| The permeability of shale gas reservoir is lower than that of conventional reservoir.While complex fracture network generated by horizontal well volume fracturing can improve reservoir structure.Hydraulic repetitive fracturing is one of a technical means to improve reservoir and permeability,it is of great significance to accelerate the development of unconventional energy.Therefore,whether the complex fracture network can be formed and then the reasonable characterization and statistics of the fracture network after shale fracturing are the key issues for hydraulic fracturing construction design and improving recovery rate.In this paper,by means of the combination of indoor physical experiment and numerical simulation,combined with the theory of fracture mechanics,the law of fracture network expansion of layered rocks is researched,and the mechanism of its formation is revealed.Firstly,a set of laboratory repetitive fracturing test equipment system for layered rock is assembled and established.When the test blocks have been manufactured and their strength meets the requirements,Re-fracturing is carried out on test blocks with different number of beddings such as 2,3 and 4 beddings.After that,different test blocks containing guide crack and precast crack were tested,and the angle between prefabricated crack and horizontal direction is 60 and 90 degrees respectively.The strain gauge is pasted in different directions of the specimen,and the strain in each direction is monitored and analyzed with the strain gauge,and combining with fracture mechanics theory,the propagation law of crack net after cracking is researched.Secondly,with the aid of digital image theory,the crack propagation law is described by fractal theory,and the fractal evolution law of fracturing network is analyzed.The method of predicting and evaluating the evolution law of hydraulic fracturing network is found,and the fracturability of shale is evaluated.Finally,three different models,including 4-layer bedding,135 degrees angle between bedding and horizontal direction,and 90 degrees angle between prefabricated cracks and horizontal direction,which are established by using RFPA2D-Flow numerical simulation software.The propagation law of crack network under repeated fracturing is researched and analyzed.Combined with laboratory physical test and fracture mechanics theory,the effects of hydraulic pressure,direction of bedding,number of bedding and primary cracks on the propagation of crack network are obtained.The results show that: After repeated fracturing,it is easier to generate reticulated fracture network around wellbore,and the front main fracture expands forward continuously,while it extends along the weak bedding plane when it encounters the weak bedding plane.The lateral main crack continues to propagate forward with a small number of secondary cracks,and there are main cracks around the water injection hole.When the angle between the prefabricated crack and the bedding surface is large,the main crack will continue to extend forward through the weak bedding surface.Fracture network has fractal characteristics after hydraulic fracturing,and the fractal theory can be used to make qualitative and quantitative statistical characterization of complex fracturing network,so as to evaluate the law of fracturing network expansion of layered rocks.Hydraulic fracturing can be divided into three stages,stress accumulation stage,crack stable forward propagation stage and rock instability failure stage.The results of numerical simulation show that the strain in X direction decreases gradually and the strain in Y direction increases gradually when the rock contains prefabricated cracks.With laboratory tests and numerical analysis,the law and mechanism of crack network expansion in layered rocks are researched.It is of great significance for accelerating the shale gas revolution and increasing shale gas production,and can provide reference for fracturing construction design. |
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