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Deep in-situ stress measurements by hydrofracturing
Tectonophysics, 29 (1975) 41-47 @ Elsevier Scientific Publishing Company,
41 Amsterdam
DEEP IN-SITU STRESS MEASUREMENTS
BEZALEL
in The Netherlands
BY HYDROFRACTURING
C. HAIMSON
Department of Metallurgical Wise. (U.S.A.)
(Accepted
- Printed
for publication
and Mineral
Engineering,
University
of Wisconsin,
Madison,
April 18, 1975)
ABSTRACT Haimson, B.C., 1975. Deep in-situ stress measurements and R. Green (Editors), Recent Crustal Movements.
by hydrofracturing. In: N. Pavoni Tectonophysics, 29 (l-4): 41-47.
Hydrofracturing is a recently developed in-situ stress-measuring method. It consists of hydraulically pressurizing a sealed-off interval inside of a borehole until fracture initiates. The fratiture is then extended by additional pumping of fluids from the surface. The pressures recorded during the test can be directly related to the magnitudes of the in-situ principal stresses. The orientation of the fracture in the borehole yields the directions of the principal stresses. The method overcomes some limitations of the more conventional strain-relief techniques: stresses can be determined at any distance from the access point (limited only by hole length), stresses are measured directly, no elastic parameters are required in determining the minimum principal stress, no delicate instruments are used in the borehole, no overcoring is necessary. The hydrofracturing method was theoretically developed for both high- and low-permeability rocks, and was extensively tested in the laboratory under simulated in-situ conditions. These tests confirmed the theoretical assumptions and results. Field measurements were conducted in some ten states within the continental U.S., at depths varying from 100 m (Nevada Test Site) to 1900 m (Rangely, Colorado). The hydrofracturing results were very consistent within each site, and the calculated principal stresses and their directions could, in most cases, be correlated to tectonic phenomena or other measurements and observations. In particular, the stresses determined at Rangely were in accord with the expected condition for the right lateral slip of the existing strike-slip fault, and were used to correctly predict the critical pore pressure necessary to trigger local earthquakes. At the Nevada Test Site, the results of twelve hydrofracturing measurements conducted both from the surface and from underground tunnels indicated a linear increase with depth in all the principal stresses. Hydrofracturing is a simple method of stress determination. It utilizes commercially available packers, pressure lines and pumps, which can be either rented or purchased. It is now being tested in fault zones, in anomalously hot formations, in extremely anisotropic rocks. It is being considered as a major tool in plate-tectonics studies, earthquake control, geothermal-energy extraction from hot dry rock, and new methods of mining.
INTRODUCTION
It is now widely accepted that knowledge of in-situ stresses is the key to understanding the driving mechanisms of plate tectonics, to predicting and
42
controlling earthquakes, to designing stable structures in rock, to safely extracting energy and minerals from the earth. The measurement of these stresses has fascinated many rock-mechanics scientists in the last twenty years. However, the perfect method of determining in-situ stresses has thus far eluded the ingenuity of field experimentalists. The great majority of the methods developed call for the insertion of an instrument containing strain-measuring devices into a borehole drilled from the access point to the location where stresses are to be determined. A variety of strain and deformation gages, borehole locations and application techniques are used. After initial readings are taken, the borehole is overcored in order to relieve the stresses. The difference in readings before and after overcoring can be used to determine the biaxial or triaxial state of stress, depending on the arrangement of the gages inside the hole. These methods, however, suffer from a number of distinct disadvantages. They are limited in depth because of the precise overcoring required (practical depth lo-20 m). They require precise knowledge of the rock elastic parameters, they do not account for residual stresses, and are ineffective in areas