Effects of chemically active solutions on shearing behavior of a sandstone

To evaluate the effect of chemical solutions on the frictional properties of a quartz-rich sandstone, triaxial compression tests have been performed on a sandstone saturated with distilled water solutions of NaCl, CaCl2 and Na2SO4 at varying ionic strengths and pH values. The sandstone, although containing 90% quartz, also bears about 7% clay. Results from these tests indicate that the presence of water alone reduces the ultimate strength by 33%. The NaCl solution of low ionic strength (0.2 M) produces similar reductions in strength. The samples saturated with NaCl solutions of intermediate ionic strength (1.0 M) are up to 20% stronger than those samples saturated with distilled water. The samples saturated with solutions of the high ionic strength (5.0 M) display the greatest weakening (41–46%) from the case of the dry specimen; this is a reduction of up to 20% compared to the water-saturated sample. Reductions in ultimate strength due to changes in pH are only apparent for samples saturated with solutions of intermediate ionic strength. Results from the samples saturated with the divalent solutions show strength reductions of up to 52% compared to dry specimens. In addition, the tests demonstrate that the presence of either cations or anions appears to produce an equivalent weakening in the samples. In all cases the samples saturated with the solutions of intermediate ionic strength (1.0 M) display the greatest ultimate strength. Results from fracture tests on intact specimens indicate that the ultimate strength of nominally dry samples is approximately 15% greater than that of samples from all other environments. There is no discernable difference between the strengths of specimens saturated with distilled water and those treated with more chemically active solutions. Scanning electron microscopic observations of the friction surfaces demonstrate that those solutions which produce the weakest mechanical behavior generate either very rough surfaces (high ionic strengths) or significant amounts of gouge (low ionic strengths). The former appear to be the result of solution of quartz, which is enhanced by the clays in the sandstone. These results suggest that the frictional resistance to sliding of the sandstone seems to be primarily controlled by the ionic strength of the pore fluid under most conditions, with secondary control by the pH. The same solution chemistry may weaken a fault zone at low concentrations but strengthen it at higher concentrations. In the former case, fault displacement would be facilitated and strain accumulated reduced. This is believed to be more common in crustal environments.