Discrete Element Numerical Simulation Of Hydraulic Fracture Propagation In Anisotropic Shale

Shale gas extraction is related to the environment and energy,and it is related to the people’s livelihood in the country.The contradiction between sustainable energy dependence and nonrenewable resources and ecological damage is becoming more and more prominent,which makes the development of domestic oil and gas resources more and more urgent.Shale gas,as an important clean energy development direction,has typical characteristics of ultra-low pore,ultralow permeability,long production cycle,and large gas-bearing area.Currently,the complex fracture network formed by hydraulic fracturing is one of the key technologies for efficient shale gas development,but the anisotropic formation structure of the reservoir shale,the distribution of natural fractures,and the existence of a large number of defective structures such as faulted weak surfaces lead to the expansion mechanism and spatial spreading pattern of fractures are still unclear.It is difficult to monitor the distribution of cracks and the evolution of rocks at the microscopic scale in laboratory tests,and numerical simulations,as an effective supplement to indoor tests,has shown obvious advantages in characterizing the microscopic properties of rocks.In this paper,theoretical analysis is combined with the discrete element numerical simulation method to establish a numerical model of shale from the perspective of macro and micromechanics,and the reliability of the model is verified by combining the theoretical model and laboratory data.After completing the model validation,the shale model with laminated faces,the shale model with natural fractures and the 3D shale model with weak faces are studied under different loads:(1)A2D anisotropic mathematical model considering shale laminae was established to analyze the evolution characteristics of shale displacement field,distribution pattern of cracks,number of cracks generated and accumulated changes of elastic potential energy in shales containing laminae of different dip angles under the action of different axial circumferential pressure and water pressure;(2)A 2D near-well zone anisotropic mathematical model considering natural fractures in shale was developed to analyze the effect of wellbore size on shale fracture extension and well wall fracture pressure,and also to analyze the interaction behavior of hydraulic fractures with natural fractures at different dip angles and fracture number generation under different ground stress ratios and different water injection rates;(3)A 3D anisotropic mathematical model considering the shale weak surface is established to analyze the shale displacement field evolution,fracture extension pattern and number variation under different lateral principal stresses and hydraulic pressure by setting up the shale structural weak surface.The development of this work will provide some new ideas and feasible numerical research tools to study the shale hydraulic fracture extension problem,and provide reference for the optimal design of shale gas reservoir fracturing.This paper has 60 figures,6 tables,and 119 references.