perforating and fracturing some savenor eight intervals in each well. We can,

pressure in the rock at the point of fracture is therefore, compare ISIP’a meaeured during thethe same as in the well bore (P = Pb), and (2)where there is no flow at all so that the pore

microfrac tests with those recorded at the time ofhydraulically fracturing corresponding intervals

preseure at the point of fracture ia the ambient during completion of the wells. The ISIP’S for thereservoir pore pressure (P = PO)* completion fractures were computed by adding to the

A few Brazilian tensile strength measurementssurface pressura recorded at shut-in thehydrostatic pressure of the fluid column from the

were made on quarried samples of diatomite. Valuaswere all l~ss than 100 psi (689 kPa). In this

surface to a depth corresponding to that at whichthe microfrac teat had earlier been made, Thi S

etudy a value of 100 psi is used, although in viewof other uncertainties in applying (1), tha

column waa compoeed of a fluid of danaity 8.5 Ppg

tensile-strength term could have been ignoted.(0.44 psi/ft = 9.95k?a/m) fr’uu eurface to 150 feet(45.7 m) above the top perforation. Below this was

Reservoir pore pressure P. ia not known verythe @and-laden gel umed In the final etagee of the

precisely and has been estimated fromfracturing operation, which had a density of about13 ppg (0.67 pai/ft = 15.2 kPa/m). In Test No. 2,

P. - 0.44 (z - 400) (2)for example, the midpoint of the 8-foot (2.4-m)intsmal between straddle packera was at adepthof

where P. is in pai and Z is tha depth in feet.1309 feet (399oO m). The corresponding intervtl

This aesumea a normal hydraulic gradient and athat was fractured in this well during completionoperationa was 1179-1319 feet (359.4-402.0 m)

SPE 15641 K. S. HANSENANUW. R. PURCELL 4

1Subsurface. me recorded pressure at the surface the outcrop strikes approximately N60°W and dipsat shut-in waa 100 psi (689 kPa). Hence, from 75* to 45° to the northeast. Rotation of the

bedding planes back to horizontal, as nuld beISIP = 100 +0.44 (1179 - 150) appropriate for interpreting fracturing that

occurred prior to deformation of the sedimentary+ 0.67 [1309 - (1179 - 150)] beds, results in the bedding-corrected fracture

= 740 psi (5110 kPa).orientations ltsted in Table 3. The rotated datagive generally improved standard deviation in

This is compared to the ISIP recorded during thecomparison with the original data, but tne differ-

not significant according to Fisher~s Fmicrofrac of 780 pai (5380 kPa). :2 ●i? This suggesta that fracturing has occurred

In Table 2, a comparison ia mada betweensimultaneously with, or partially overlapped, the

ISIP’a determined from microfrac tests and thoseperiod of active folding rather than completely

later measured during completion operations. It ispredating or postdating anticlinal development.

seen that there is close agreement between theISIP’a measured under the two different circum-

We have interpreted fracture acts I and 11 aa

Exact agreement would not necesearfly benear-vertical conjugate seta representing anti-

Stances, thetic and synthetic Riedel shears, respectively,expected because the microfrac interval was only a

activity~2asaociate with San Andreas fault zone

small part, and within the lower portion, of the Their subtended angle is equal to 70”,corresponding conventionally completed interval. and the bisector of this angle implies a direction

of N4°E for S . For bedding-corrected data, the

STRESS DIRECTIONSFsubtended ang e is equal to 86’ and the inferred SH

azimuth is N7°W. Fractures may have been more

Regional Structural Geologynearly vertical at the time of their formation thanindicated in Table 3, with contemporeneoue or

The regional state of stress along the Sansubsequent folding ●ccounting for their presently

Andreas fault, which pasaes within 12 miles (20 km)observed devlatione from vertical. From conalder-atlons of both the original and bedding-corrected

6 tfiof south Belridge F ure l), has been reviewed byZoback and 2oback. ‘

data, we conclude that the orientation of S haeNest of their stress data averaged about N-S during the pest four mil~ion

are from geologic indicators (active and recentfault movements), but thesa are generally cOnSfS-

yeara, which ia within the period of activedefozmatlon.

tent with observations of borehole ellipticity anddirect atresa measurements from hydraulicfracturing that have been made at a few

Borehole Elliptfctty

locations. According to linear, isotropic, poroelastic

The predominant stress regime is strike-slip,stress-strafn theory, state of etress at the bore-hole wall dependa on the far-field earth atresaes,

aa anticipated f’ m influence of the San Andreasfault itself. However, mixed thrusting and strfke-

density and fluid-loss properties of the drilling

slip faults coexist in porttons of the Californiamud, reservoir pore pressure, and elaetic conatanta

Coast Rangea as well as in the Big Send region ofof the formation. If there is a difference in

the San Andreas located to the south and southeastmagnitude between the two horizontal principal

of Belridge. This auggeats that the regional stateearth stresses (SH > Sh), then state of stress will

of stress near Belridge is such that S > s“ ~ Sh,vary around the well bore circumference depending

Ysimilar to reaulta obtained from our m crof]actureon position with respect to the trajectories of S

!/and S . Certain portions of the borahole wall WI 1tests (SH > Sv > Sh). The azimuth of S ia

!Vthere ore be more susceptible to failure than

generally N-S to NNE-SSWalong the San An reasfault, which is likewise consfs~ant with our

others. As mentioned previously, probability of

results to be discussed below.shear failure is greatest along an axis alignedwith the direction of Sh and least in tha directionof SH (see Appendix).

