UNITED STATESDE

PARTMENT OF THE INTERIOR

GE O LOGI CAL S URVEY

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Area 12-43 UNITED STATES USGS-474-2221976 DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

Federal Center, Denver, Colorado 80225

A METHOD FOR STRESS DETERMINATION IN N, E, AND T TUNNELS,NEVADA TEST SITE, BY HYDRAULIC FRACTURING,WITH A COMPARISON OF OVERCORING METHODS

By

C. H. Miller

ABSTRACT

Twenty-nine intervals in 10 core holes were hydraulically

fractured in N, E, and T tunnels, Nevada Test Site, during 1974.

Certain pressures were determined and related to the ambient stressfield, but the orientation of the hydraulic fractures was not meas-

ured. These data and data from previous investigations in G tunnel

indicated that both the magnitude of the hydraulic pressures and

the direction of fracturing are independent of the orientation of

the core holes. The maximum and minimum principal compressive

stresses determined by the hydraulic fracturing methods are good

approximations of those determined by nearby overcore methods.The data show that a good approximation of the magnitudes of themaximum and minimum principal stress axes can be obtained from

several hydrofractured intervals in one core hole. Furthermore,if fracture orientation can be measured, then the direction of

minimum principal compressive stress can be determined and the

orientation of the plane of the maximum and intermediate principal

compressive stresses can also be determined.

INTRODUCTION

During the interval between June 1974 and February 1975, 29zones in 10 core holes were hydraulically fractured (hydrofractured)in three DNA (Defense Nuclear Agency) tunnels (N, E, and T) at NTS(Nevada Test Site). The purpose of the hydrofracturing was torelate certain pressures used in fracturing rocks to the ambientstress fields. The project was funded and originated by DNA incooperation with SLA (Sandia Laboratories) at Albuquerque and theUSGS (U.S. Geological Survey). The following analysis andconclusions fulfill the USGS role in the project.

THEORY OF HYDROFRACTURE MEASUREMENTS

The theory of hydrofracturing is discussed by Haimson (1974)and Obert and Duvall (1967). Some other hydrofracturing-for-stressdeterminations at NTS have been reported by Haimson, LaComb, Jones,and Green (1974).

Theory employed for analysis of the hydrofracturing experimentassumes that the rocks are elastic, isotropic, and porous but im-

permeable between the rock grains. Intergranular pore pressure ofthe volcanic rocks approaches zero; any permeability is related tofractures.

The method further assumes that the core hole used in fractur-ing is parallel to one of the principal stress axes of a differen-tial stress field. The holes used in the present study arerandomly oriented, and the relation of the magnitude of hydraulicpressures to the orientation of the holes is discussed in this

report.

In a vertical hole drilled along the maximum principal stressaxis the breakdown pressure, Pc, required to fracture impermeablerocks along an interval isolated by straddle packers is given by

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where T is the hydraulic fracturing strength of the rocks; 03 is

the least horizontal stress, psi (pounds per square inch), normalto the borehole; a1 is the maximum horizontal stress (psi) normalto the borehole; and Pf is the pore-fluid pressure (psi), which iszero in the present study. Direction of fracture extension is

assumed to be parallel to a1 and normal to a3' The factors Pc anda3 (instantaneous shut-inpressure) are interpreted from the time-pressure graphs; T is measured in the laboratory (Haimson andothers, 1974, p. 562), and a3 may be calculated when the otherelements of equation (1) are known.

Figure 1 shows a generalized time-pressure model of a hydro-

fracturing test. The time-pressure graph of figure 1 and equation

(1) are models used as a basis of discussion in this report.It is assumed that this model approximates the conditions encountered

in the randomly oriented holes discussed herein.

RESULTS AND DISCUSSION

Table 1 summarizes drilling and pressure data for 29 inter-vals isolated by packers in 10 core holes in N, E, and T tunnels.The hydrofracture pressures are interpreted from records taken byFenix and Scisson, Inc. Orientations by hydrofractures were not

measured.

Apparent breakdown and fracture-extension pressures

Many of the breakdown pressures (Pc), that is, the "hump," arenot evident on the hydrofracture records run in the tunnels. Instead

the pressure rises steadily with time until it reaches a plateaupressure. The beginning of the plateau is called Pc and fracture-extension pressures were interpolated further along the plateau.The magnitudes of the Pc humps are indicated in table 1 by the

difference between the apparent breakdown-pressure and fracture-

extension-pressure columns. These data and previous experience

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Figure 1.--Time-pressure model of a hydrofracturing test.

