The Effects of Confining Pressure and Fluid Saturation on Ultrasonic Velocities in Rocks

Abstract

Laboratory measurements of ultrasonic p- and S-wave velocities were made as a function of confining pressure for vacuum dry, benzene-, and water-saturated samples of Westerly granite, Bedford limestone, and Weber, Navajo, Berea, and Kayenta sandstones. The measurements indicate: 1) water-saturated bulk moduli are higher than benzene-saturated values, 2) fluid-saturated shear moduli are always greater than or equal to dry values, and 3) water-saturated shear moduli for the sandstones are higher than benzene values at low pressure while lower than both benzene and dry values at higher pressure, Indicating that an apparent water-softening effect Is concentrated In the shear modulus. Modelling of the velocity measurements with the Blot (1956a) and Gassmann (1951) equations for static effective bulk modulus indicates that it underestimates the increase in bulk modulus and velocities caused by fluid saturation. Inertial effects of the pore fluid as treated by Blot (1956a, 1956b) are also shown to give minimal improvement to predicted velocities, which are underestimated. Velocity measurements are modelled with the Cheng-Kuster-Toksoz ellipsoidal pore and crack model using the inversion technique developed by Cheng (1978). Fits of dry and benzene-saturated velocities are shown along with pore aspect ratio distributions at zero pressure. Water-saturated velocity data and measured porosity reductions with pressure are compared with predictions of the model.