Experimental Investigation of Mechanical Properties and Failure Behavior of Fluid-Saturated Hot Dry Rocks

Thousands of cubic meters of fluid are continuously injected for a long term to create complex fracture patterns in hydraulic fracturing of hot dry rocks. However, the physics and mechanics behind the interaction of fluid–rock are not fully understood at present. To reveal the related damage mechanisms of saturated rock samples such as damage initiation and evolution at various alternative stress levels, a series of in-house laboratory tests were performed on a TAW-series triaxial rock mechanics testing system, combined with ultrasound measurement and acoustic emission (AE) monitoring. After saturation with nano-emulsion and distilled water, ultrasound velocity of longitudinal wave was increased by 40%. Saturation weakens these mechanical parameters such as the crack damage stress ratio, fracture toughness and cohesive strength under different stress conditions. Fluid-saturated rock sample has higher AE hit rate than dry rock sample. Meanwhile, many step-like jumps appeared on the curve of cumulative AE events. Failure envelop, b-value and frequency spectrum were analyzed out to compare the mechanical difference between fluid-saturated and dry rock samples. The experimental results demonstrate that the saturation increased the pore pressure in rocks and further promoted crack propagation in hydraulic fracturing. Moreover, nano-emulsion liquid is more advantageous than distilled water for enhancing fracture complexity. This investigation provides for better understanding of the mechanisms of complex fracture formation in deep geothermal reservoirs.