Laboratory assessment of the use of borehole pressure transients to measure the permeability of fractured rock masses

Abstract

A laboratory program is described that was designed (1) to evaluate the degree of correlation between permeability values determined from steady-state and transient tests on the same samples and (2) to determine the effects of packer compliance on pressure pulse tests performed on low permeability rocks. The basic theory of pressure pulse testing is reviewed and modifications are proposed that may account for packer compliance effects. The laboratory set-up simulates a full-scale field situation using standard field packers in a 76 mm steel pipe. Cylindrical samples 5 cm in diameter and 11 cm long can be subjected to hydrostatic confining pressure up to 34 MPa and pore pressures up to 10 MPa. Using this equipment, transient pressure pulse tests and stead-state flow tests have been performed on (1) two samples of Berea sandstone having conductivities of 6.1 x 10/sup -5/ cm/sec and 2.4 x 10/sup -4/ cm/sec. and (2) one sample of Stripa granite containing a single fracture parallel to the core axis. Flow through the fracture varied from 1.0 to 0.05 cm/sup 3//min when the confining pressure varied from about 3 MPa to 14 MPa for pressure differentials of 0.14 MPa. Transient tests on the porous media samples consistentlymore » gave lower permeability values than steady-state tests on the same samples. All samples showed distinct compliance effects that increased with decreasing permeability. The laboratory results demonstrated that transient tests are very sensitive to minor leaks in the test assembly and to temperature variations as slight as +- 0.05/sup 0/C in the cavity fluid. Thus pressure-pulse borehole equipment must be carefully checked in full-scale test assemblies and must incorporate temperature measuring devices that can detect changes of +- 0.01/sup 0/C. « less

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