No abstract

This study was conducted in order to characterize the frequency, orientation, and aperture of macroscopic fractures in the crust and their effect on physical properties over an appreciable depth interval (1829–3450 m). The following are our major findings: (1) Over the range of apparent apertures measured with confidence, the frequency of fractures with a given aperture decreases as aperture increases. With applied corrections for sampling bias, the observed distribution of fracture aperture has a power law form providing evidence of the self‐similar nature of fractures in the crystalline crust. Fractal analysis of the fracture aperture data yields a fractal dimension of 1.4 over the range of reliable aperture measurements in this study from 15 to 100 mm. (2) Fracture frequency does not systematically decrease with depth in the study interval. (3) No significant correlation was found between fracture occurrence and lithology, and both fracture spacing and aperture are uncorrelated with fracture orientation or depth. (4) The majority of fractures encountered in the well strike NNW‐SSE and dip steeply to the west. One set of steeply dipping fractures appears to be related to the NW striking San Andreas fault and appears to be related to steeply dipping, NW striking shear fractures observed in nearby outcrops that are characterized by laumontitic alteration. (5) The fractures bear no obvious relation to the current northeast direction of maximum horizontal compression but do correlate with anomalies in physical properties measurements of compressional and shear velocity, porosity, and resistivity. (6) Macroscopic fractures strike in a direction nearly orthogonal to the fast propagation direction of seismic wave anisotropy determined from vertical seismic profiling experiments in the well. These fractures appear to be unrelated to the observed seismic anisotropy. (7) Hydraulically conductive fractures and major faults indicate that fluid‐conducting fractures are a subset of the overall statistically significant population and not related to the San Andreas fault or to the orientation of SHmax in an obvious way.

No abstract

The acoustic borehole televiewer provides excellent data for the detection and measurement of stress‐induced wellbore breakouts. Analog televiewer data from the Fenton Geothermal well EE‐3 in New Mexico were digitized and interactively processed for detection and analysis of azimuth and shape of stress‐induced breakouts occurring in the well at depths of about 2.9–3.5 km. A statistical analysis of the measured breakout azimuths yields a well resolved orientation of least horizontal principal stress of 119°, consistent with least principal stress data from the Rio Grande Rift. As the magnitude of the least horizontal compressive stress, Shmin, in EE‐3 is known from hydraulic fracturing, we present a new method in which Shmin and data on breakout width are used to estimate the magnitude of the maximum horizontal principal stress.

Detailed studies of stress-induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements. These discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole. This phenomenon is observed in a variety of tectonic environments and rock types. Utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements, we use three-dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults. Case studies from normal, strike-slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies. Hydraulic fracturing data independently confirm the occurrence of nearcomplete stress drop on some faults penetrated by drilling. Modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress. ! ' I BARTON AND ZOBACK: STRESS PERTURBATIONS AT ACTIVE FAULTS 9375 AND ZOBACK: STRESS PERTURBATIONS AT ACTIVE FAULTS 9383 ods, lnt. J. Rock Mech. Min. $ci. Geomech. Abstr., 26(6), 461-469, 1989. Baumgfirtner, J., F. Rummel, and M.D. Zoback, Hydraulic fracturing in situ stress measurements to 3 km depth in the KTB pilot hole VB, KTB Rep. 90-6a, edited by K. Bram, J. K. Draxler, W. Kessels, and G. Zoth, Niedersfichsisches Landesamt far Bodenforschung, Hannover, 1990.