Outcrop Fractures

Orientation of the principal earth stressesIn this study, a number of oriented four-arm

was inferred from fractures observed in outcrop ofcaliper logs from Sections 33 and 34 were examined

the reservoir rock in Chico-Nartinez Creek locatedfor instancea of borehole elongation. Out-of-round

2.5 miles (4,0 km) southwest of Belridgs (Figuraintervala werz generally considered to be those in

1). Fracturas were observed in both Belridgewhich the two recorded diametars differed from one

Dfatomite and Chico-Nartinez Chert, which is theanother by more than 1}4 inch (1/2 cm) on averaga

outcrop equivalent of Selridge Porcelanite. Strikeover a continuous depth interval of at least 50feet (15 m). All data were determined from vieual

and dip were recorded for a total of 24 groups offractures located at several different stationa.

inspection of the printed logs, from which boreholadiamater can be read to the neareet 1/8 inch (1/4

The outcrop fractures ware grouped into twocm) end caliper azimuth to the naarest 5°.

aete striking NE-SW and NW-SE, respectively, asshown in Table 3. At a given location, most

In addition to stress. +nduced sloughing, out-

fractures occur in parallel sets, although someo!-round boreholes may reeuit from factors such askey-seating of the drill stem in highly deviated

conjugate sate are alao present. Bedding within

5 BARTNSTRESS MSASURENEN’fSIN TNB SOUTHBEmIDOE OIL FIBLD, KERNCOUNTY,CALIFORNIA SPE 15641

well boree or froa drilling rock formations that within the range N1O”E to N25*E. The greater vari-are efficiently anieotropic in their material andelastic propartiee. The following additional

ation among intervala of margfnal ellipttcity mayindicate that some of these intervale are out of

criteria were therefore considered to determinewhich out-of-round intervals would be the probable

round for reaeons other than strese-related ehaar

indicator for etrese orientation.failure and that they are therefore not reliableindicator of atresa orientation.

1) The difference between the two diametera In eummary, the azimuth of SK inferred fromindicated on the four-arm caliper should be borehole ellipticity in eeveral wells of Sectioneat leaet 1 inch (2.5 cm) over a continuousdepth interval of 100 feet (30 m) or more.

33 and 34 ie N15°E~100 (eee Table 4), which ie inreasonable agreement with the regional streee field

2) Deviation of the hole from vertical shouldde8cribed in the literature. Mvantagea of thie

be leae than 4° within the interval oftechnique are its relatively moderate coet and the

interest.fact that it can be run routinely without disrupt-ing the normal well-completion procedure. One

3) The wells should not be located neardisadvantage is that not all holes become

significant faults.elliptical, 8(. it may require many loge to locatejust a few out-of-round intervals. In this study,for example, etress-related elllpticity was

Based on criteria (2) and (3), several wells observed in 4 of the 16 wells considered (25%). Onwere eliminated from coneideratfon as Lndlcatore ofatrees orientation. The remaining walls were then

the basis of total log feet, strese information wae

analyzed for etresa orientation subject toobtained from only 1660 feet (506 m) out of 32,000

restriction (1) above. Elliptical intervals, iftotal feet (9750 m) examined (5%).

present at all, were generally confined todiatomite. The underlying porcelanite wee

Surface Tiltmetera

apparently more stable with reepect to compreeslve Deformation of the earth’e eurface above ashear failure, with the well bore in most fnetancesbeing very nearly round and to gauge. The

propagating hydraultc fracture at depth generally

diatomite/porcelanlte transition was oftenreeults In an elongated depraeaion parallel to theetrike of a high-angle or vertical fractura and a

characterized by an intarval of borehole rugosfty.or horizontal fracture. t9:~tr2p~P~etL1;E2E?nearly symmetric bulge

Within each Interval of significant deformation field during hydraulic fracturing andellipticlty, orientation of the ehorter caliperpair wee considered to be the trajectory of greater

comparing it with that predicted from linearly

horizontal etreee SK. An example Is shown inelastic modele of fracture growth therefore

Figure 5. Note that the hole Is to gauge in theprovides a technique for inferring orient(etrika and dip) of the induced fracture. 1!:?8

minimum direction and wlthfn 1° of vertical. In Bacause resulting tilt magnitudes are on the orderthie instance, the reference electrode Cl iS track-ing the smaller recorded diameter, and its azimuth

of nanoradians to microradiane, depending on eixe

ie therefore a direct measure of S orientation (O”and depth of tha fracture, eensitive surface tilt-

Y+ 15° magnetic declination = N15°E .meters are required to adequately resolve thisdeformation; the sensors are usually confined in

The azimuths of SH inferred from each interval shallow boreholes just below the eurface for addedof significant borehole elliptlclty, as defined by stability. Thay are typically deployed in one or

criterion (1) above, are given in Table 4. only 4 more concentric arraya about the well, with radiiof 16 total wells considered contained significant of the rings chosen to coincide with the

intervals, but in these intervale the indicated anticipated maximum surface tilt based on theorientation of S waa consistently N1O”E to

Idepths of interest (greater distances from the well

N25”E. One of t e four wane was located on the for deeper fractursa).northeaet flank of the South Belridge anticlinenear the present productive limit of the field, several completion zones in each of two Southwhile the other three welle were cloeely spaced Belridge wells (Figure 2) have been monitored withnear the structural crest in Section 34 (see Figure surfaca tiltmeter artaya during routine hydraulic2). Well G contaified a continuous interval of 660 fracturing. The first teet (Well F) was designed

fact (201 m) in which the inferred azimuth for S to optimize tilt eignale from completions at aboutwas within ~lOO of N15”E (Figure 5), while Well ~ 1500 feet (460 m); eix equally spaced senaora were

contained a 740-foot (226-m) interval in which the deployed on each of two concentric circles locatedSH azimuth wae wlthin~20° of N15°E. 500 and 700 feet (150 and 210 m) from the well.

Based on successful reeults from this experiment,streee orientation ie displayed graphically in

Figure 6 as a 10° rose diagram for all measurablethe second teet array (Well E) was configured to

intarvals contained within the 16 wane consideredracover signals from fracturee ae deep aa 3000 feet

in thle etudy. Radial axis of the rose diagram(910 m). In this instance, a total of fourtaen

gives the composite depth intarval (In ntnzber oftiltmeters was distributed over three concentric

fact from all walls) that is orianted with smallringe located at radii of 400, 650, and 1150 feet

borehole dfameter in a given direction. For all(120. 200, and 350 m). The Innermost ring wae

data, azimuth varies from N55”W to N35”E, but fordesigned to respond to the ehallowaet zones ofintereet (around 1000 feet = 300 m), while the

aigniftcant out-of-round intervals ths boreholeellipticity (small axis) ie oriented consistently

outermoet ring was designed to recover signals fromthe deepest zones.

