Acoustic emissions and hydraulic performance during injection-rate- and pressure-rate-controlled laboratory hydraulic fracturing experiments on granite

Hydraulic fracturing is a widely used reservoir engineering practice to generate new flow paths in the subsurface by fluid injection. While most research focuses on fluid type and proppant transport, the impact of the injection scheme on hydraulic water fracturing remains largely unexplored. To expand the range of suitable hydraulic fracturing options, the present work reports on hydraulic fracturing experiments with cubic granite samples (100 mm side length) under true triaxial stress conditions and compares six injection schemes divided into two groups, focusing on the acoustic emission (AE) and hydraulic performance. In the first group, the injection rate was controlled by comparing constant continuous-, stepwise continuous-, and cyclic progressive injection. The second group examined pressure-rate-controlled tests and compared stepwise pressure, stepwise pulse pressure, and cyclic pulse pressure. Cyclic progressive injection had the lowest cumulative AE energy and injectivity gains, but higher injection rates in subsequent cycles can enhance fracture growth and injectivity. Overall, pressure-rate-controlled schemes yielded greater injectivity improvement than injection-rate-controlled schemes due to rapid pressure restoration after breakdown. Moreover, the inclusion of pressure pulses improved injectivity and reduced failure time compared to tests without pulses. Pressure pulses enhanced injectivity even at similar hydraulic energy levels, highlighting their advantage in efficient injection delivery. Pressure pulses also concentrated AE activity, with high-energy AEs occurring mainly during the rising phase, explaining the relatively lower cumulative AE energy despite the highest injectivity gains. Field stimulations could benefit from the incorporation of pressure pulses, which optimize hydraulic energy use and improve injectivity.