岩石脉冲压裂的增压现象与机制

Supercharging phenomenon and mechanism of rock pulse hydrofracturing

脉冲压裂是实现缝网压裂的有效途径.现场进行脉冲压裂时管路存在高频振动的现象,同时室内实验与现场试验均发现,与恒排量水力压裂相比,脉冲压裂可以在较低泵注压力下将岩石压裂,即存在脉冲低压破岩现象.针对此现象,本文通过原理性实验和模拟,发现脉冲压裂中存在增压现象,具体包括管路中的增压以及裂缝内的增压.从管道内水波叠加、水柱振动和裂缝内水压汇聚三个角度解释了脉冲增压现象和机制.揭示了“水波叠压增压”、“水柱共振增压”、“缝尖汇聚增压”三类增压机制,很好地解释了岩石脉冲压裂过程中的“低压脉冲致裂”现象.对于脉冲压裂,多数泵注条件下,管道内存在水波叠加增压,只是增压幅值有差别,且增压效果有限.少数泵注条件下(泵注频率等于或接近水柱固有频率),才会存在水柱共振增压,且增压效果显著.对于类似楔形结构的裂缝,其内通常存在汇聚增压,且增压效果明显.

Pulse hydrofracturing is an effective way to form a fracture network.During on-site pulse hydrofracturing,a highfrequency 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.