Table l.--Drilling and pressure data for hydrofracture holes in N; E, and T tunnels(Queried where quality of data is in doubt)

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suggest that tight fractures exist in the isolated zone between

packers. Apparently there is some strength to the tight fractures.

Haimson, LaComb, Jones, and Green (1974, p. 559) also noted in theirlists that no breakdown pressures were recorded in 4 out of 12 casesat NTS. In these four cases it was believed that breakdown pressureswere nonexistent because of preexisting fractures in the rocks.

Time dependency of apparent breakdownand fracture-extension pressures

Duration of pumping from beginning pressure to apparent break-

down of pressure during apparent fracture extension was also analyzed.There is no indication that either the apparent breakdown pressuresor the apparent fracture-extension pressures are time dependent.

However, if the breakdown and fracture-extension pressures are

rather low, there is a tendency for the operator to pump for a longerperiod of time.

Dependency of apparent breakdown pressure on depth andorientation of hydrofracture holes

Underground openings influence the virgin stress field only to orie

about two diameters of the opening. The tunnels in which the hydro-fracturing was done are usually less than about 10 feet in diameter.Except possibly for the 7.2-13.8-foot interval of the Ul2t.02 HF-1hole, there is no correlation between the depth of the hydrofractureinterval and the magnitude of the breakdown pressure.

Figure 2 shows the apparent breakdown pressures of the 29 hydro-fracture intervals in 10 holes as a function of the orientation of

the holes. There is no correlation between the orientation of theholes and the magnitude of apparent breakdown pressure.

Geologic mapping along hydrofractured holes in G tunnel (C. H.Miller, D. R. Townsend, and G. R. Terry, written commun., Jan. 24, 1974;

C. H. Miller, G. R. Terry, and S. S. Terry, written commun., July 11,1974) has shown that the direction of fracture propagation is independentof the orientation of the hole.

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Figure 2.-Twenty-nine hydrofracture intervals in 10 holes showingthe orientation of the holes and the apparent breakdownpressures in the intervals.

Comparison of stresses determined by the hydrofracturemethod with those determined by the overcoring method

Because the hydrofractures apparently propagate parallel to the

maximum principal compressive stress (Smax) and, therefore, normalto the minimum principal compressive stress (Smin), then theinstantaneous shut-in pressure in equation (1), can be equated toSmin. Likewise, a1 becomes Smax and equation (1) can be rewritten:

Smax-T+3 Smin-Pc . (2)

Using an average T value of 435 psi (Haimson and others, 1974,p. 559), Smax was computed for each hydrofracture interval shown intable 1. These values and Smin are compared to the equivalent pre-

liminary stresses determined by the USBM (U.S. Bureau of Mines) over-core method. These comparisons are shown in figure 3. The overcore

method has been described by Hooker and Bickel (1974) and Hooker,Aggson, and Bickel (1974). The comparisons are grouped by proximityin the tunnel complexes. The stresses determined by hydrofracture

methods at Ul2n.09 bypass and Ul2t.03 main drift are conspicuously

high and may be in error. There are no nearby overcoming data to

support this assumption. The hydrofracture intervals were done withdyed water and then the intervals were overmined with an Alpine Miner.The advancing face was mapped in detail during the overmining,but no

dyed fractures were observed. Perhaps the anomalously high "break-

down" pressures of the Ul2t.03 drift are related to the apparent

lack of liquid injection. The other stresses, except possibly Smaxin Ul2e.06 drift determined by hydrofracture methods, compare wellwith the equivalent overcore stresses.

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Figure 3.-Comparison of maximum and minimum stresses determined by the hydrofracture method withthose determined by the overcoring method.

The data summarized in figure 3 imply that good approximations

of maximum and minimum principal compressive stresses can be deter-

mined by the hydrofracture method by testing several intervals in

only one core hole. The data of figure 3 and previously discussed

data indicate that the direction of hydrofracturing is parallel to

the maximum principal stress regardless of the orientation of the

hole. If this is true,then the maximum principal compressivestress (Smax) and minimum principal compressive stress (Smin) canbe computed. Furthermore, if fracture orientation can be measured,then the direction of Smin can be determined and the orientationof the plane of the maximum and intermediate principal compressive

stress axes can also be determined.

