Velocity-effective stress laws and hysteresis in Carnarvon Basin sandstones

Abstract

P-wave and S-wave velocities, attenuation and the Vp/Vs ratio show different responses in saturated and dry Carnarvon basin sandstones under increasing differential pressures. At low differential pressures, Vp/Vs ratios are high under oil saturation but low in the same rocks under dry (gas-saturated) conditions. Velocities and attenuation are also shown to be sensitive to pore pressure, not just differential pressure. Under identical low differential pressure conditions, velocity is higher in sandstones when pore pressure is high during simulation of inflationary overpressure conditions. Thus the velocity- differential pressure relationship is not unique. However, the use of the measured Biot effective stress coefficient results in a unique relationship. It is postulated that higher velocity and different Vp/Vs ratios are caused by loss of compliance in microfractures due to stiffening by increasing fluid pressure.