Effect of liquid nitrogen pre-treatment on the fracture behaviour of shale

Liquid nitrogen enhancement has been successfully applied in petroleum extraction engineering. In order to explore its feasibility in shale gas production, the three-point bending experiments of shale were conducted before and after liquid nitrogen treatment in this paper. The results showed that the fracture toughness of shale decreased by about 18.8%, the wave velocity decreased by about 6%, and CT scans showed transgranular fracture occurring inside the shale. After destruction, the tortuosity of crack path significantly increases, and the fracture area increases, but applied energy demand reduced by 35%. It can be seen that the application of liquid nitrogen in shale reservoir transformation is beneficial for increasing the fracture volume of the reservoir, increasing the complexity of the fracture network, and reducing the design parameter requirements of fracturing equipment, thus saving costs.