Experimental study on fracture propagation and induced earthquake reduction by pulse hydraulic fracturing in shale reservoirs

Large-scale and complex cracks have always been the eternal goal of hydraulic fracturing (HF) technology in shale reservoirs, but there has been the public concern that the HF could trigger earthquake disasters. It is urgent to develop the new HF technology that can generate a large number of complex cracks and reduce the risk of induced earthquake. Herein, the fracture propagation, fluid pressure distribution and acoustic emission (AE) energy of four types of hydraulic fracturing are analyzed through laboratory experiment and numerical simulation in this study. The new rectangular pulse HF is proposed. It is concluded that the pulse HF with higher fluid pressure gradient takes a shorter time to fracture shale. It is easier to form complex cracks. Relying on reciprocating impact and strong “Water Hammer Effect”, the pulse HF can form a large number of micro-cracks and reduce the energy released at the moment of complete failure of shale. The research shows that the pulse HF with the larger pressure gradient performs better in fracturing shale and reducing the risk of induced earthquake. The rectangular pulse HF has more significant advantages than the sine and triangle pulse HF. It relies on low instantaneous rock rupture energy, long rupture time and widely distributed low energy rupture events to reduce induced earthquakes. These can help to develop the pulse HF technology and reduce the risk of induced earthquakes for shale gas exploitation.