Fluid pressure response to undrained compression in saturated sedimentary rock

The pore pressure response of saturated porous rock subjected to undrained compression at low effective stresses are investigated theoretically and experimentally. This behavior is quantified by the undrained pore pressure buildup coefficient, B=(dP f /dP c ) dmf =O' where P f is fluid pressure, P c is confining pressure, and m f is the mass of fluid per unit bulk volume. The measured values for B for three sandstones and a dolomite are near 1.0 at zero effective stress and decrease with increasing effective stress. In one sandstone, B is 0.62 at 13 MPa effective stress. These results agree with the theories of Gassmann (1951) and Bishop (1966), which assume a locally homogeneous solid framework. The decrease of B with increasing effective stress is probably related to crack closure and to high-compressibility materials within the rock framework. The more general theories of Biot (1955) and Brown and Korringa (1975) introduce an additional parameter, the unjacketed pore compressibility, which can be determined from induced pore pressure. Values of B close to 1 imply that under appropriate conditions within the crust, zones of low effective pressure, with low seismic wave velocity and high wave attenuation could exist. Also, in confined aquifer-reservoir systems at very low effective stress, the calculated specific storage coefficient is an order of magnitude larger than if less overpressured conditions prevailed.--Modified journal abstract