A New Simulating Method Based On Energy Criterion For Network Fracturing In Shale Gas Reservoirs

The horizontal well with segmented volume fracturing is the main techniques for effective exploitation of shale gas.The micro-seismic monitoring shows that complex fracture network is formed during the process of the large-scale hydraulic fracturing for shale reservoir.Aiming at large-scale fracture networks with multiple expansion frontiers,the fracture network propagation paths can't be known in advance and the network structure is very complex.When the fracture network expansion is simulated with extended finite element based on the stress intensity factor,the computation is difficult and very consuming which can't be applied in fracturing design.Therefore,it is of great significance to study the rock damage and fracture extension criterion suitable for on-site fracturing construction to simulate the formation of fracture network and to guide the fracturing design.To solve the above problem,a new simulating method based on energy criterion is presented.The author uses fluid solid coupling method to calculate fracturing fluid flow rate and pressure distribution in the fracture network.Considering from the view of energy balance,the energy criterion controlling extending frontiers is established by balancing hydraulic fracturing injection energy,strain energy,surface energy,rock fluctuations and friction dissipation energy.The new method can avoid the calculation of stress intensity factor in the fracture network edge.The fluid migration equation and rock mass deformation coupling equation are considered in the fracture network propagation model combined with the proposed energy criterion and crack interaction.The new energy method is established for the fracturing simulation in shale gas horizontal wells.The mathematical model established in this paper is solved numerically and the calculation program of fracture network expansion is also developed.The rationality of energy criterion and the mathematical model are validated by combining with the field data.The result is basically consistent about the simulation of fracture network morphology and micro seismic monitoring.The error of SRV calculation is about 13.7%.Under the same conditions,the efficiency is improved by 4 orders of magnitude compared with the extended finite element method.The influence of construction factors(pump rate and viscosity of fracturing fluid)on geometric parameters of fracture network and SRV are simulated by using compiled simulation software.The high pump rate and low viscosity of fracturing fluid are required to obtain large volume of reservoir reconstruction for shale reservoir fracturing.The energy method provides a new idea and basis for improving the shale reservoir fracturing design and simulation.