Research On Characterization Of Multi-scale Media And Flow Simulation Of Shale Gas Reservoirs

Shale gas resources are widely distributed and abundant in our country,which is an important field for the strategic replacement and development of oil and gas resources.The shale gas reservoir has adsorption gas,free gas,and some of inorganic pores filled with film water and bulk water.The structure of different scale media,such as organic pores,inorganic pores,and induced-fractures,are difficult to describe.Therefore,flow behavoir cannot be simulated by conventional theory.Based on the characterization of the apparent porosity/permeability of organic matter,inorganic matter,and fracture system in shale gas reservoirs,this paper proposed a method to describing the flow behavior of multi-fractured horizontal wells in shale gas reservoirs,in which the fluid transport mechanisms in multi-scale media and mass transfer between different scale media were coupled.Firstly,considering the migration characteristics of gas molecules in the adsorption layer,the slip coefficient in micro-scale pores of shale gas reservoir was modified,and the model for micro-scale fluid migration in shale gas reservoirs was established,which coupled adsorption layer,Knudsen layer,and bulk layer.Based on the comprehensive characterization of static structure characteristics of porous media,such as pore size distribution,pore shapes,tortuosity and surface roughness,and the dynamic pore size influenced by various stresses,the apparent porosity/permeability model of organic matter considering single-phase gas flow was established.Based on the apparent porosity/permeability model,the gas migration in organic porous media was analyzed.The results show that the small pores in organic matter are the main storage space of gas(more than 95% of the gas is stored in pores less than 10nm),and the large pores are gas flow channel.At the same time,the apparent porosity/permeability model combined with the conventional Darcy equation can be used to describe the single-phase gas flow in shale gas reservoirs.Secondly,based on the static structure and dynamic pore size of porous media,water membrane force on pore surface was analyzed to study the distribution of water film thickness and water film saturation in different pores.The apparent porosity/permeability and gas-water relative permeability model of inorganic porous media with different water saturation and pore pressure was established.The results show that with the increase of water saturation,the apparent porosity/permeability decreases greatly.The gas storage in the reservoir with water saturation of 0.1952 is about 25% lower than that of nonwater reservoir,and the apparent permeability decreases by 40%.Then,the model of orthogonal discrete fracture coupled dual media of organic matter-inorganic matter was constructed,and the effectively induced-fracture spacing of the “shortest distance” flow in shale gas reservoir was studied.Combined with the distribution of induced-fracture spacing from microseismic data inversion,an effective stimulated reservoir volume(ESRV)evaluation method for shale gas reservoirs was proposed,and the fractal dimension for induced-fracture spacing and fractal dimension for induced-fracture aperture were proposed to calculate the apparent porosity/permeability of ESRV.Studies have shown that a multi-fractured horizontal well develops a fracture network with typical reverse bifurcation characteristics.The farther away from the hydraulic fracture is,the smaller the number and density of induced-fracture in the ESRV region will be.Finally,Comprehensive considering the apparent porosity/permeability of organic matter,inorganic matter and fracture system in ESRV and the distribution of inducedfracture spacing,the DFDE equation considering flow mechanism in multi-scale media and the heterogeneity of mass transfer between different scale media in ESRV was proposed,and the Bessel-Spline method was proposed to solve the DFDE equation semi-analytically.On the basis of the above studies,a coupled fractal trilinear flow model for single-phase gas of multifractured horizontal wells in shale gas reservoir was proposed,and a water-gas two-phase numerical model considering multi-scale media of multi-fractured horizontal wells in shale gas reservoir was also proposed.The flow behavior of multi-fractured horizontal well in shale gas reservoirs was studied with the models.The results show that the DFDE equation can accurately characterize the influence of organic matter,inorganic matter and induced fracture on flow behavior.The proposed models can accurately predict reservoir variation and fluid flow in the production of shale gas reservoir,and can provide theoretical support for the efficient development of shale gas reservoirs.