of high differential stresses where overcoring can produce discing. A new method has recently been suggested based on theoretical considerations which have been verified in the laboratory and successfully tested and used in the field. It is the hydraulic fracturing (hydrofracturing) technique which consists of sealing-off a section of a borehole at the required depth by means of two commercially available rubber packers, and pressurizing the packed-off segment by pumping in a fluid. At some critical (breakdown) pressure, the rock surrounding the borehole fails in tension and develops a fracture. This fracture can be extended away from the hole by continuous pumping. When the pump is shut-off with the hydraulic circuit kept closed, a ‘shut-in’ pressure is recorded. This is the pressure necessary to keep the fracture open. The breakdown and shut-in pressures can be’related to the prevailing in-situ stresses. A commercial impression packer is finally used to determine the exact direction and inclination of the hydrofracture which develops along a plane perpendicular to the direction of the least principal stress. In this manner, both the magnitudes and the directions of the principal stresses can be evaluated. The hydrofracturing technique can be used in deep holes drilled from the surface of the earth, or in short holes arohnd tunnels, requires simple off-the-shelf equipment, and can be carried out by unskilled personnel. In the following, hydrofracturing theory, laboratory results and selected field experience will be reported. RELATIONSHIP
BETWEEN
HYDRAULIC
FRACTURING
AND IN-SITU
STRESSES
We assume that the rock is brittle, linearly elastic, homogeneous, isotropic. We also assume that one of the principal tectonic stresses acts in a direction parallel with the axis of the borehole. The state of stress around the borehole can be found by superposing the three individual stress fields
43
generated by: (1) the three principal in-situ stresses; (2) the pressurization of the borehole; and (3) the fluid flow from the pressurized hole into the formation. As the borehole pressure in the sealed-off interval is increased, the compressive tangential effective stress at the borehole wall gradually diminishes and eventually becomes tensile. A longitudinal or vertical fracture will be initiated at the wellbore when the tangential effective stress becomes equal to or larger than the tensile strength of the rock (7’). The minimum wellbore critical pressure (P,” ) required to induce a vertical fracture is given by (Haimson and Fairhurst, 1967): p:--0
T+ ~cJ,, __L._L -e,,,, = -.-____ 2 - o1L.S? l-v
(I)
where u, *‘, uy3 are the least and largest horizontal principal effective stresses, respectively; (Yis the Biot constant (a = 1 - C,./Cb and C,, C,, are the rock matrix and rock mass compressibilities, respectively); v is the Poisson’s ratio; P,, is the pore pressure in the rock. If the rock is completely impermeable to the fracturing fluid the equation is simplified (Hubbert and Willis, 1957): Pi-PO
=T+3o,,
-cJ,,,
The fracture initiates and usually extends along a plane perpendicular to the direction of the smallest horizontal principal stress. The shut-in pressure needed to keep this fracture open (P, ) is given by:
The two equations (1) and (3) or (2) and (3) yield the two unknown horizontal principal stresses. The vertical principal stress is determined by calculating the weight of the overburden. As long as the rock is not prefractured or jointed in the hydrofractured interval, fracture will always initiate in the vertical direction. Because a hydrofracture propagates along the path of least resistance, it could initiate with a vertical orientation and extend in a horizontal plane, if the vertical stress were the least principal stress. In such a case only the lower limit of the two horizontal principal stresses and their directions could be determined. Other possibilities are detailed by Haimson (1974). LABORATORY
RESULTS
Extensive,laboratory simulated hydrofracturing tests have been conducted in different rock types in an attempt to verify the theoretical assumptions
