Stress orientations and magnitudes in the SAFOD pilot hole

Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ± 10 ° at 1000–1150 m to a maximum of 69 ± 14 ° at 2050–2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust

IN-SITU STRESS AND FRACTURE CHARACTERIZATION FOR PLANNING OF AN EGS STIMULATION IN THE DESERT PEAK GEOTHERMAL FIELD, NEVADA

An integrated study of natural fracture geometry, fluid flow and stress was conducted in Desert Peak well 27-15 in preparation for development of an Enhanced Geothermal System (EGS) through hydraulic stimulation. This stimulation will be carried out at depths of ~3000 to 3500 ft in units comprised of silicified rhyolite tuffs and metamorphosed mudstones at ambient temperatures of ~180 to 195° C. Our previous analyses of borehole image logs from this well showed that the current minimum horizontal principal stress, Shmin, is oriented 114 ± 17º and that numerous fractures in the planned stimulation interval are optimally oriented for normal faulting. As an extension of this earlier work, a hydraulic fracturing stress measurement was conducted at the top of the intended stimulation interval and indicates that the magnitude of Shmin is 1995 ± 60 psi, which is ~0.61 of the calculated vertical (overburden) stress at this depth. This Shmin magnitude is somewhat higher than expected for frictional failure on optimally oriented normal faults under current reservoir pressures given typical laboratory measurements of sliding friction (Byerlee’s Law). However, Coulomb failure calculations using coefficients of friction derived from laboratory tests on representative core samples from a nearby well (Lutz et al., 2010) indicate that shear failure could be induced on well-oriented preexisting fractures in well 27-15 once fluid pressures are increased by several hundred psi above the ambient formation fluid pressure. This geomechanical model will be tested during hydraulic stimulation of well 27-15, which is intended to enhance formation permeability through selfpropping shear failure. If this stimulation is successful, then preferential activation of normal faults should generate a zone of enhanced permeability propagating to the SSW, in the direction of nearby geothermal injection and production wells, and to the NNE, into an unexploited portion of the field

Experimental studies of pressure solution and crack healing in halite and calcite

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1989.Includes bibliographical references (leaves 156-158).by Stephen Hoyt Hickman.Ph.D