*E 15641 K. So NANSENANOW. R. PURCELL 6

Analyaie of the tilt data was performed by K.F. 8vana of M. D. Wood, Inc. (for Well F) and by G.R. Nolzhaueen of Tera Corporation (Wall Q. Theinduced fracture orientation determined from tilt-netar reeulte in theee two walla are lieted inTable 5. The fracture azimutha, pre8umedequivalent to trajectory of S , range from N?OE toN16”Ewlth an average of NIOO~. These resulte aresummarized graphically in the 5° roeediagram ofFigure 7.

In Wall F, a high-angle to near-verticalfracture with azimuth of N7”E wee indicated foreach of the five uppermost intervals, althoughuncertainty of the measurements increaeed wfthdepth because of an increasingly weaker signal.These results imply a nearly uniform azimuth for SHextending over the entire diatomite interval andinto the upper porcelanite, consistent with reaultefrom borehole ellipticity in nearby Walls G andL. Fracture-related tilting was aleo observed forthe interval 1849-1969 feet (563.6-600.2 m), butthe maasured ttlt wee too zmall to permit a moredetailed analyeie of the data. For the threedeepest completion intervals of Wail F, no obviousfracture-related tiltlng wee observed, which ie noteurprieing eince the particular tiltmater arrayused tmre wee deefgnad primarily for testingshallower compilation zonea.

Sinilar results tmre obtained from MOl E, inwhich caae analyaieof the tilt 8ignal vaa poaaibleto deptha of 2400-2530 feet (731.5-771.1 m).Fracture-related @igna.ls observed from two deepercompletion zones mre too tmak for datailedenalyals.

Impraaeion Packara fromMicrofrac Tests

The microfrac teate ware each preceedad andlorfollowed by the running of Impression packere.Tha pre- and postbreakdown packers ware requiredto distinguish the induced fractures from anypreexisting natural fractures that might bepresant.

The impreeeLon packera wre about 10 feet(3 m) in length, ao they covered the entirefractured interval with some overlap at each end.The packars were oriented after they had been runin on drill pipe to the appropriate depth but priorto their being inflated. Inflation wae accomplish-ed by pumping fraehwater on top of the mud in thedrill stem until the deeired preseure waeattained. The impression packera mre set at sur-face preseurao ranging from 300 to 450 pei (2070 to3100 ItPa), depending on dapth of the testinterval. Thie praesure wae maintained for about30 minutes to allow the deformable, outer rubberwrap to pick up impreeeione of any surface featuraepretent on the borehole wall. Az soon ae thepackere ware removedfrom the hole, they ware●xamined for Lmpramaione. Significant feature.ware marked imzadlately since much of the reliefinitially present on tha packerwrap disappearedwithin a few hours ae the rubber ralzxed,

Naasurementa of relative bearing with reepactto ●n orientation mark ●cribad on the peckar waremade for all significant high-angle and near-vertical faaturee obeerved on each packer.

Azimuths of the orientation mark mre than addadto the reletive bearings to obtain ezimuths of theobeerved Impreeeione with respect to trw north.

The significant featuree--those which extendedin length for a few feet or more-were of ttmdistinct types: very thin, faint tracea, and ~/4-inch to l/2-inch (1/2-cm to l-cm) wideimpreeelone. Tha “faint”’ tracae ware observable aeelightly raised features at the time the packerswere removed from the borehole. The “wide”impressions were alao raised featuree, but theywere ueually delineated by grooves or teare in therubber wrap running along each side. The width ofthese **wide” impreealone does not necessarily indi-cate width of the obearved fracture. A more likelyexplanation la that tha packer wrap waa deformedinto a fracture thereby stretching the rubber,aometimee efficiently to causa a tear.

Of those featurea tentatively identified aepreexisting natural fractures, based on examinationof the prebraakdown impreeaion peckara, one wee a“faint” treca about 5 feet (1.5 m) in length whileall the others wre “wide” impreeeione.bnver@@lYg apart fran one “wide” Impreealon, allof the induced fracturea appearing on poatbreakdownpackereware represented by “trace” impression.In the one ●xception cited, a prebreakdown packerwa$ not run, so it wee not pocsible to unembig-uouely diatinguleh natural and induced featureaeIt may be that all the “wide” Impreeeione repreeentnatural fractures

Azide from difficulty in distlnguiehing withcertainty fnducad fromnatural fracturee, readtngeof the impreeoion packers ware beset with otherdifflcultlee. Three postbreakdown packera, allfrom the deepaet Interval tested at about 2100 feet(640 m), ware either loet in the hole or aobadlytorn that no useful Information could beobtained, In instances where the aama preexistingfracturea could be identified on both pre- andpoatbreakdown packars, the measured azimutha aome-timee differed by ae much aa 45*. This probablyindicatee that the packere, after being oriented,rotated while being inflated. An inherent diffi-culty ie that, at beet, tha packer records thetraceof a fractura only at the borehole wall, Ithaa been ahown that the direction of fracturepropagation may wander somewhat in the immediatevicinity of tha well before it becomae a planar

:::::lcirndicular to the leaat principal ●arth

Recognizing all these problems, ahigh dagree of uncertainty must be attached tostreaa diractiona Inferred from our etudy of theimpression packara run during these teste. For thacake of completenasa, however, we giva our raadingain Table 6, data from which ara plotted in Figure 8ns a 15° rosediagram, The fracture-orientationdata averaga about N-Si75’.

Geophone Teets

If ●coustic signals recorded during hydraulicfracturing ●re generated by the induced fracturea,then mapping diatributfon of the source locationawI1l provida information ●bout fracture orientation

7 BARTN STRB8S MEASURBNBNl%IN TM SO~H BELRIDGBOIL FIELD, WBRNCOUNTY,CALIFORNIA SPB 15641

(strike, dip) end fracture geometry (height,length). In theory, ttiisource locations can be

presumed to represent the orientation of neer-

determined from a eingle three-component geo hone,vertical fractures intersected by the wall bore.

