Shale Swelling, Osmosis, and Acoustic Changes Measured Under Simulated Downhole Conditions

Preserved shale samples from four different shale cores were exposed to various aqueous fluids while under simulated downhole stress conditions. Before fluid exposure, the samples were not contacted by any aqueous fluid, including simulated pore fluid. Time-dependent measurements of pore pressure, swelling, and acoustic velocities were made. Pore pressures both less than and greater than the applied fluid pressure were observed, and apparent osmotic membrane efficiencies were calculated. Swelling was found to depend not only on the shale type and the fluid but also on the level of confining stress; sufficient confining stress can prevent swelling. Swelling anisotropy was observed. Acoustic velocities were found to change as a result of fluid exposure, but significant velocity decrease occurred only in the presence of chemically induced swelling. S-wave velocities were found to be more sensitive than P-wave velocities. Because swelling is expected to occur mostly in the radial direction around a wellbore and only very close to the wellbore, the best detection method would be either (1) well-parallel S-velocities processed to yield the high-frequency (near-wellbore) slowness or (2) S-velocities measured in the radial direction instead of the well-parallel direction.