Experimental deformation of sedimentary rocks under confining pressure; tests at high temperature

Short-time triaxial compression tests of dry anhydrite, dolomite rock, limestone, sandstone, shale, siltstone, slate, and halite single crystals under pressure-temperature conditions simulating depths down to 30,000 feet reveal the following. Invariably an increase of pressure at constant temperature increases the yield stress, but an increase of temperature at constant pressure reduces it.An increase of pressure at constant temperature always enhances the ultimate strength. However, heating at constant pressure may raise the ultimate strength by increasing the ductility of work-hardening rocks. More commonly heating lowers the ultimate strength by eliminating work-hardening, so that even though a rock is more ductile, it is weaker because its yield stress is reduced.In any event for all materials tested except the halite, the ultimate strength at any simulated depth exceeds the crushing strength at atmospheric conditions. Below about 15,000 feet the strength of halite is less than that at the surface.The effect of heating up to 300 degrees C. on the strength and ductility of anhydrite, dolomite, sandstone, and slate is small. However, the strength of limestone, shale, and siltstone at room temperature exceeds that at 300 degrees C. by about 50 per cent, and of halite by nearly 7 times. Work in progress will more realistically simulate the natural environment of deeply buried rocks by introducing an independently controlled fluid pore pressure. It will be possible to assess the effects of confining pressure, pore pressure, and temperature separately and to predict the short-time deformational behavior of most sedimentary rocks for a wide variety of conditions.