Unstressed and laboratory-stressed samples of graywacke sandstone from the site of the Rio Blanco gas-stimulation experiment were studied, both optically and with a scanning electron microscope, to relate imposed stress to pore and microcrack structure. This sandstone consisted of 100-300-μm-diameter clasts (principally quartz and feld¬spar) in a fine-grained (<10 μm diameter) matrix of clay and cementing minerals. The porosity of the rock was contained in tortuous networks of narrow (<10 μm diameter) channels around and between cement grains. Samples deformed in both uniaxial-strain and uniaxial-stress experi¬ments were studied. The microscopic effects of uniaxial-strain conditions were occasional short (<0.5 grain diameter) transgranular fractures, partial cement breakdown, and narrow cracks at the grain boundaries. Increased strain appeared to increase the degree of fracturing. The effects of uniaxial-stress conditions varied with the confining pressure of the test.
展开▼