and results. Prismatic specimens (12 cm X 16 cm X 15 cm and 10 cm X 10 cm X 20 cm) were subjected to three uniform and unequal principal stresses, induced via a polyaxial cell and a compression testing machine (Haimson, 1968, 1974). The simulated in-situ stresses (O-2 kbar) were kept constant during the hydrofracturing test, which consisted of pressurizing a central axial hole, 8 mm in diameter, until fracture occurred. The major results of the laboratory tests were: (1) All the induced hydraulic fractures were tensile ruptures, as expected, and no shear fractures were observed. (2) The inclination of all the hydrofractures was always vertical when rubber packers were used. (3) Vertical fractures were always perpendicular to the smaller horizontal principal stress. It is emphasized that this result was true regardless of the amount of fracturing-fluid penetration into the rock, or of the amount of preexisting discontinuities or weak bedding (foliation) planes. (4) The critical (breakdown) pressure for vertical fracture initiation was approximately as predicted by equations (1) or (2) depending on rock types. It should be emphasized, however, that the critical pressure was dependent on the rate of pressurization. (5) In general, the laboratory studies undertaken to date have confirmed the basic soundness of the method of hydraulic fracturing. The studies have also indicated that because of the nature of rock, the method cannot be expected to be very precise. It is rather a technique of estimating the principal stresses and their directions. I’IEI,D
RESULTS
The first indications of the potential of hydraulic fracturing as a stressmeasuring technique came from calculations reported by Scheidegger (1962) and Kehle (1964) based on routine oil-well hydraulic-fracturing jobs run for production-stimulation purposes. Haimson and Stahl (1970) reported on three series of oil-field fracturing jobs (in the states of New York, Illinois and Ohio) in open holes, where impression packers and bottom-hole pressure transducers were used to determine crack orientation and fracturing pressures. The results within each group of tests were very consistent with respect to critical and shut-in pressures, and fracture direction. The significance of the consistency of results was the strong indication that they were closely related to local in-situ stresses. Each group of fracturing jobs was run in a different oil-field. Within each group, the wells were located no further apart than a mile and were fractured at approximately the same depth in an area that was geologically undisturbed. Hence, the in-situ stresses were not expected to vary considerably from one well to the other. The consistent pressure and fracture-direction results confirmed this expectation. Stress calculations based on these fracturing jobs were subsequently published (Haimson, 1974).
The first major utilization of the method of hydraulic fracturing purely as a deep borehole stress-measuring technique was in connection with the earthquake control research at Rangely, Colorado (Haimson, 1973). Intense seismic activity centered in and around the Rangely Oil Field in the vicinity of a strike-slip fault had been recorded in the area in the last decade. A primary objective of the research program was to determine whether a correlation existed between earthquake triggering and formation pore pressure. The latter had been artificially raised through water flooding of the field. A quantitative solution relating pore pressure to slip initiation along the fault, which in turn could have triggered the earthquakes, required knowledge of the in-situ stress condition in or below the oil-bearing strata. Hydraulic fracturing was selected as the stress-determining method. The borehole used was a newly drilled oil-well located approximately 1 km west of the fault by the southern boundary of the oil-field, near the earthquake-prone region. Extracted cores showed that the lowest segment of the well at 1900 m below surface was in solid rock, free of apparent discontinuities. The bottom of the hole was used as one end of the fracturing interval and an inflatable rubber packer was placed at the upper end of the segment. A mixture of water and gel was pumped into the interval until fracturing and shut-in were obtained. The impression packer showed a vertical fracture oriented at N 70” E. The permanently recorded pressures monitored both at depth and on the surface yielded P, = 330 bars, P, = 295 bars. The oil company provided information on the reservoir pressure (PO = 150 bars). Since laboratory tests in specimens prepared from the extracted cores showed that the formation