Study On Shale Gas Multi-scale Transport Characteristics And Flow Consistency Mechanism

Shale gas reservoirs are characterized by low porosity and low permeability.Therefore,the large-scale commercial development of shale gas must employ reservoir reconstruction technology,such as horizontal well drilling and staged hydraulic fracturing technologies.The gas transfer capacity of the reconstructed reservoir is significantly improved,however,the efficiency and sustainability of shale gas recovery are the result of multi-scale,multi-time and multi-physics coupling processes.From the microscopic scale to the macroscopic scale,the characteristics of desorption and diffusion of gas in the nanopores within the matrix and the seepage in the natural fractures and hydraulic fractures can influence or even determine the efficiency of shale gas recovery.Therefore,to reveal the multi-scale gas transport mechanism during shale gas recovery has important scientific background and engineering significance.To this end,this paper employed experimental tests,mathematical model derivation,numerical simulation analysis and comparisons with field data to systematically study the different scales transport characteristics,flow consistency mechanism,yield-increasing measure and mechanism for shale gas.It aims to provide theoretical support for the efficient and sustainable development of shale gas.The main research contents and conclusions of this paper are as follows:(1)This study quantitatively depicted the pore distribution characteristics of shale matrix.A new method for characterizing matrix gas transport characteristics was proposed and the gas diffusion mechanism in heterogeneous shale matrix was revealed.First,by XRD and FE-SEM,the shale mineral composition and matrix pores structure characteristics were qualitatively characterized.The pore size distribution and multifractal characteristics were quantitatively analyzed by nitrogen adsorption isotherm and mercury intrusion porosimetry.Then,the matrix nonlinear diffusion model was established by coupling multiple gas transport mechanisms,such as viscous flow with slip boundary,Knudsen diffusion,molecular diffusion and surface diffusion.Finally,by reconstructing the two-dimensional shale matrix,and employing the fractal theory of porous media and image recognition technology,a homogenization method for matrix diffusion at microscopic scale was proposed.The gas transport differences between organic matter and inorganic matrix pores were discussed.(2)A new analytic model of zonal trilinear shale gas productivity was deduced,and the relationships between gas production rate and multi-scale transport parameters were clarified.Based on the linear flow hypothesis,the shale gas seepage behaviors for different zones were characterized by Fick's second diffusion law,modified dual-porosity model and seepage equation of porous medium,respectively.The analytic productivity model for shale gas was obtained by Laplace transformation and Taylor series simplification.The model quantitatively evaluated the influence of bottom hole pressure,hydraulic fracture half-length and permeability,free and adsorption index,matrix block size and matrix nano-pore size on gas productivity.The consistency mechanism of multi-scale transport parameters was revealed preliminarily.(3)A multi-scale hydro-mechanical coupled numerical model considering discrete fracture networks was developed to investigate the evolution of coupled seepage and stress during shale gas recovery.This model incorporated the deformation of the matrix,discrete natural fractures and hydraulic fractures filled with proppant packs.The local discontinuities on the macroscopic scale were transformed into anisotropic continuums on the cell scale.The coupled hydro-mechanical fracture equivalent continuum–dual porosity model was developed.The fracture permeability tensor was modified by DFM.On the basis of reasonable description of the heterogeneity of the fractured shale gas reservoirs,the amount of calculation was significantly optimized.The directionalities of the fracture seepage and deformation were described by less simple mesh element.It has significant advantages when simulating large-scale and complex fracture networks.It provided a powerful tool for productivity simulation of fractured shale gas reservoirs.(4)Based on the efficient and controllable multi-parameter optimization method,the effect of multi-scale gas transport consistency on shale gas recovery was quantificationally evaluated.Combined multi-scale coupled hydro-mechanical numerical model,optimal design of experimental and response surface methodology,a multi-parameter optimization model for a typical shale gas multi-stage fractured horizontal well was established by setting the reasonable ranges of seven multi-scale transport parameters.These parameters were optimized to maximize short-term and long-term shale gas recovery.The key influencing factors of these parameters were determined,and the consistency effects of matrix,natural fractures and hydraulic fractures properties on shale gas recovery were discussed.The research results can provide reference for the high-efficient recovery of shale gas.(5)The study proposed a yield-increasing measure for shale gas based on formation heat treatment.The coupled thermo-hydro-mechanical mechanism during yield-increasing process was explored.According to the experimental data fitting,the shale gas temperature-dependent adsorption model and the thermally induced matrix fracturing model were presented.The microwave heating was used as the heating source to establish a coupled thermo-hydro-mechanical model.The study revealed the mechanisms of influences of thermal desorption and thermally induced fracturing on the evolutions of matrix porosity and diffusivity.The simulation evaluated the efficiency of microwave heating enhanced shale gas recovery.The numerical simulation research can provide new ideas and theoretical calculation methods or tools for sustainable production of shale gas.