Effects of pore fluid chemistry on stable sliding of Berea sandstone

Single-cycle and multiple-cycle frictional-sliding experiments were employed to evaluate the effects of pore fluid environments on yield strength, frictional-sliding dynamics, and gouge production and morphology. Circular right cylinders cored from Berea sandstone sawcut at 35° to the axes were saturated in water, an inorganic brine, and various anionic, cationic, and nonionic aqueous surface-active agents. Samples were deformed under an effective confining pressure of 50 MPa and an axial strain rate of 6×10^–5 sec^–1 until a 2% axial strain beyond yield (defined as the onset of sliding) was achieved. All samples were displaced by stable sliding. In the single-cycle tests the unsaturated and water-saturated samples displayed small stress peaks at yield. During stable sliding samples saturated with DTAB and SDS displayed slight increases in differential stress and statistically significant higher frictional coefficients than other environments (including water) but were very similar in behavior to dry, unsaturated samples. In the multiple-cycle tests, samples were loaded to 2% strain beyond yield and unloaded to a differential stress of approximately 5–10 MPa a total of four times. These results further suggest that DTAB exerts a strengthening effect on the sandstone relative to water which, to a limiting value, increased with displacement. The DTAB and SDS environments also produced a coarser grain-size distribution in the gouge relative to gouge produced in the other environments. Investigation of the gouge by scanning electron microscope revealed that these larger grains were composed of dense, apparently cemented aggregates of ultrafine, platy quartz particles.