had no measurable permeability equations for impermeable rock were used in estimating the effective stresses. The results obtained were: u, = 255 bars (vertical), u, = 150 bars (horizontal at N20”W), u, = 430 bars (horizontal at N70”E). The results of the hydraulic-fracturing test showed that the vertical principal stress was intermediate in magnitude, strongly indicating strike-slip faulting. The horizontal principal-stress directions were in accord with both the N50”E strike of the fault and its right-lateral slip direction. The magnitudes of the measured principal stresses were used to predict a critical pore pressure of 240 bars for fault slip to occur. This value was within 10% of the earthquake-related pressures monitored at the site. The Rangely stress measurement was considered very successful because of the close relationship between the determined stresses and other field observations (Raleigh et al., 1972). A recent complete set of stress measurements using hydraulic fracturing was undertaken in tunnel complex U12n at the Nevada Test Site (Haimson et al., 1974). The stresses were required for the calculation of tunnel and mountain stability during nuclear tests. A total of twelve hydrofracturing measurements were conducted: one in each of two horizontal and three vertical holes (drilled some 25 m from the tunnel wall), and seven at different elevations in a 250-m deep vertical borehole drilled from the surface of the earth to the tunnel level. All the holes were approximately 12.5 cm in
diameter and commercial straddle packers were used to isolate the fracturing interval. Two pressurization cycles were run per test to double-check the value of the shut-in pressure. The two horizontal holes were drilled in the general direction of the expected minimum stress and only shut-in pressures were obtained because instead of fracturing, existing discontinuities perpendicular to the hole axis were opened up. Were the horizontal holes drilled in the direction of the larger principal stress, more information might have been gained. All the other tests, both in the tunnel wall and in the deep borehole, enabled a full evaluation of the stresses. Surprisingly, the calculated stresses show a high sensitivity to minor changes with depth, and a linear elevationstress relationship appears to exist for the two horizontal principal stresses. The in-situ stresses around the tunnel were determined at CJ,.= 70 bars (vertical), u_ = 35 bars (horizontal at N55”W), u, = 90 bars (horizontal at N35” E). Both the magnitudes and the directions of the principal stresses were in agreement with borehole deformation measurements (Hooker et al., 1971), the topography of the area, and seismic investigation results (Hamilton and Healy, 1969), indicating again the reliability of the method. CONCLUSIONS
Hydraulic fracturing is a new concept in stress-measuring techniques. It combines simple elasticity and the tensile failure criteria to provide an estimate of the in-situ stress magnitudes and directions. It does not require overcoring or sophisticated equipment in the hole, and is therefore ideal for deep hole measurements. It is, however, as effective in short holes around tunnels. Recent field case histories demonstrate the validity of the theoretical and laboratory results and indicate the great potential of the method as a routine stress-measuring tool.
REFERENCES Haimson, B.C., 1968. Hydraulic Fracturing in Porous and Nonporous Rock and its potential for Determining In-Situ Stresses at Great Depth. Tech. Rep. ‘l-68, Missouri River Div. (brps of Eng.; also Thesis, Univ. of Minnesota, 234 p. Haimson, B.C., 1973. Earthquake-related stresses at Rangely, Colorado. In: H.R. Hardy and R. Stefanko (Editors), New Horizons in Rock Mechanics. Proc. 1 .Ith Symp. Rock Mechanics. American Society of Civil Engineers, New York, p, 689-70H. Haimson, B.C., 1974. A simple method for estimating in-situ stresses at great depth, field testing and instrumentation of rock. STP 554, American Society for Testing of Materials, Philadelphia, p. 156-I 82. Haimson, B.C. and Fairhurst, C., 1967. Initiation and extension of hydraulic fractures in rock. Sot. Pet. Eng. J., p. 310-318. Haimson, B.C. and Stahl, E., 1970. Hydraulic fracturing and the extraction of minerals through wells, In: 3rd Symposium on Salt. The Northern Ohio Geological Society, Cleveland, Ohio, p. 421-432.