18 Apart from Test No. 7 (Wall L), the &de Imedlanaapart from an smblguity of 180° Ln azimuth.D%atance La obtained from the difference between

range from N41°E to N66°E, and the Node 11 madiansrange from N41SWto N62”W. Test No. 7, which con-

ahaar- and pressure-wave arrival timsa, and orien-tation la obtained from the relative amplitude

aieted of nonaeiemic events only, axhlbita a sharpunimodal peak at N13°E; this direction ia inter-

recorded by the three geophona components for agiven acouatic signal.

mediate between the other Mode I and Mode IIWith the uee of offset medians. The composite data aet of events from all

instruments, tha source locations can be determined tests is depicted in Ffgure 9 ae e 10° roeeunambiguously. diagram.

There ara alight differences of lese than 5°

Natio=p=~~~~$8;$1~~e~~d=Z~$~ia ~:d::lf determinedbetween the fracture azimuthe presented in Table 7

intervals of Wall N during Auguet 1980 and in one from the same baeic data byzone each of four additional welle durtng November These differences ariee in part because

1980 (Figure 2). The acousttc measurements ware Sandia analyzed the eeismlc end nonseismfc aignalemade with a three-component geophone hung in the separately, generally preferring the seiemic datewell whi;.e it was being fractured. During the for determination of fracture orientation. Sandla

second aet of experiments (November), offeet aecribed the Mods I azimuths to “major” fracturea

listening devicee, consisting of two geophonea and that presumably align themaelvea with the greatertwo hydrophores, were hung In each of the three d the Mode II azimuths towslle that were not being fractured and in one

yi;~t::a:;:e:.f~,$? This conc~ue~on ~e

edditio.~al neighboring well. Because of the acous- partially supported by the obeervatton that most ofticallyabaorptfve nature of diatomlte, howsver, no the events (34L of 529 = 65%) occur in the NErecognizable signals were recorded on any of these quadrent rathar than the NW. In soae of ths indl-

offaet instruments, the neareet of which were 240- vidual sone8, however, the Mode 11 ●fgnals ware the

37o feet (73-113 m) distant from the teet -11. more common ones. There thus ●ppeara to be no

Also, it was often difficult to diatinguieh arrival overriding reaeon for favoring o~e mode or theof the p:eeeure wave from that of the shear wave, other as being repreeentatlve of SM orientation.

end in mveral inatancea the vertlcel geophonecomponent In the teat wall failed to operate.

An alternative explanation of the geophone

Sacause of these difflcultiee, asimuthsof acousticdata is that they represent slippage along pre-

eignale (ae projected on a horizontal plane) wereexisting natural fractures intercepted by or pese-ing neat the well bore. Similar ectlvetlon of

the only usable information obtained at Bslridge.ThIe eufflces to infer azimuth of induced

natural fractures by induced hydraulic fract ~lng

fracturea, however, provided that the fractures arehas been obearvad in othergeophone survey.. ‘The

aeeumed to be vertical.bimodal azimuth diatrlbutton at Belrldge ie euggea-tlveof natural fracturee obeerved in the Chico-

Data or fracture azimuth have been publishedMertfnez Creek outcrop, which occurred in two eeta

by Sandia2i ae the number of events originatingoriented approximately NW-SE and NE-SW. Ths Node

within each 10° azimuth interval from O“ to 180°e11 avents are also comparable to a predominant NW-

The data are tabulated separately for background,SE orientation of natural fracturee obeerved in

breakdown, prepad, and pad atagea of each teetcores at South Balridge; minor fracture satsobeervsd in coree vary frcm well to well, but some

zone, and they are further eubdivtded as to whetherthay represent

of them are oriented NE-SW.“eelamic” or “’noneeismi.c” events.

Seismic events are defined by a dietinct P-wavearrival followed by an S*ave, while noneeismic

Baeed on our analyeia of the data recorded by

events are acouetic eignale that contain noSandia, it appaara that geophone taata by

recognizable P-wave.themselves cannot be used aa indicators of stressorientation at Belridge. Some of the geophone

Apart from one teet zone (Well L) whoee data, however, may represent induced hydraultcreeulte coneiet entirely of noneeiamic events, fractures aligned with S1l. For example, thethere does not appear to be any obvioue difference unfmodal azimuth of N13° recorded in Well L iein the azimuth distribution of eelemic awi non- virtually the same as the N15°E azimuth infarredeeiamic returns. Aleo, thara are no eigniticent for SH from borehole ellipticity within tha samedifference in distribution of azimuth among the well and depth zone (eee Table 4).various stagee of each teet. In our analyeie, wehave therefore lumped together all eeiamic andnonaaiemic events for each tast, except thet CONCLUSIONSeignale recorded during the “background” etageprior to fracturing have been omitted. At depths of about 850 end 1300 feat in

Section 33 of the South Belridge field, the megni-The data are genarally bimodal, with meet tect tude of the vertical etress (overburden) lies

zones exhibiting dietlnct psak@ in both tha NE between those of the greatet and leeeer horizontal(Node 1) and NW(Mode 11) quadrante. Average etreaeee determined fromopan-hola microfracdirections (mediane) have been aeoigned to tha two teateg At ●bout2100feet,however, the verticalmodee in eech test zone forwhich ●ufficlent data and leaeer hori~ontal otreaaee●re equal, to withinexist (eee Table 7). ~eee median azimutha are the limite of accuracy of our measurements, If

trende of stress vereue depth eatabliehed at the

SPE 15641 K. S, HANSENANDW. R. PURCSLL 8

three measuramant points continue downward, there NOMENCLATUREie the poaatbflity that at depths below 2100 feetthe mlnlmum etreae may be vertical. A = poroelastic constant = (1 - 2vm )(1 -

l%e orientation of greater horizontal atreaeKm/Kg)/(l - Um )

SH and of hydraulically induced vertical fracturea,ae meaaured or Inferred by several different c = borehole diameter indicated by calipertechniques, ie N15eE&150 within Sections 33 and34. This ie conaiatant with tha orientation of

log

about N-S inferred for S from natural fractureH

71 - mean effective atresepetterne In the nearby C ico+iertinez Creekoutcrop. It also agraee with valuee reported in ;/2 -the literature for the regional stress orientation, ‘2

root-mean-square shear atreas

which range from N-S to NNE-SSWalong the SanAndreas fault. ‘g = bulk modulus of solid grain material

The two moat reliable methode for determiningKm - bulk modulus of dry or drained rock

In-eitu atresa orientation at South Belridge areframe (“matrix”)

surface tiltmetera and orientation of out-of-round(elliptical) boraholes.

