Physical properties of shale at temperature and pressure

Abstract

Deformational properties, P‐wave and S‐wave velocities, and electrical resistivity were measured for three North Sea Malm shales in the laboratory under pressures to 800 bars and temperatures to 100 °C. These data were used to evaluate how factors such as mineralogy, microstructure, compaction, and pore‐fluid conductivity affect a shale’s seismic and electrical responses. Deformation in the shales is dominated by inelastic processes which cause time‐dependent changes in velocity, resistivity, and pore pressure. Overall, shales are less sensitive to pressure changes as compared to sandstones of similar porosity. However, changes in temperature result in large changes in physical properties as compared to sandstones or shaly sands. P‐wave and S‐wave velocities may decrease by as much as 10 percent over the temperature range studied, and calculated activation energies for surface conduction are nearly twice those observed in shaly sands. These comparisons emphasize fundamental differences in fabric among the clastic rocks and suggest that solid‐liquid physical and electrical interactions may play an important role in controlling a shale’s seismic or electrical response to compaction. The results of this study have impact on the well‐log interpretation of shaly sands and the determination of shale properties from seismic data.