A Discrete Fracture-dual Porosity Coupling Model For Shale Gas Reservoirs

In the process of shale gas reservoir, hydraulic fracturing is an important method. Fluid flow in large fractures is obviously different from natural fractures, the scale of matrix pores, natural fractures and artificial fractures are different. Considering the multi-scale effects of the seepage flow in the shale gas reservoir, a specific discrete-fracture model based upon the fracture-matrix dual-porosity is proposed, which describe the development of shale gas reservoir.For analyzing the reliability, this model is compared with two typical dual-porosity models, which are separately base upon the permeability upscaling and infinite diverting capacity in artificial fractures. Numerical examples show that the proposed model and the dual porosity model with accurate upscaling permeability both have the same accuracy with the good convergence. In the actual geological conditions, the artificial fractures with complex shapes, calculating equivalent permeability would be very hard in permeability upscaling. But proposed model avoid this problem effectively, and is easily extended to the simulations for the multiphase flows. Constant-pressure model is simple and easy to analysis and calculation, however, this model assumes that infinite diverting capacity in artificial fractures, which will overestimate gas production rate. So constant-pressure would bring error in simulation of shale gas reservoir.In this paper, we analyzed the influence of numerical results with the size of computational area and different boundary conditions. The results show that:as the computational area is not big enough, the results are greatly influenced with the boundary conditions. Therefore, in the process of simulation, we should select enough computational area.The proposed model is applied to the development of shale gas reservoir, which study the effects of the geological parameters and the process parameters for the gas well production, expecting that can provide reference for the development of shale gas reservoir. In the geological parameters, we simulate the influence of matrix porosity, natural fracture porosity diffusion coefficient of matrix, natural fracture permeability for gas rate production. In the process parameters, we study influence of diverting capacity, the length and the number of artificial fractures for gas rate production. The results show that:the natural fracture permeability and diffusion coefficient of matrix have the important influence on gas production rate. While as the increasing of diverting capacity, the length and the number of artificial fractures, gas production rate will also increase, but the rate of increase will slow down gradually.In summary, a specific discrete-fracture model based upon the fracture-matrix dual-porosity is proposed in this work. As numerical simulation for shale gas reservoir development, proposed model can describe the multi-scale effects of the seepage flow in the shale gas reservoir. In the process of simulation which don’t need very huge CPU time, therefore, it applied to actual easily. We expect proposed model which provide reference for the development of shale gas reservoir.