PThese indicate directions

= pore preesure of formation adjacent to

of N7”E to N16°E and NIOOE to N25°E, respectively,well bore, or internal pore pressure in

for azimuth of SH. Impression packera run duringlaboratory triaxial test

the microfrac tests and seismic data recordedduring routine hydraulic fracturing procedures

P. - ambiant pore preseure of formation awayfrom well

yielded leae deffnittve information concerningfracture and strese orientation. Astmuth of:nduced fracturee recorded on the impression

pb = well bore preaeure required to initiatehydraulic fracture at borehole wall

packerw averaged about N-S&75°. Bimodal distribu-tions of geophone data in moat teat intervals Pw = well bore pressureeuggest that the geophones were responding pre-dominantly to elippege along preexleting naturalfrecturaa near the well rather then to tnduced

S1,S2BS3 = greateat, intermediate, and Ieaet

hydraulic fractures.compreaeive principal streeeee

‘r$S&sA =The Ban Andreae fault ●trikee approximately

radial, tangential, ●nd axial streseea

NW-SE about 12 mflas southwest of Belridge; theat the borehole well

ralatlve orientation between our meaeured S andTha La

Shthe fault strike is therefore about 60”.

= leeaer compressive horizontal principal

somewhat greater then the relative orientation ofearth etress

45” expected within aedlmencs overlying a purestrike-alfp zone and la consistent with a regional ‘H = greater compreasfve horizontal

convergence acroee the San Andreas fault in theprincipal earth etress

aet Ranges as indicated by previousZ~~~IYIP Zoback and Zoback propee that the

Sv = vertical principal earth stress

principal atrees trajectories rotate aa they(overburden)

approach the fault eo that the relative orientation Tof s“ is 450 at the fault itself, thus providing

= tansile strength of formation

maximum hear stress favorable for strike-slipmotion.18

zHowevar, if our azimuth data reflect the

= dapth below surface

regional sitrase stata rather than local stress 8-variations, they indicate that near Belridge, at

azimuth on borehole circumference

leaat, the regional compression extends closer torelative to direction of Sh

the San Andreaa fault than pravioualy believed, v = Poieson’a ratio of dry or drained rockm frame (“matrtx”)

ACKNOWLEDGMENTS‘N = effective normal strees acting on plane

We would like to thank Shell Developmentof maximum shear

Company and Shell California Production Inc. forpermission to publish thie paper. R. T, Miller

T- maximum ehear straeamex

providsd valuable assistance in planning and imple-menting the field experiment, while W, E. Hottmanand D. E. Schwartz collected and aasiated withanalysia of the outcrop fracture data. The tri- REFERENCESaxial testsof diatomtte were designed andconducted by H. Hsi, G. LO MoWrey, and G, M, 1, Hubbert, M, K. and Wlllfa, D. G.: “hchenicsTiller. of Hydraulic Fracturing”, Trans, Sot. Petr.

~., V. 210 (L957) 153-66.

9 BARTHSTRESSWEASURBMENTSINTNB SOUTMBELRIDCBOILFIELD,KBRNCOUNTY,CALIFORNIA BPS 15641

29

3*

4*

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11.

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Bradley, W. B: “Failure of InclinedBorehol~s’*, J. !3nergy Resources Technol.,V. 101 (1979) 232-39,

Bell, J. S. and Cough, D. I.: “Northeast-Southweet Compressive Strees in ALberta:Evidence from Oil Welle”. Earth and Planet,scio Lett., V. 45 (1979) 475-82.

Zoback, MOD., Moos, D., Nestin, L, andAnderson, R. N.: “Well Bora Breakouts andIn Situ Stress”, J. Ceophy. Rem., v. 90(1985) 5523-30.

Fordjor, C. K., Bell, J. S. and Cough, D.1.: “Breakoute in Alberta and Streaa in theNorth American Plate”, can. J. Barth Bci.,V. 20 (1983) 1445-55,

Plumb, R. A. end Hickman, S. H.: “Streee-Induced Borehole Elongation: A CompertaonBetween the Four-Ara M$xeeter and theBorehole Televiewer in the Auburn GeothermalWell”, J. GaoDhye. Rae., v. 90 (1985) 5513-21*

Haimzon, B. and Fairhuret, C.: “HydraulicFrecturlnu in Porous-Permeable Matertala”, J.Pet. Tec~ol., v. 21 (1%9) 811-17.

Zoback, M, L. and &back, M. D.: “State ofStresie in the Conterminoue united States”, ~.Geophys. Rae., v. 85 (1980) 6113-56.

end : “Tectonic Streaa Field of~ntinental U. S.”, eubmftted toGeophysical Framework of the ContinentalUnited Statea (L. Pakiser and W, Mooney,lids.), Ceol. Sot. Am. Memoir (1986).

Jenkins, G. M. and Watta, D. G.: SpactralAnalyafa and Its Application, Holden-Day,San Francisco (1968) 85-87.

Harding, T. P,: “Petroleum Traps Azaociatedwith Wrench Faults”, Am. Assoc. Pet. Gael*Bull., V. 58 (1974) 1290-1304.

Pollard, D. P. and Holzhausen, G.: “On theMechanical Interaction between a Fluid-FilLedFracture and the Earth’a Surface”,Tectonophyaicm, v, 53 (1979) 27-57.

