Numerical Simulation Of Proped Fracture Conductivity Of Shale With Coupled Discrete Element Method And Computational Fluid Dynamics Method

Shale gas is resourceful in our country,as a typical unconventional gas,its development potential is huge.Due to the extremely low permeability of shale matrix even to nano-darcy,large-scale hydraulic fracturing is usually required to generate a complex hydrofracture network in shale reservoirs,to improve the permeability of horizontal wells around shale gas and realize the commercial development of shale gas.However,after the production of shale gas reservoir,the reservoir pressure decreases rapidly and the fracture closure pressure increases.Under high closure stress,proppant under high closure stress is easy to embed into shale or break,which significantly reduces the conductivity of fractures and shale gas production.Therefore,it is urgent to carry out the research on the mechanism of embedment and fracture of proppant in complex fractures in shale.In this paper,discrete element numerical simulation method was used to study the fluid-solid coupling mechanism of fracture conductivity,as a result,the variation rule of propped fracture width and conductivity was revealed.The model was verified by laboratory tests.The main research contents are as follows:(1)The mechanical characteristics of shale and the fracture characteristics of proppant were obtained through the uniaxial and triaxial mechanical experiment of shale and the fracture experiment of proppant.(2)The embedding and crushing model of proppant was established by discrete element method,and the fracture rate of proppant under different closure pressures was obtained through numerical simulation,that was,the fracture rate of proppant increased with the increase of closure pressure.Different factors affecting the change of fracture width were studied and the variation rule of fracture width was obtained.(3)The nonlinear contact model of seepage-stress coupling shale-monolayer proppant-shale was established by integrating DEM with CFD,and through the model research could be carried out on the influence mechanism of proppant parameters,shale mechanical parameters,closed stress changes and other factors on the embedding and crushing of proppant.(4)Based on the above research,the nonlinear contact model of seepage-stress coupling shale-multilayer proppant-shale was established,similarly,through the model research could be carried out on the influence mechanism of multilayer proppant parameters,shale mechanical parameters,closed stress changes and other factors on the embedding and crushing of proppant.(5)The experiment of fracture conductivity of shale core was carried out by using split core,and the variation rule of flow conductivity is studied through different influencing factors,the numerical model established in this paper is verified at last.Through arrying out the numerieal simulation research on the highly nonlinear eontact of shale-proppant-shale within shale fracture,the mechanism of proppant embedding and crushing in shale gas reservoir exploited before was revealed.Finally,the conductivity experiment of shale supporting fractures was carried out to verify the correctness of the established model.The results of this paper provide a new method for the prediction of conductivity of Shale fracture.