Evaluation of acoustic and mechanical properties of sedimentary rock under different saturation conditions

The growing demand and depletion of deep-earth mineral resources has increased the potential of considering mineral recovery in low-grade ore-bodies. In-situ leaching is an effective method of recovering resources such as uranium, and copper from low-grade ore by injecting a leaching solution to the deposit to dissolve minerals. Most common sedimentary ore-deposits and oilfields, however, needs a well-developed fracture network for enhanced recovery. Fracturing the ore-deposit for enhanced recovery requires sound knowledge of strength properties of sedimentary deposits. This study presents experimental results of mechanical properties of sandstone saturated in water, oil, and NaCl brines to represent strength characteristics of sandstone oilfields and saline, mineral-bearing sandstone formations. Unconfined compressive strength tests were conducted on sandstone samples and the stress-strain characteristics were captured through a non-contact digital image correlation system. Acoustic velocities and acoustic energy release of samples during failure were measured using a PCI-2 based AE system. Compared to water saturated samples, brine saturated ones showed further strength deterioration at lower brine concentrations and this influence diminished with increasing salinity. A significant strength retention in sandstone saturated in oil was observed compared to water and brine saturated samples. The acoustic velocities measured can be correlated with strength variation pattern of tested specimens.