47 Haimson, B.C., LaComb, J., Green, S.J. and Jones, A.H., 1974. Deep stress measurements in tuff at the Nevada test site. Proc. 3rd Int. Congr. Rock Mechanics, Denver. National Academy of Sciences, Washington, D.C., 2: 557-562. Hamilton, R.M. and Healy, J.H., 1969. Aftershocks of the Benham nuclear explosion. Bull. Seismol. Sot. Am., 59: 2271-2281. Hooker, V.E., Aggson, J.R. and Bickel, D.C., 1971. In-situ determination of stresses in Rainier Mesa, Nevada test site. U.S. Bur. Mines Rep. Hubbert, M.K. and Willis, D.G., 1957. Mechanics of hydraulic fracturing. Trans. Sot. Pet. Eng. AIME, 210: 153-168. Kehle, R.O., 1964. Determination of tectonic stresses through analysis of hydraulic well fracturing. J. Geophys. Res., 69: 259. Raleigh, C.B., Healy, J.H. and Bredehoeft, J.D., 1972. Faulting and crustal stress at Rangely, Colorado. In: Flow and Fracture of Rocks. Am. Geophys. Union, Washington, D.C., pp. 275-284. Scheidegger, A.E., 1962. Stresses in earth’s crust as determined from hydraulic fracturing data. Geol. Bauwesen, 27: 45.
41 Amsterdam
DEEP IN-SITU STRESS MEASUREMENTS
BEZALEL
in The Netherlands
BY HYDROFRACTURING
C. HAIMSON
Department of Metallurgical Wise. (U.S.A.)
(Accepted
- Printed
for publication
and Mineral
Engineering,
University
of Wisconsin,
Madison,
April 18, 1975)
ABSTRACT Haimson, B.C., 1975. Deep in-situ stress measurements and R. Green (Editors), Recent Crustal Movements.
by hydrofracturing. In: N. Pavoni Tectonophysics, 29 (l-4): 41-47.
Hydrofracturing is a recently developed in-situ stress-measuring method. It consists of hydraulically pressurizing a sealed-off interval inside of a borehole until fracture initiates. The fratiture is then extended by additional pumping of fluids from the surface. The pressures recorded during the test can be directly related to the magnitudes of the in-situ principal stresses. The orientation of the fracture in the borehole yields the directions of the principal stresses. The method overcomes some limitations of the more conventional strain-relief techniques: stresses can be determined at any distance from the access point (limited only by hole length), stresses are measured directly, no elastic parameters are required in determining the minimum principal stress, no delicate instruments are used in the borehole, no overcoring is necessary. The hydrofracturing method was theoretically developed for both high- and low-permeability rocks, and was extensively tested in the laboratory under simulated in-situ conditions. These tests confirmed the theoretical assumptions and results. Field measurements were conducted in some ten states within the continental U.S., at depths varying from 100 m (Nevada Test Site) to 1900 m (Rangely, Colorado). The hydrofracturing results were very consistent within each site, and the calculated principal stresses and their directions could, in most cases, be correlated to tectonic phenomena or other measurements and observations. In particular, the stresses determined at Rangely were in accord with the expected condition for the right lateral slip of the existing strike-slip fault, and were used to correctly predict the critical pore pressure necessary to trigger local earthquakes. At the Nevada Test Site, the results of twelve hydrofracturing measurements conducted both from the surface and from underground tunnels indicated a linear increase with depth in all the principal stresses. Hydrofracturing is a simple method of stress determination. It utilizes commercially available packers, pressure lines and pumps, which can be either rented or purchased. It is now being tested in fault zones, in anomalously hot formations, in extremely anisotropic rocks. It is being considered as a major tool in plate-tectonics studies, earthquake control, geothermal-energy extraction from hot dry rock, and new methods of mining.
