Research On Hydro-mechanical Coupled Simulation In Shale Matrix And Parameter Optimization Of Hydraulic Fracture

Maximizing the shale gas production is the ultimate goal of well completion in the shale gas reservoir.The multi-mechanism of shale gas flow considering nano-scale and multiphysics is the foundation of establishing accurate production model.The theoretical analysis and numerical simulation have been carried out to study the hydro-mechanical fully coupled process and optimize the parameters of hydraulic fracture in the shale gas reservoir.The main contents and results are as follows:(1)A fully coupled gas flow and deformation numerical model has been developed based on the poroelastic theory with consideration of properties of real methane gas and the non-Darcy flow characteristics under nano-scale.It is found that the matrix porosity and apparent permeability are affected by both the matrix compressibility and pore pressure.The matix compaction caused by pore pressure depletion makes the porosity and apparent permeability decrease.However,they increase when desorption and non-Darcy flow become apparent as the pore pressure decreases a lot.(2)The numerical scheme of shale gas flow in matrix has been established based on the finite element method.Incremental version of finite element method and nonlinear iteration has been performed to guarantee the convergence in every iteration process in the time step.The computer code is writeen following the numerical scheme.The analysis of fully coupled gas flow and deformation process near a wellbore reveals that the matrix porosity and intrinsic permeability decreases near the wellbore with the depletion of pore pressure.The tension stress induced by pore pressure depletion reduces the total compression stress near the wall.The reduction of compression stress increases with the increase of pressure depletion amplitude.The decrease of bottom-hole pressure during production process alters the effective stress and consequently futhur reduces the matrix porosity and intrinsic permeability.(3)The productivity model of fractured horizontal well considering discrete fracture and fracture conductivity loss due to fracture normal deformation has been established.The numerical hydra-mechanical model is solved by a finite element simulator.The influence of hydraulic fracture on shale gas production has been analyzed and the results indicate that: the consideration of geomechanics on matrix and fracture conductivity will improve the accuracy of production prediction.The loss of fracture conductivity,including both the primary fractures and secondary fractures,impairs the production distinctly only when the initial conductivities are small.Fracture network can significantly improve the cumulative production of shale gas only when the initial fracture conductivity is larger.(4)Based on the hydro-mechanical characteristics of shale gas reservoir,various parameters of hydraulic fracture are optimized through design of experiment and the productivity model in the paper.The optimization results are listed as following: the fracture half-length is the most significant factor affecting the shale gas production,followed by the fracture stage spacing and cluster spacing.The fracture conductivity has the least influence on the production.There is a linear relationship between fracture half-length and cumulative production.Optimal fracture stage spacing and cluster spacing exists.Reference values of hydraulic fracture design are provided under the typical simulation conditions in this paper.This study would provide theoretical basis for optimizing the hydraulic fracture parameters of horizontal well with multistage fracturing in the shale gas reservoir.