Permeability of granitic drill core tested by steady flow and transient flow gas permeameter probes: Improvements to methods and applications

Abstract

The permeability of undeformed, deformed and/or altered granites (Toki, Inada), and artificial porous ceramics, were tested by gas-probe permeameters (steady-state gas flow and transient pressure-pulse). The gas leak evaluation is essential in such tests. Here we demonstrate that stabilized apparent permeability vs. applied load does not guarantee no-leak condition, but the gas leaks are effectively prevented by an applied epoxy-resin seal-ring around each planned test spot, including on rough and vuggy rocks. The steady-state gas flow and the transient pressure pulse-decay tests show very close agreement in permeability results, if the differential test pressures and gas flow rates are approximately the same. The permeabilities begin to differ if the pressures do not match resulting from non-Darcy gas flow effects that depend on the gas flow rate. The pressure-decay test is analyzed at small data segments to show the change of apparent permeability with differential pressure (from non-Darcy to Darcy regime). In tests on rocks of low permeability, the correction for environmental temperature variation is simple to implement. For rocks with permeability larger than about 10⁻¹⁷ m², the temperature variation does not affect the flow regime and no corrections need to be applied. Gas cooling due to depressurization did not significantly affect the transient pressure pulse-decay test results from later-time analysis of test data as low injection pressures are commonly used in pulse tests. However, impacts on early time data analysis need to be considered. The results of the absolute permeability from pressure-decay gas probe tests are matching published water-based permeameters within a half order of magnitude, which is within the uncertainty of these published data. On low-permeability granite, wider tests spots are better at testing the few dominant microfracture conduits than small spots. With appropriate probe-rock sealing methods and corrections, the probe-gas tip permeameter has been shown to correctly measure permeability in granite down to 10⁻²⁰ m² absolute permeability.