INTRODUCTION
It is now widely accepted that knowledge of in-situ stresses is the key to understanding the driving mechanisms of plate tectonics, to predicting and
42
controlling earthquakes, to designing stable structures in rock, to safely extracting energy and minerals from the earth. The measurement of these stresses has fascinated many rock-mechanics scientists in the last twenty years. However, the perfect method of determining in-situ stresses has thus far eluded the ingenuity of field experimentalists. The great majority of the methods developed call for the insertion of an instrument containing strain-measuring devices into a borehole drilled from the access point to the location where stresses are to be determined. A variety of strain and deformation gages, borehole locations and application techniques are used. After initial readings are taken, the borehole is overcored in order to relieve the stresses. The difference in readings before and after overcoring can be used to determine the biaxial or triaxial state of stress, depending on the arrangement of the gages inside the hole. These methods, however, suffer from a number of distinct disadvantages. They are limited in depth because of the precise overcoring required (practical depth lo-20 m). They require precise knowledge of the rock elastic parameters, they do not account for residual stresses, and are ineffective in areas of high differential stresses where overcoring can produce discing. A new method has recently been suggested based on theoretical considerations which have been verified in the laboratory and successfully tested and used in the field. It is the hydraulic fracturing (hydrofracturing) technique which consists of sealing-off a section of a borehole at the required depth by means of two commercially available rubber packers, and pressurizing the packed-off segment by pumping in a fluid. At some critical (breakdown) pressure, the rock surrounding the borehole fails in tension and develops a fracture. This fracture can be extended away from the hole by continuous pumping. When the pump is shut-off with the hydraulic circuit kept closed, a ‘shut-in’ pressure is recorded. This is the pressure necessary to keep the fracture open. The breakdown and shut-in pressures can be’related to the prevailing in-situ stresses. A commercial impression packer is finally used to determine the exact direction and inclination of the hydrofracture which develops along a plane perpendicular to the direction of the least principal stress. In this manner, both the magnitudes and the directions of the principal stresses can be evaluated. The hydrofracturing technique can be used in deep holes drilled from the surface of the earth, or in short holes arohnd tunnels, requires simple off-the-shelf equipment, and can be carried out by unskilled personnel. In the following, hydrofracturing theory, laboratory results and selected field experience will be reported. RELATIONSHIP
BETWEEN
HYDRAULIC
FRACTURING
AND IN-SITU
STRESSES
We assume that the rock is brittle, linearly elastic, homogeneous, isotropic. We also assume that one of the principal tectonic stresses acts in a direction parallel with the axis of the borehole. The state of stress around the borehole can be found by superposing the three individual stress fields
43
generated by: (1) the three principal in-situ stresses; (2) the pressurization of the borehole; and (3) the fluid flow from the pressurized hole into the formation. As the borehole pressure in the sealed-off interval is increased, the compressive tangential effective stress at the borehole wall gradually diminishes and eventually becomes tensile. A longitudinal or vertical fracture will be initiated at the wellbore when the tangential effective stress becomes equal to or larger than the tensile strength of the rock (7’). The minimum wellbore critical pressure (P,” ) required to induce a vertical fracture is given by (Haimson and Fairhurst, 1967): p:--0
T+ ~cJ,, __L._L -e,,,, = -.-____ 2 - o1L.S? l-v
(I)
where u, *‘, uy3 are the least and largest horizontal principal effective stresses, respectively; (Yis the Biot constant (a = 1 - C,./Cb and C,, C,, are the rock matrix and rock mass compressibilities, respectively); v is the Poisson’s ratio; P,, is the pore pressure in the rock. If the rock is completely impermeable to the fracturing fluid the equation is simplified (Hubbert and Willis, 1957): Pi-PO
=T+3o,,
-cJ,,,
The fracture initiates and usually extends along a plane perpendicular to the direction of the smallest horizontal principal stress. The shut-in pressure needed to keep this fracture open (P, ) is given by:
