Fluid-solid Coupling Theory Between Multiple Flows Of Shale Gas And Fractal Structure Of Pores And Fractures In Fractured Shale Reserviors

The pores and fractures of shale gas reservoirs have strong heterogeneity characteristics at different scales and can be quantitatively characterized by fractal theory.The shale gas flow experiences the gas desorption from kerogen,gas diffusion in shale matrix,and nonlinear flow in microfractures and hydraulic fracture.Therefore,the gas flow in shale gas reservoirs is a typical multi-scale and multi-flow regime process.The fractal structure of pores and fractures in shale gas reservoirs have significant impacts on gas desorption,gas diffusion,slip flow and gas seepage.On the other hand,the desorption and flow of shale gas will also modify the fractal structure of pores and fractures in shale gas reservoirs.In order to realize the efficient exploitation of shale gas,the theories and technologies of shale gas exploitation should be carefully examined and improved.The coupling mechanism between the multi-flow regimes of shale gas and fractal structure characteristics of pores and fractures in fractured shale gas reservoirs should be carefully studied.In this thesis,firstly,the experimental observations on the fractal structure of micropores in shale matrix were carried out.Then,a modified lattice Boltzmann simulation method and a theoretical fractal permeability model were proposed.The evolution of the fractal dimension was explored through the coupling between the multiple flow regimes of shale gas and fractal structure of pores and fractures.After that,the fluid-solid coupling mechanism was applied to the macroscopic exploitation of shale gas reservoirs.The cross-scale and multiple flow regime process of shale gas was simulated by using the fractal discrete fracture network and the triple-porosity model of"organic pore,inorganic pore,and fracture network".These studies are to provide theoretical guidance for the efficient production of shale gas.The main research contents and results are as follows:（1）A quantitative method is proposed to characterize the heterogeneity of shale matrix pores at nano and micro scales.Firstly,field emission scanning electron microscopy（FE-SEM）and micro-CT test（Xμ-CT）are used to obtain the images of shale matrix microstructures at nano-scale and micro-scale,respectively.The pore morphology,mineral composition and distribution characteristics are qualitatively analyzed.Secondly,a quantitative method is proposed to characterize the heterogeneity of pore structure in terms of the lacunarity,fractal dimension and pore size distribution.The results show that for the FE-SEM images at nano-scale,the change of fractal dimension is"U"type and the change of lacunarity is"∩"type;For the Xμ-CT images at micro-scale,the fractal dimension and the lacunarity all change in a logarithmic function.A negative linear relationship between fractal dimension and lacunarity is observed,and the heterogeneity of shale matrix can be more comprehensively characterized by the combination of these two parameters quantitatively.For the 3D connected pores,the structural parameters mainly include pore number,pore volume,pore diameter,pore surface area,throat number,throat diameter,throat surface area and throat length.They all are of strong heterogeneity.The average coordination number range of the connected pore network model is2.92-4.36.This indicates the poor pore connectivity in the shale matrix.（2）A modified multi-relaxation time lattice Boltzmann model（MRT-LBM）is proposed to simulate the shale gas flow in complex structures through the consideration of both Knudsen layer effect and effective gas viscosity.The quantitative relationships between lacunarity,fractal dimension,average pore diameter and permeability are established.A series of shale gas flow in the heterogeneous structures are carried out by the modified MRT-LBM.This modified MRT-LBM model is verified by the comparison with three theoretical permeability models.The results show that this modified MRT-LBM model can well simulate the gas transport properties in complex structures and accurately predict the permeability of fractal shale matrix at nano-scale.Then,the effects of lacunarity,fractal dimension and average pore size on permeability are analyzed and the quantitative relationship between these three structural parameters and permeability is determined.It is also found that the permeability of shale matrix is directional and depends on the anisotropy ratio AR of shale matrix structure.With the increase of AR,the permeability k_x increases first and then decreases,reaching the peak when AR is about20,while the permeability k_y decreases monotonically.（3）A new fractal permeability model of shale matrix is proposed to include the effects of multilayer adsorption surface diffusion and effective porosity.The permeability experiments of helium（He）,argon（Ar）and methane（CH₄）are used to verify the correctness of the fractal permeability model.Sensitivity analysis revealed that pore diameter fractal dimension D_λincreases with the increases of matrix porosity;the permeability of shale matrix decreases with the increases of tortuosity fractal dimension D_τ.When the parameters of multi-layer BET adsorption model（monolayer saturated adsorption volume V_m,adsorption layer number n and adsorption constant C）increase,the adsorption volume of shale gas will increase and the surface diffusion of shale gas will be further enhanced.（4）An evolution model of is established in the development process of shale gas,and the evolution mechanism of fractal dimension in the process of shale gas exploitation is revealed.Based on the multi-physical coupling mechanisms in the process of shale gas production,the evolution of fractal dimension（D_λand D_τ）is realized through numerical simulation,and the effects of volumetric strain,adsorption induced expansion strain,gas pressure and porosity of adsorbed gas on the evolution of fractal dimension are analyzed.The results show that the deformation of shale gas reservoir and the desorption of shale gas control the evolution of fractal dimension,and the porosity of adsorbed gas is the most important factor affecting the evolution of fractal dimension.With the development of shale gas exploitation,the adsorbed gas gradually desorbs,and the pore diameter fractal dimension D_λgradually decreases,while the tortuosity fractal dimension D_τgradually increases.The monolayer saturated adsorption volume V_m is the most sensitive parameter to the change of pore diameter fractal dimension D_λand tortuosity fractal dimension D_τ,followed by the number of adsorption layers n,and the sensitivity of adsorption constant C is the lowest.（5）A random fractal discrete multi-scale fracture network is generated to effectively characterize the multi-scale characteristics of fractures and clarify the contribution of fractal discrete fracture network to shale gas productivity.According to the scaling law of fracture length,a fractal distribution expression of fracture length is derived,a random fractal discrete multi-scale fracture network is generated,and a numerical simulation model is further established to explore the key factors affecting shale gas production capacity.This numerical model for fractured shale gas reservoirs realizes the full coupling of fractal discrete multi-scale fractures,fractal characteristics of matrix pores,slip flow,Knudsen diffusion,surface diffusion,multilayer adsorption.The reliability and accuracy of the simulation results are verified by the field data of two shale gas wells.Then the influences of fractal dimension of fracture length,pore size distribution and fracture permeability on shale gas production are analyzed,and the contribution of fractal discrete fracture network to shale gas production is clarified.（6）A new triple-porosity model of‘organic pore,inorganic pore,and fracture network’is established to reveal the multi-scale transport mechanism in the process of shale gas exploitation.This new triple-porosity model comprehensively considers the fractal characteristics of organic pores,inorganic pores and fracture networks in shale gas reservoirs,multiple migration mechanisms,multi-scale flow processes and multi-layer adsorption/desorption effects.The comparison of the triple-porosity model with the traditional single-porosity and dual-porosity models and the gas production rate of two shale gas wells show that it is more accurate than the traditional single-porosity model and dual-porosity model.The flow process of shale gas,the evolution of pressure and permeability,and the contribution of different transport mechanisms to shale gas production are studied,and the interaction mechanism between organic pores,inorganic pores and fracture networks and the time-space relationship of their contribution to productivity in the process of shale gas exploitation are revealed.There are 89 figures,20 tables and 207 references.