Supercharging phenomenon and mechanism of rock pulse hydrofracturing
Pulse hydrofracturing is an effective way to form a fracture network.During on-site pulse hydrofracturing,a high-frequency vibration phenomenon occurs in the pipeline.Both indoor and on-site experiments have shown that compared with constant hydraulic fracturing,pulse hydrofracturing can fracture rocks at lower pump pressure.In response to this phenomenon,we conducted experiments and simulations and observed the occurrence of a pressure-increase phenomenon during pulse hydrofracturing in the pipeline and crack.The phenomenon and mechanism of pulse pressurization are explained from three perspectives:water wave superposition in pipelines,water column vibration,and water pressure convergence in cracks.Three types of supercharging mechanisms have been revealed,namely"water wave superposition pressurization","water column resonance pressurization",and"fracture tip convergence pressurization",which can well explain the occurrence of the"low-pressure pulse hydrofracturing"phenomenon in the process of rock pulse hydrofracturing.For pulse hydrofracturing,water-wave superposition pressurization occurs in the pipeline under most pumping conditions,with differences only in the pressurization amplitude and limited pressurization effect.Only under a few pumping conditions(i.e.,pumping frequency equal to or close to the natural frequency of the water column)can resonance pressurization of the water column and a significant supercharging effect be observed.For cracks similar to wedge-shaped structures,converging pressurization usually occurs inside,and the pressurization effect is obvious.
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