Devle, P. M.: “Surface DeformationAeaoctated with a Dipping Hydrofracture”, J,GeODhYe. Rea., v. 88 (19S3) 5826-34.

Evans, K.t “on the Development of ShallowHydraulic Fracturee as Viewed through theSurface Dafocmation Field, Part 1,Principles”, J. Pet. Technol., v. 35 (1983),406-10.

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end Holzhauaen, G.: “On the=Pment of Shallow Hydraulic Fracturea aaViewed through tha Surface Deformation Field,Part 2, Caee Histories”, J. Pet. Technol., v.35 (19S3) 411-20.

Smith, C., Vollendorf, W. C. and Warren, W.E,: “In-Situ Streee from Hydraulic FractureMeaauramenta in G Tunnel, Nevada Test Site”,Report BAND80-1138, Sandia NationalLaboratories, Albuquerque (1981) 72 pp.

Warpineki, N. R., Sclanidt, R. A., andNorthrup, D. A.: “In-Situ Streeses: ThePredominant Influence on Hydraulic FractureContainment”, Proc., First Annual Symposiumon Unconventional Gas Recovery, Sot. Petr.Engr. AIME, Dellaa, Texaa (1981) 83-94.

Saavey, R. W.: “Borehole Belemlc Unit,Report SAND82-0373, Bandia National”Laboratories, Albuquerque (1982) 44 pp.

U. S. Oapertaent of lkmrgy: “Weetern GeeSande ProjectStatue Ueport, October-Noveaber-Decateber 4%0”, refarenceDOE/BC/10003-18, Bartleeville EnergyTechnology Center,LasVagae(1980)100.

: ‘“tbetern Gea Sande Project Status~, April-May-June 1981”, referenceDOE/BC/10003-24, Bartleeville BnergyTechnology Center. Las Vega. (1981) 88-104.

Lacy, L. L.: “Comparteon of HydraulicFracture Orientation Techniques”, SPS paper13225, 59th Ann. Tech. Conf. and Exh. Sot.Pet. Eng., Nouston, Sept. 16-19 (1984).

HandIn, J., Hager, R. V., Jr., Friedman, M.and Feather, J. N.: “Experimental Deforma-tion of Sedimentary Rockz under ConfiningPressure: Pore Preeaure Teete”, Am. Aaaoc.Pet. Ceol. Bull., v. 47 (1963) 71-

Jaeger, J, C. and Cook, N. G, W.:Fundementale of Rock Nechanicn. 3rd Sd,Chapman and Hall, London (1979) 593 pp.

APPENDIX- BOREHOLESTABILITY ANALYSIS

2!!S?IY

When a hole 18 drilled in the earth thepraexistfng streseea are alter d in the nearvicinity of the borehole wall. 9 If the rock beingdrilled ie modeled as an isotropic, linearlyelaetic and continuous porous eolid in ● conditionof plane etrain ●long the exie of the borehole,then the @tree*as induced ●t the borahola wall canbe written ●a functions of the undisturbed earth●treaeea, the fluid (mud) preaeure in tha borehole,the rock’e ambiant pore praeeure ●nd certain of iteelaatic conatante. If the borehole ie aligned withone of the principal eerth #treeaae (e.g.o ●

vertical hole in an ●srth streaa field that iovertically and horizontally oriented--conditions

SPE 15641 K. S, HANSENANDW. R. PURCELL 10

thought appropriate for Salridge), than there are obtained. A constant rate of axial displacementno ahaar atreeaea acting at the wall of the (100 microatraina per eecond) was then appliedvertical hole and the normal atreeeee are, until the plug had failed and was in the reaidual-therefore, principal etreeeee. For theee strength ragime. Raeulta are given in Table A-1.conditions, the etreeeee at the borehole wall The average composition (volume percent)of theee(Figure A-1) are: eleven eemplee ia: 65.0% poroeity, 22.6% bioganic

Sr - pw (A-1 )silica, and 12.4% terrigenoua material.

Sorehole StreseeaSe= S“+Sh+2(S” - ‘h)cos 2e-pW

(A-2 ) To determine if shear failure will occur at+ A(P - Po) the borehole wall, computations of the atreas state

as a function of position around the borehole areSA - Sv+ 2Vm (SH - Sh)cos 2e

(A-3 )made in accordance with (A-1) to (A-3). For thiswe use earth stress data as determined from the

+ A(P - Po) microfrac tests. The procedure ia illustrated withdata from Table 1 for the middle interval et abo~”

To determine over what pert, if any, of the 1300 feet (400 m). The ambient pore pressure P. atborehole wall compressive ehear failure might this depth is about 390 psi (2690 kPa). Welle areoccur, the strese etate at the wall, ae computed by(A-I) to (A-3), ia compared witha rock strength

routinely drilled through the diatomite section

cwve appropriate for the particular rock underwith8.9 ppg (1.1 kg/L) mud eo that at 1300 feat Pw

etudy. These failure condition can be e ablished= 600 pei (4140 kPa), ALso, S ie about 950 pai

by laboratory trtaxial compraeaion teeta.1! (6550 kPa) and

2Iabout 775 pe (5340 kPa). We

Varioue mathods have been used to generalise thecompute streee e ●tes for the two limiting caeee:(1) no flow to the fomation, in which caee P -PO

reeulte of the laboratory teote ao that they can be ●nd S ie 1150 psi (7930 kPa); ●nd (2) unreetrictaduaad to predict ●treae statee required for rockfsilure under conditions other then thoee spec -

i4

!!flow o the formation, in which caaa P = Pw ●nd SH

fically applied in the laboratory ●xparimente”.ia about 1045 pei (7210 kPa). Strains meaaurad

Two of these aethode deserve ●pacial mention, One,during triaxial defamation teats gave ●n average

based on the von Mae. failure theory, utilizee thevalue for v of 0.18; ~is very much smaller than

nagnitudee of ●ll three principal etreeaee ●t fail-Kg so that (? - ~/Kg) ia nearly ●qual to 1. ThusA ia about0,77.

