6:15 PM - 7:30 PM
[SCG57-P07] Measurements of elastic wave velocity and conductivity in a brine-saturated sandstone under confining pressures
Keywords:seismic velocity, conductivity, resistivity, sandstone, fluid
Berea sandstone (OH, USA) was selected as a rock sample for its high porosity (~20%) and permeability (~10-13 m2). It is mainly composed of subangular quartz grains, with small amounts of feldspar grains. Microstructural examinations showed that clay minerals (e.g., kaolinite) and carbonates (e.g., calcite) fill many gaps between grains. The grain size is 100-200 micrometers. Cylindrical samples (D=26 mm, L=30 mm) were saturated with 0.1 M KCl aqueous solution. Measurements have been made using a 200 MPa hydrostatic pressure vessel, in which confining and pore-fluid pressures can be separately controlled. An aqueous pore-fluid is electrically insulated from the metal work by using a plastic devices. Elastic wave velocity was measured with the pulse transmission technique (PZT transducers, f=2 MHz), and electrical conductivity the four-electrode method (f=100 mHz - 100 kHz) to minimize the influence of polarization on electrodes.
Confining and pore-fluid pressures work in opposite ways. Increasing confining pressure closes pores, while increasing pore-fluid pressure opens them. For a given pore-fluid pressure, both compressional and shear velocities increase with increasing confining pressure, while electrical conductivity decreases. When confining pressure is fixed, velocity decreases with increasing pore-fluid pressure while conductivity increases. The closure and opening of pores can explain observed changes of velocity and conductivity. In contrast to a granitic rock, a brine-filled sandstone showed only relatively small changes in conductivity. These contrasting behaviors might reflect the difference in pore geometry between two rock samples.