Numerical Simulation Study On Self-propped Fracture Conductivity In Shale Reservoir

Shale gas is the focus of unconventional gas exploration and development,hydraulic fracturing is the key technology to develop these reservoirs.Self-propped fractures are important gas flow channels formed during the fracturing of shale gas,and the fracture conductivity is the key indicators to evaluate the effectiveness of self-propped fractures.The existing conductivity evaluation method is mainly based on experimental researches,which is greatly restricted by the sizes and quantities of cores.At the same time,it is difficult to analyze the influence of rock mechanics,surface topography and other factors on the fracture conductivity only by means of experiment,and can not fully understand the flow characteristics of the fluid in self-propped fractures.Therefore,this paper carried out research on the numerical simulation of the conductivity of self-propped fractures,especially on the deformation of self-propped fractures and flow simulation inside fractures.Firstly,the plastic deformation of the fracture surface was considered,and the elastic-plastic deformation model of the fracture was established.Then,a corresponding model solving method was put forward to calculate the deformation of self-propped fractures under different closure stresses.Taking into account of the interference of the deformation of microfractures inside rock and interspace between contact interfaces,an experiment method was established to measure the deformation of self-propped fractures under closure stress.The reliability of the deformation model was verified by comparing the experimental results and the model calculation results.Secondly,based on the elastic-plastic deformation model,the topography of self-propped fractures under different closure stresses was reconstructed.Then,the geometrical model of the flow channels inside fractures were built.The flow of the fluid in the self-propped fractures was described by the three-dimensional incompressible stationary Navier-Stokes equation,the fluid area was discretized by using unstructured grids,and the finite volume method was used to discretize the governing partial differential equations,momentum equations was solved with SIMPLE algorithm.As a result,the numerical simulation of self-propped fracture conductivity was realized.Finally,according to the simulation results of the flow field,the flow characteristics of the fluid in the self-propped fracture were analyzed.It was found that the size and shape of the contact area had important influence on the flow velocity,flow path and flow pressure difference.Based on the single variable principle,the influence of Young's modulus,Poisson's ratio,geometrical surface of fracture surface and shear slippage on the conductivity was studied by using the numerical simulation method of self-propped fracture conductivity.The simulation results showed that,under high closure stress,Young's modulus was the main factor influencing the conductivity,and the conductivity was positively correlated with Young's modulus.In this paper,a numerical simulation method of conductivity under different closure stresses was established.The method could predict the conductivity of self-propped fractures and reveal the flow characteristics of the fluid inside fractures.The research results provided a new approach for the study of self-propped fracture conductivity in shale reservoir.