REFERENCES CITED

Haimson, B. C., 1974, A simple method for estimating in situ stresses

at great depths, in Field testing and instrumentation of rock:

Am. Soc. Testing and Materials Spec. Tech. Pub. 554, p. 156-182.

Haimson, B. C., LaComb, J. W., Jones, A. H., and Green, S. J., 1974,

Deep stress measurements in tuff at the Nevada Test Site, inReports of current research, v. 2, pt. A of Advances in rockmechanics: Third Cong. Internat. Soc. Rock Mechanics, Proc.,

Denver, Colo., Sept. 1-7, 1974, Themes 1-2, p. 557-562.

Hooker, V. E., Aggson, J. R., and Bickel, D. L., 1974, Improvements

in the three-component borehole deformation gage and overcoring.

techniques, with an appendix on Stress relief by center hole,by W. I. Duvall: U.S. Bur. Mines Rept. Inv. 7894, 29 p.

Hooker, V. E., and Bickel, D. L., 1974, Overcoring equipment andtechniques used in rock stress determination: U.S. Bur. Mines

Inf. Circ. 8618, 32 p.Obert, Leonard, and Duvall, W. I., 1967, Rock mechanics and the

design of structures in rock: New York John Wiley and Sons,

Inc., 650 p.

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Distribution

Defense Nuclear Agency:

USGS-474-222Area 12-43

Test Construction Division, FCTD-N (Attn: J. W. LaComb,Clifford Snow), Mercury, NV

Director (Attn: SPSS, David Oakley, Eugene Sevin,Clifton MacFarland), Washington, DC

Field Command, FCTD-T2 (Attn: Benton L. Tibbetts),Kirtland AFB, NM

Las Vegas Liaison Office, Las Vegas, NV

U.S. Energy Research & Development Administration, NevadaOperations Office, Las Vegas, NV:

Elaine Bickerstaff, NV Technical Library (3)E. M. Douthett (3)M. E. GatesD. G. Jackson (3)R. R. Loux (3)Roger RayA. J. Whitman

U.S. Energy Research & Development Administration, Nevada TestSite Support Office, Mercury, NV:

J. O. Cummings

U.S. Energy Research & Development Administration, TechnicalInformation Center, Oak Ridge, TN: (27)

BrownleeBryantCampbellHouseOlsenSharpSowder

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Lawrence Livermore Laboratory, Livermore, CA:

J. E. CarothersL. S. GermainR. S. GuidoN. W. HowardRoger IdeA. E. LewisL. D. RamspottD. L. SpringerTechnical Information Division

Lawrence Livermore Laboratory, Mercury, NV:

W. B. McKinnis

Sandia Laboratories, Albuquerque, NM:

J. R. BanisterC. D. BroylesM. L. MerrittL. D. TylerW. C. VollendorfW. D. Weart

Sandia Laboratories, Mercury, NV:

B. G. Edwards

Defense Advanced Research Projects Agency, Arlington, VA:

G. H. Heilmeier

Environmental Protection Agency, National Environmental ResearchCenter, Las Vegas, NV:

D. S. Barth (3)

Fenix & Scisson, Inc.:

Grant Bruesch, Mercury, NVM. H. May, Las Vegas, NVF. D. Waltman, Mercury, NV (2)

Holmes & Narver, Inc.:

R. P. Kennedy, Anaheim, CALibrary, Las Vegas, NV (2)Resident Engineer, Mercury, NV

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Pacifica Technology, Del Mar, CA:

Robert BjorkG. I. Kent

R & D Associates, Santa Monica, CA:

John Lewis

Rand Corp., Santa Monica, CA:

Olen Nance

Systems, Science & Software, Inc., San Diego, CA:

Charles DismukesRussell Duff

Terra Tek, Inc., Salt Lake City, UT:

Scott ButtersS. J. Green

U.S. Army Corps of Engineers, Waterways Experiment Station,Vicksburg, MS:

LibraryJ. H. Scanlon, Jr.William Steinriede, Jr.

U.S. Geological Survey:

Geologic Data Center, Mercury, NV (15)Library, Denver, COLibrary, Menlo Park, CA

U.S. Geological Survey, Reston, VA:

Chief Hydrologist, WRD (Attn: Radiohydrology Section)LibraryMilitary Geology UnitJ. C. Reed, Jr.

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