ure, Theotheris baead on the Mohr-Coulomb fail-ure theory in which only the greateat ●nd leaet From the ●bove-cited datt, valuae of the threeprincipal streeeea●re thoughtto controlfailure. If the von Nieeo theory is used, the

principal ●treaaea ●cting ●t the borehole wall have

failure envelope may be dieplayad in tema of abeen calculated for several valuae of 0 from0“ to90’. (Secauee of aymetry we need consider only

root-maan-square ehear etreee, thie one quadrant of the borehole.) Valuee of the

J;’2 ‘{[(s, - S2)2 + (s2 - S3)2borehole etreases are given in ~~~li~-2~t0;~~~~e

(A-4 )with corresponding valuee of J2

+ (s3 1- S1)21/6 1’2 ,von Nisea repreeentation, calculated froml(A-4) to(A-5), ●ndof T and aN for the Mohr-Coulomb

and an effective confining preseure,criterion, calcu~ed from (A-6) to (A-7).

$ ‘(S~+S2+S3)/3-p* (A-5)State of atreee at the borehole wall is shown

in Figure A-2 as e function of position around the

If the Mohr-Coulomb theory ie used, failurehole for the criteria of both von Miees and Mohr-

conditions may be expressed in terms of the ehearCoulomb. Superimposed on these linee of atreas

and normal componante of atreaa acting on the planeetatea are the corresponding failure line. plottedfromtha data of Table A-1. Where the etate of

of maximum shear stresei strese at the borehole wall lice below the failure

T = (S1- S3)/2~ (A-6)line the borehole @hould not fail; where the stateof etressfalla abova the failure line, that

% = (S1 +S3)/2 -P . (A-7 )portion of the hole is unetable and will undergocompreeeive ehear failure. Conclueiona reached

Because it is not yet certain which of the twoconcerning borehole ctability ●re eomewhat

theories ie to be prefarred for failure of porouedifferent for the two failure criteria. The von

rocke, we will uee and compare both thaories.Nises treatment shows tha boreholeto be lecesusceptible to failurethan dose Mhr-hulomb, In

Rock Strengthcase tharc ie no fluid flowto the fomation thevon Niaes criterion ehowo the borehole to be etabla

Elaven pluge were cut from ● core taken in theover ●bout 57* of ●ach quadrant whereae the @bhr-

diatomit~ section ●t ● depth of ●bout 1300 feetCoulomb theory givee stabilityovar only ●bout

(400 m). ‘fheee right circular cylindrical pluge50°. The difference for the unrestricted flow caee

ware coredwith fraeh water ●nd ●aturated with ●

;s more pronounced. Herethe Blohr-Coulomb theory

eimulated field brine, Seth plug wet jacketed withshows the ●ntire borehole to be unetabl~s whereaa

heat-shrinkable tubing ●nd placed in ● vented tri-the von Nicee theory indicate. that only ●bout 40’

●xial compreeeion cell. A emall axial load waeof ●ach quedrmt ia Unstable, Howav@rjboth

initially ●pplied ●nd then confining preaeure wasthooriee predict that come degree of compressive

elowly ●pplied until the deeired preecure we.failureshould occur in the diatomite ●t a depth of●bout 1300 feet (400 m!,

Table 1

PR1~CIPALEARTH STRESSES AT THREE INTERVALSIN FOUR WELLS OF SOUTH SELRIDCE SECTION 33

Table 2

CO+IPARISON OF SHt2T-IN PIWSSURES (IN PSIC) FROH HICROFRAC UITHTHOSE RECOSDCD DURING cOHPLETION FRACS

Sy (pSin) t

TOIL Depth* lSIP ■ sh P Unr@8tricted Nowell NO, (feet) (P:!s) (Psi8) (PS!S) (P:!s) FIOw Flow

<ISIP froll h lSIP

ISIPRatio

COmpl*tiOn Frac tticrOfrac- lsicrofraclTent from Stfcrr.d to Dapth Complccion Completion

No, lsicrofrac of Hicrofrac Frac Frac

TOP [nterval rOp Intwv4i

3 530 465 656

1,144s0 45) 25

9 4s01.05

525 -45 0.9111 470 45s 15 1.03

A 3lb6c 90 11

Avercge

AZB5c s

Avarase

AB:cD J

Avwase

865845859S4S

054

130912711267

12s2

2099206221232103

2091

675665610660

66s

965940940

948

15h51s151$601543

1$41

530 6S0480 6504s0 7s0470 550

20-1 105 805595 695430 >60685 760

604 705

195200200

490 665 200 Middle Interval

2 7s0 160 40 1,0$5 730 730 0 1,00s 810 715 95 1,13

Bottom [nt*rval

1 1500 1710 -190 0.894 14ss 166S -210 0.8?1 1s05 1610 -105

100.93

1610 1720 -110 0.94

Middla Interwl

780 970730 790810 880

4003s51s0

945 10701030 11151160 1210

1045 1152113 880

SOttOm Interv*l

1580 1700145s 16501s05 17101610 1920

1s0730760750

2180 23901883 20S5193$ 21452005 2260

2001 22201538 1745 748

●14idpoint of O-foot isolated tett interval,

$Calculated from riq. (1) with Wn = 0,18, A = 0.77, ●nd T ● 100 psis.

