Physical effects in rock under negative effective pressures: sound speeds and hydraulic diffusivity : Shankland, T J; Halleck, P M Proc 22nd US Symposium on Rock Mechanics, Cambridge, Mass, 29 June-2 July 1981, P105–109 Publ Cambridge, Mass: MIT, 1981

Abstract

A method for creating negative effective pressure in the laboratory has been developed and longitudinal sound speed and gas diffusivity under these conditions have been measured. The pressure field is created inside an annulus of fluid injection holes in a cylindrical sample; fluid flows radially outward down the pore pressure gradient to the sample's exterior. Pore pressure is monitored through capillary probes and the pressure field is uniform inside the injection ring. With pore fluid pressures to 1.2 MPa, decreases in both acoustic velocity and amplitude in an unconfined sample of fine-grained granite are observed. The velocity decrease is about 1.5% per MPa while amplitude decreases by approximately 10% per MPa. By measuring pressure relaxation after stepwise pore pressure increases, increases in gas diffusivity of approximately a factor of three between 0 and -1 MPa effective pressure are determined.