Pressure testing of fractured rocks - a methodology employing three-dimensional cross-hole tests

A comprehensive methodology of hydraulic testing in fractured rocks is presented. The methodology utilizes geological and geophysical information as background. It consists of conventional single-hole packer tests in conjunction with a newly developed cross-hole packer-testing procedure. The cross-hole procedure consists of injecting fluid at a constant rate into a packed-off interval in one borehole and monitoring pressure variations in packed-off intervals within neighboring boreholes. If injection takes place at a constant pressure, the analysis considers only those data obtained after the flow rate stabilizes. Borehole orientation is generally unrelated to the principal hydraulic conductivity directions which, therefore, need not be known a priori. The method yields complete information about the directional nature of hydraulic conductivity in three dimensions on a scale comparable to the distance between the test boreholes. In addition to providing all six components of the hydraulic conductivity tensor, the cross-hole method also yields the specific storage of the fractured rock mass. While the theory behind this method treats the rock as a uniform anisotropic porous medium, the test provides detailed information about the degree to which such assumptions may actually be valid in the field. The method may also be useful as a tool for detecting major fractures, faults, or dikes in the vicinity of the test area that have not been intercepted by boreholes. Preliminary results from a granite site near Oracle in southern Arizona are presented together with details of the instrumentation designed and constructed for the site