Laboratory-scale hydraulic pulse testing: influence of air fraction in cavity on estimation of permeability

When performing transient hydraulic pulse tests on porous materials in the laboratory it is implicitly assumed that the accessible pore space of a porous medium and the pressurised fluid-filled cavity are completely saturated with the permeating fluid. In certain instances the pressurised fluid cavity in a hydraulic pulse test can contain an air fraction introduced through either the experimental procedure or released from the unsaturated regions of the porous medium. The purpose of this paper is to examine the influence of an air fraction within the pressurised region, on the estimation of permeability derived from transient hydraulic pulse tests. A series of hydraulic pulse tests conducted on Stanstead Granite was used to estimate the permeability of the rock. It is shown that if the effect of the cavity air fraction is omitted from the analysis of the hydraulic pulse test, the permeability can be underestimated. The paper also presents a discussion of the potential role the air fraction can contribute to discrepancies in the permeabilities estimated from steady-state tests and hydraulic pulse tests.