The two equations (1) and (3) or (2) and (3) yield the two unknown horizontal principal stresses. The vertical principal stress is determined by calculating the weight of the overburden. As long as the rock is not prefractured or jointed in the hydrofractured interval, fracture will always initiate in the vertical direction. Because a hydrofracture propagates along the path of least resistance, it could initiate with a vertical orientation and extend in a horizontal plane, if the vertical stress were the least principal stress. In such a case only the lower limit of the two horizontal principal stresses and their directions could be determined. Other possibilities are detailed by Haimson (1974). LABORATORY
RESULTS
Extensive,laboratory simulated hydrofracturing tests have been conducted in different rock types in an attempt to verify the theoretical assumptions
and results. Prismatic specimens (12 cm X 16 cm X 15 cm and 10 cm X 10 cm X 20 cm) were subjected to three uniform and unequal principal stresses, induced via a polyaxial cell and a compression testing machine (Haimson, 1968, 1974). The simulated in-situ stresses (O-2 kbar) were kept constant during the hydrofracturing test, which consisted of pressurizing a central axial hole, 8 mm in diameter, until fracture occurred. The major results of the laboratory tests were: (1) All the induced hydraulic fractures were tensile ruptures, as expected, and no shear fractures were observed. (2) The inclination of all the hydrofractures was always vertical when rubber packers were used. (3) Vertical fractures were always perpendicular to the smaller horizontal principal stress. It is emphasized that this result was true regardless of the amount of fracturing-fluid penetration into the rock, or of the amount of preexisting discontinuities or weak bedding (foliation) planes. (4) The critical (breakdown) pressure for vertical fracture initiation was approximately as predicted by equations (1) or (2) depending on rock types. It should be emphasized, however, that the critical pressure was dependent on the rate of pressurization. (5) In general, the laboratory studies undertaken to date have confirmed the basic soundness of the method of hydraulic fracturing. The studies have also indicated that because of the nature of rock, the method cannot be expected to be very precise. It is rather a technique of estimating the principal stresses and their directions. I’IEI,D
RESULTS
The first indications of the potential of hydraulic fracturing as a stressmeasuring technique came from calculations reported by Scheidegger (1962) and Kehle (1964) based on routine oil-well hydraulic-fracturing jobs run for production-stimulation purposes. Haimson and Stahl (1970) reported on three series of oil-field fracturing jobs (in the states of New York, Illinois and Ohio) in open holes, where impression packers and bottom-hole pressure transducers were used to determine crack orientation and fracturing pressures. The results within each group of tests were very consistent with respect to critical and shut-in pressures, and fracture direction. The significance of the consistency of results was the strong indication that they were closely related to local in-situ stresses. Each group of fracturing jobs was run in a different oil-field. Within each group, the wells were located no further apart than a mile and were fractured at approximately the same depth in an area that was geologically undisturbed. Hence, the in-situ stresses were not expected to vary considerably from one well to the other. The consistent pressure and fracture-direction results confirmed this expectation. Stress calculations based on these fracturing jobs were subsequently published (Haimson, 1974).
The first major utilization of the method of hydraulic fracturing purely as a deep borehole stress-measuring technique was in connection with the earthquake control research at Rangely, Colorado (Haimson, 1973). Intense seismic activity centered in and around the Rangely Oil Field in the vicinity of a strike-slip fault had been recorded in the area in the last decade. A primary objective of the research program was to determine whether a correlation existed between earthquake triggering and formation pore pressure. The latter had been artificially raised through water flooding of the field. A quantitative solution relating pore pressure to slip initiation along the fault, which in turn could have triggered the earthquakes, required knowledge of the in-situ stress condition in or below the oil-bearing strata. Hydraulic fracturing was selected as the stress-determining method. The borehole used was a newly drilled oil-well located approximately 1 km west of the fault by the southern boundary of the oil-field, near the earthquake-prone region. Extracted cores showed that the lowest segment of the well at 1900 m below surface was in solid rock, free of apparent discontinuities. The bottom of the hole was used as one end of the fracturing interval and an inflatable rubber packer was placed at the upper end of the segment. A mixture of water and