Effects of Fluid Saturation on Waves in Porous Rock and Relations to Hydraulic Permeability

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Abstract
Detailed and very accurate measurements of compressional Vp and shear Vs wave velocities and their attenuation in dry, partly saturated and fully saturated clay-bearing sandstone were carried out at 20, 145, and 198°C, under variable confining and pore pressures. The velocity results show that water saturated rock has high Vp and relatively low Vs not only at room temperature but also at 145 and l98°C. Varying the pore pressure reveals that the transition from the fully saturated to the dry state involves a gradual transition of Vs and a very irregular transition of Vp – with a minimum in Vp relative to either the dry or the fully saturated cases. The attenuation of S and P waves – Qs−1 and Qp−1, respectively - shows similar patterns: in dry rock, both Qs−1 and Qp−1 are low. In fully saturated rock, Qp−1 is low and Qs−1 is high. In partly saturated rock, Qs−1 changes gradually with saturation, but Qp−1 shows a very strong maximum-suggesting strong energy dissipation. The same behavior is observed at 145°C, with the steam/hot-water transition. It was also found that attenuation of shear waves increases with permeability in some sandstones, consistent with squirting attenuation mechanism. The results suggest diagnostic characteristics for determining the degree of saturation in situ from seismic logs, with particular applicability to evaluation of steam content in geothermal reservoirs and of gas content in oil reservoirs.