Experimental Study And Mechanism Analysis Of Shale Under Multistage Hydraulic Fracturing

At present,in the actual shale gas exploitation project,there are few studies on the development of shale gas channels,ie,the development and variation of microcracks in hydraulic fracturing mud shales,and most of them rely on construction experience,which severely restricts the efficiency of shale gas extraction..In this paper,the evolution law of microcracks was studied by using multistage hydraulic fracturing mud shale simulation samples and discrete element numerical simulation.First,a system for mud shale fracturing simulation and Monitoring methods was designed and assembled to conduct multi-stage hydraulic fracturing mud shale tests.The shale fracturing simulation device and the detection method system are divided into four parts: the water inlet device,the pressurization generating device,the pressure fixing device and the deformation detection device.Four sets of shale shale similar material samples containing 2 layers of bedding,4 layers of bedding,2 layers of 60° pre-crack,and 4 layers of 30° pre-crack were prepared.Then,the hydraulic fracturing tests of each group were performed in turn,and the development and evolution of microcrack under different structural conditions were compared and analyzed.Strain gauges were used for strain monitoring.At the same time,using the particle flow software PFC2 D to establish a mud shale test block model,including the non-border parallel adhesion model,the parallel adhesion model with 2 layers of bedding,and the rational parallel adhesion model with a 90° pre-crack layer.Then simulate waterflood fracturing for different models,select different time nodes to analyze the process of microcrack development,and record the particle position and displacement map during the fracturing process.The results show that in the absence of prefabricated cracks,the direction of the hydraulic crack tends to extend toward the weak bedding plane until it reaches the weak bedding plane,and then the main crack is directly generated along the weak bedding plane.At the same time there are prefabricated primary cracks and weak In the case of bedding planes,a large area of network-like cracks is preferentially formed in the region between the rifting hole and the pre-existing crack;the cracking is mainly caused by the tensile stress inside the sample,and the shear stress plays a minor role;The growth rate of the cut cracks is less than the growth rate of tension cracks.After the tension cracks develop to a certain amount,shear cracks are generated.The research in this paper analyzes the influence of bedding characteristics and original crack characteristics on the crack propagation of mud shale,and provides theoretical support for the exploitation of mud shale in practical projects.