gel was pumped into the interval until fracturing and shut-in were obtained. The impression packer showed a vertical fracture oriented at N 70” E. The permanently recorded pressures monitored both at depth and on the surface yielded P, = 330 bars, P, = 295 bars. The oil company provided information on the reservoir pressure (PO = 150 bars). Since laboratory tests in specimens prepared from the extracted cores showed that the formation had no measurable permeability equations for impermeable rock were used in estimating the effective stresses. The results obtained were: u, = 255 bars (vertical), u, = 150 bars (horizontal at N20”W), u, = 430 bars (horizontal at N70”E). The results of the hydraulic-fracturing test showed that the vertical principal stress was intermediate in magnitude, strongly indicating strike-slip faulting. The horizontal principal-stress directions were in accord with both the N50”E strike of the fault and its right-lateral slip direction. The magnitudes of the measured principal stresses were used to predict a critical pore pressure of 240 bars for fault slip to occur. This value was within 10% of the earthquake-related pressures monitored at the site. The Rangely stress measurement was considered very successful because of the close relationship between the determined stresses and other field observations (Raleigh et al., 1972). A recent complete set of stress measurements using hydraulic fracturing was undertaken in tunnel complex U12n at the Nevada Test Site (Haimson et al., 1974). The stresses were required for the calculation of tunnel and mountain stability during nuclear tests. A total of twelve hydrofracturing measurements were conducted: one in each of two horizontal and three vertical holes (drilled some 25 m from the tunnel wall), and seven at different elevations in a 250-m deep vertical borehole drilled from the surface of the earth to the tunnel level. All the holes were approximately 12.5 cm in
diameter and commercial straddle packers were used to isolate the fracturing interval. Two pressurization cycles were run per test to double-check the value of the shut-in pressure. The two horizontal holes were drilled in the general direction of the expected minimum stress and only shut-in pressures were obtained because instead of fracturing, existing discontinuities perpendicular to the hole axis were opened up. Were the horizontal holes drilled in the direction of the larger principal stress, more information might have been gained. All the other tests, both in the tunnel wall and in the deep borehole, enabled a full evaluation of the stresses. Surprisingly, the calculated stresses show a high sensitivity to minor changes with depth, and a linear elevationstress relationship appears to exist for the two horizontal principal stresses. The in-situ stresses around the tunnel were determined at CJ,.= 70 bars (vertical), u_ = 35 bars (horizontal at N55”W), u, = 90 bars (horizontal at N35” E). Both the magnitudes and the directions of the principal stresses were in agreement with borehole deformation measurements (Hooker et al., 1971), the topography of the area, and seismic investigation results (Hamilton and Healy, 1969), indicating again the reliability of the method. CONCLUSIONS
Hydraulic fracturing is a new concept in stress-measuring techniques. It combines simple elasticity and the tensile failure criteria to provide an estimate of the in-situ stress magnitudes and directions. It does not require overcoring or sophisticated equipment in the hole, and is therefore ideal for deep hole measurements. It is, however, as effective in short holes around tunnels. Recent field case histories demonstrate the validity of the theoretical and laboratory results and indicate the great potential of the method as a routine stress-measuring tool.
REFERENCES Haimson, B.C., 1968. Hydraulic Fracturing in Porous and Nonporous Rock and its potential for Determining In-Situ Stresses at Great Depth. Tech. Rep. ‘l-68, Missouri River Div. (brps of Eng.; also Thesis, Univ. of Minnesota, 234 p. Haimson, B.C., 1973. Earthquake-related stresses at Rangely, Colorado. In: H.R. Hardy and R. Stefanko (Editors), New Horizons in Rock Mechanics. Proc. 1 .Ith Symp. Rock Mechanics. American Society of Civil Engineers, New York, p, 689-70H. Haimson, B.C., 1974. A simple method for estimating in-situ stresses at great depth, field testing and instrumentation of rock. STP 554, American Society for Testing of Materials, Philadelphia, p. 156-I 82. Haimson, B.C. and Fairhurst, C., 1967. Initiation and extension of hydraulic fractures in rock. Sot. Pet. Eng. J., p. 310-318. Haimson, B.C. and Stahl, E., 1970. Hydraulic fracturing and the extraction of minerals through wells, In: 3rd Symposium on Salt. The Northern Ohio Geological Society, Cleveland, Ohio, p. 421-432.
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