Hydraulic Fracturing of Soft and Hard Rocks: Part 1—Rock Behavior Due to Fluid Penetration Rate

Hydraulic fracturing (HF) can initiate, propagate, and coalesce cracks, by injecting fluids into rocks. Two papers provide experimental results on the performance of rocks during HF, by varying between a high and low injection rate of 20 and 2 ml/min, respectively, into the matrix of two different rock types (soft gypsum and hard granite. In Paper 1, by using a novel transparent membrane, the evolution of the fluid fronts and the fracturing patters was observed. The results showed that the breakdown pressures were larger for the higher injection rates in both materials, and granite showed larger breakdown pressures than gypsum. The injected fluid volumes were larger for the specimens injected with lower rate and were larger for gypsum specimens. Larger injected volumes into the matrices caused lower fluid pore pressures and larger fluid fronts, where the shape of the fluid fronts was elliptical and uniform for gypsum, and irregular for granite specimens. The fluid pore pressures were smaller for gypsum specimens due to the smaller breakdown pressures and were constant around the tips of the pre-fabricated flaws. For granite specimens, the fluid pore pressures were larger but were not constant around the tips of the pre-fabricated flaws. The fluid pore pressures did not significantly contributed to the crack initiation and propagation for gypsum specimens but showed a larger contribution for granite specimens. Finally, gypsum specimens subjected to the lower injection rate showed a larger connectivity between cracks in the bridge zone, while granite specimens showed the opposite behavior.