ABSTRACT:

This study examined the relationship between in situ stress, fractures and fluid flow in the Long Valley Exploratory Well (LVEW) which penetrates fractured and faulted volcanic rocks of the Long Valley Caldera in the eastern Sierra Nevada. Utilizing data from detailed analyses of stress magnitude, fracture geometry and precision temperature logs (that indicate localized fluid flow) results indicate that fluid flow along potentially active faults appears to contribute substantially to the bulk hydraulic conductivity in situ.

INTRODUCTION

While fracture-enhanced permeability depends on fracture density, orientation, and most importantly, the hydraulic conductivity of the different fracture and fault planes present the relationship between fracture and fault permeability and in situ stress is poorly understood. Fractures active in the present day stress field are generally classified as either Mode I extensile fractures, oriented perpendicular to the least principal stress (e.g., Pollard and Aydin, 1988), or Mode II or lI[ shear fractures (potentially active faults). In this study we investigate whether fracture enhanced permeability principally results from flow along Mode I fractures (as commonly assumed) or shear (Mode II or Mode lI[) faults, or neither. That is, flow through highly fractured crystalline and sedimentary rock can be dominated by the orientation of faults and fractures introduced during the long geologic history of the rock mass and thus bear no genetic relation to the current stress field. In this study, we used data from a borehole that penetrates fractured and faulted igneous rocks in the LVEW to investigate the relationship between in situ stress and fracture and fault permeability.

The interaction between the state of stress and fracture characteristics is determined both by the orientation, aperture and hydraulic conductivity of the individual fractures and by the magnitudes and orientations of all three principal stresses. If active faults control fluid flow, the relative magnitudes of the three stresses determine their orientation. Laboratory studies (Jones, 1975; Nelson and Handin, 1977; Krantz et al., 1979; Trimmer et al., 1980; Tsang and Witherspoon, 1981) and theoretical models (Bawden, et al., 1980; Ayatollahi et al., 1983) agree that there is a marked decrease in fracture permeability with increasing normal load. The impact of shear displacement on permeability appears to be a function of fracture aperture and roughness (Brown and Scholz, 1985; Brown, 1987) and there is evidence that dilatancy plays the central role in fracture conductivity under shear deformation (Tchalenko, 1970; MaiM, 1971; Teufel, 1981, 1987; Makurat, 1985; Makurat, et al., 1985). Thus, the current stress field governs the current fluid activity of the fault and can explain why faults may be at different times both an avenue and a barrier to fluid flow (Hooper, 1991).

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