The tube-wave method of estimating in-situ rock fracture permeability in fluid-filled boreholes

Abstract

Studies on subsurface fracture permeabilities in igneous and/or metamorphic rock bodies are in progress as part of the AECL (Atomic Energy of Canada Limited) Nuclear Fuel Waste Management Program. A multichannel borehole hydrophone array has been used to detect open fractures and rock property changes circumjacent to deep boreholes in granitic rock. The 12-channel array, with equispaced hydrophones of approximately 1 m, is lowered at intervals in the borehole. The seismic source consists of a small dynamite charge detonated in a shallow shot hole near the well head. A 12-channel digital engineering seisomograph system is used to record high resolution data (0.2 ms sample rate) over a recording period of 200 ms. In addition to first-arrival compressional waves, high-amplitude low-velocity later events were observed, which appeared to be generated at various intervals within the borehole. These events have been identified as “tube waves”, which are interpreted as being generated by the incidence of compressional waves from the surrounding rock body onto fluid-filled fracture zones intersecting the borehole. Correlations of positions and amplitudes of tube waves with open fractures observed from TV logging, core logging and hydrogeology studies suggest that tube-wave events can be used as reliable indicators of fractures open to fluid flow. The relative peak-to-peak amplitudes of tube waves appear to be directly related to the measured hydraulic conductivities in a borehole.