T#ble 3

STATISTICAL S1211)lARY OF FSACTURE ORIENTATIONS FROH THECHICO-MARTINEZ CREEK OUTCROP

Table 4

ORI13NTATION OF GREATER HORIZONTAL STRESS SHINFERRED FROH SOREHOLE I! LLIPTICITY

Strike COnjunate DiPIStandard Standard

Mean Oeviation Mean Oevlttion

Dapth Interval Calipw Orientationwall (ft subsurface) Difference (in) of SH

Fracture Set

G 720-1040 1-2 N15*E1040-1380 2-5 N15’lrOrisinal Oat*

1 N39’I! 21’ 71’ 26°11 N31’U 20” 49’ 23”

H 1690 -1S00 2-3 Nl\’E

K 1200-1430 1-3 N25’CSeddln~-Correctod Data

L ?10-1170 N15’!21170-1400 i:: NI$*B1400-1510 1-2 NIO$B

1 N3b’t 1)’ 66* 23”11 N$O*H 22’ 62* 11”

Tablt S

ORIENTATION 0? I14DUCIlD HYDMULIC FZACTURE9AS MEASUZCO BY SURFACS TI LTWTCIM

Orientation of Induced ?racture

wall (ft %!rfm) A;imuth DiP

c 670- 811D930-1090

1150-12901340-14801530-17301780-19602390 -?5202800-29703270-3470Avera@a E

F 729-949999-1139

1189-133913s9-15091649-17991849-19692039-2139

N12’Bt3” ao*sct5’N16*Ei>5* 86”sci5*

N9’W5” 8a*sBi~5”N1O” Eb5” 88* SC*X*N13*CM’ 88* Stt~5’N12*Ei30” L17”sci15*

N9*IM(?) 82”9C*(?)? ?

irr+ ad--N7” C+4’ >75” SP,N7”!27X” >15” SEN7’C;>4” :15*8CN7”C;*4” 275*s E ‘N7*C~15* 375*SC

1- ?,, ,,

2189-2379 ,. . .2429-2499Avorasc F * *

Awra:a Al 1 Zonoo N1O*C t 759SC

~ 15641

~ORIENTATION OF MAKIltlM NOZIZONTAL STZKSD SN

AS lWCTM21191C0PROW IMPRESSION PACKERS

Dtpt h Well wall ‘d@ll wallIntgrval A s c D

Uppmr NM-I! N2*C N23”U N4*U

Hiddla N78°t N22*U N7°W ,,

Lower . . ,, ., N76”W

Xflku

FhMTURB0U2WTATIOWIUFBRMCOPROM‘NE SAffOIA CZOPNONCT88T9 TSIAXIAL FAILmt TCST2or t41AmITt SAMPLC9

4 N 179-989 N$3*W M66”E

a Is 049-969 N4?’W N49’fl

7 L 1OZ9-1129 ,. N13’E

3 N 1039-1259 . . . .

6 K 1229-1209 N6Z’W N41” E

2 w 1109-1449 N53*W N48*K

5 J l\69-1639 N41”W N60’E

1 N 1919-2029 ,. N66’U

Composita from 7 tasts (omittlnc No. 7) N53’w W51’I!Compotita of data from ●ll 8 ttstt N53’w N47’E

2 260- 400 370 215 12s 1C3 la>10: 1 7s5- 830 810 410 335 3$5 45s300 4 1035-1273 1150 490 58s 42S 725Soo 2 1310-1483 1400 520 800 450 950

Tabla A-2

BTRCSS ST4TM AROUNO TN2! DORENOLZ AT SUBSURFACE OEPTN OF 1300 FEZ?

von Miws 140hr-Coulomb

e Sr%

SA J21/2 31 ,m”‘N

(da) (P*i$) (Ptic) (pSiS) (pol) (p-l) (psi) (psi)

NO Flew (P . Pe~

o 600 20?9 10s5 150 065 740 950600 1975 1065 700 025 681 895

;; 600 1100 1020 5)5 715 550 76045 600 132$ 950 36S 310 16560

575600 9)0 889 18$ 4J0 173 305600 613 81s 120 )15

;:115 125

600 515 815 130 27) 120 305

Unr$strictad ~low (P . ?J

600 1920 1210 bto 645 660 6601: 600 1850 1 19) 62$ 615 623 62!

600 1650 k 160 529 S35::

525 525600 13s0 1110 )95 430 190

60390

600 llio 106> 180 )2$ 2S5 2!)600 915 1030 220 24S 215 215

# 600 140 101) 210 220 205 20)

td “’T-”-7---&---+--

,.-,.co.~a an ------- p

~’

e’

800,

E

i!000.

1900.

n000.

OTWSO tosel

000 !000 1000 two

-&----- _____ _____ -,----- -

c. AZIMUTH HOLE O! AMEIER–*300” 20 IN.

7Ir.-) 1200 —---

1’

hi+--Li Az; KUTH

.. —.-.— ——IiI

I1I— 1300

I

II

q-~

.-

i~

!g

j3

I

‘r OEWAWON

——

“c

00

‘ ~’”

Ii

II

.- 1- .—

-1” OEWAWON *“ za IM,

IN,

~Is..E

m“

a--

N/

/2” S AC <1”

“SAC

y

u .1IN. 1500 1000 SOo o 500 1000 15oo-

FLz.?-hlwth otgrvatw btiont#l #tr.s8SH m8%<1miwdtmratlnmdw suw*y8.

N

j

N?”t/

u— —E

FIo, O-mimulh 01owl- hodmntd Wow% so detwmhd from lm.P!QUIWI ptckom run duringW mkrd?m teats.

N

N-51” E

N-53-W /’‘\

u— ‘E

,/

s-------- tlEDIRN OF EVENTS

IN ERCH OLMDRRNT~ TEST NO. 7 ONLY

58 OF 529 EVENTS

7 OF 8 TESTS INO. 1-6.81

471 OF S29 EVENTS

400

zoa

c

400

200

a

Is“

&-$

II *%

I.:— __ Sh

FIs. A.t-OdwImHWI d shun m bamhob WM.

Unsfobke=o*,+

#,“ /

Von Mises Theorye=o*

+

,,,

Borohalo StNssoa

~ No nOW

+-----+ FUII nOW

Fallum

,200 400 600 800 1(

MEAN EFFECTIVESTRESS j, (psi)

Mohr-Codomb Theory

e=o”

e=o”Unstable ,+

+’ /,,’ /,,,+” /’,,

,,’,,’

,. +’

.,,.,’ Borshola Stistas

~ No nOwstable +.. --.+ full~o~

*

290 400 660 600 Ic

EFFECTIVE NORMAL STRESS UN (@)

Pl#,A.Z-COOWMIWrI al bomlwh SIMU utah st $,WE R WI! foch mrwI@I.

)0

10