Characterization Of Diffusion Law And Apparent Permeability Of Shale Gas Reservoirs Under Multiple Mechanism

With the continuous consumption of conventional oil and gas resources,shale gas reservoir has been widely concerned as an unconventional energy source.However,there are some problems in the development of most shale gas reservoirs,such as unsatisfactory results and high exploitation cost,which are mainly caused by insufficient understanding of the gas migration law in shale reservoirs.In the process of shale gas production,the free gas is first produced in large-scale fractures,and then the gas adsorbed on the matrix desorbs and diffuses into the fractures.The whole gas production is in the order of desorption-diffusion-seepage.In addition,a series of changes caused by shale gas exploitation,such as stress sensitivity,desorption shrinkage and fluid flow regimes change,will have a certain impact on gas migration in the reservoir.Therefore,it is of great significance to study the diffusion characteristics and seepage evolution law of shale gas reservoirs under the action of multiple mechanisms for efficient exploitation of shale gas.In this paper,by means of combining experiment with theory,focusing on theoretical research,starting from the desorption-diffusion-seepage process of shale gas output,based on multidisciplinary theories such as adsorption dynamics,diffusion dynamics,seepage mechanics and poroelastic mechanics,the adsorption-diffusion characteristics of shale gas reservoirs under the action of temperature and stress are emphatically analyzed,and the apparent permeability model of shale under the action of multiple mechanisms is further established to study the evolution mechanism of shale gas seepage under reservoir conditions.The following research progress has been made:(1)Isothermal adsorption tests of shale at different temperatures and pressures were carried out to analyze the effects of temperature and gas pressure on the adsorption characteristics of shale.Based on a suitable adsorption model,a shale nanopore gas diffusion model was constructed considering two major diffusion types: bulk gas diffusion and adsorbed gas surface diffusion,and its reliability was verified by two sets of tests with constant temperature and variable pressure and constant pressure and variable temperature,and the response characteristics of total gas diffusion coefficient,Fick diffusion coefficient,Knudsen diffusion coefficient and surface diffusion coefficient to temperature and stress were analyzed.(2)Combining the confinement effect to modify the gas thermodynamic properties,the coupling of slip flow velocity and Knudsen diffusion velocity was considered,and the energy conservation law was used to characterize the bulk gas transport.The shale nanopore apparent permeability model was constructed by combining various mechanisms,including the confinement effect,real gas effect,stress sensitivity,matrix shrinkage,surface diffusion,and pore size variation.The reliability of the new model was verified by comparing the shale permeability data with the model,and the effects of different action mechanisms,stress-sensitive constants and internal swelling coefficient on the gas percolation through shale nanopores were investigated.(3)Based on the dual pore structure of shale,the Knudsen number equation in the microfracture network and capillary network was modified by using the pore elasticity theory to further construct a dual-porosity apparent permeability model of shale.The new model was validated using shale permeability data with constant effective stress,constant confining pressure and constant pore pressure,and the mechanism of shale apparent permeability evolution under different boundary conditions was analyzed.The evolution of Knudsen number and apparent permeability in the microfracture network and capillary network were revealed,the effects of different factors on permeability and Knudsen number were quantified,and the fluid flow regimes transformation in different pore networks were discussed.And the coupling relationship between rock deformation field,seepage field of microfracture network,seepage field of capillary network and Knudsen number was summarized.Finally,according to the research results of desorption-diffusion-seepage process of the whole shale gas reservoir,the development of shale gas wells in the field was discussed accordingly.