| With the deepening of the contradiction between world energy supply and demand,as well as the increasing consumption of conventional oil and gas resources,unconventional oil and gas is an important strategic reserve resource of conventional oil and gas,and shale gas has become a hot topic for exploration and development due to its wide distribution range,large resource reserves and long stable production cycle.Although our country has achieved stage achievements in shale gas development,however,most of the blocks still show poor development effect and large deviation between predicted production and actual reserves.The main reason lies in the unclear understanding of the microscopic flow mechanism in the reservoir and large errors of the production prediction model.Therefore,it's urgent to understand the gas flow mechanisms in different scales of porous media at a high level,so as to establish a more accurate evaluation model system of shale gas productivity based on the shale reservoir characteristics and fluid distribution.By means of numerical simulations and experiments,this paper focused on theoretical analysis,from microscopic pore structures and occurrence mechanics of fluid in shale,combined with the accumulation of oil and gas geology,kerogen geochemistry,adsorption,thermo dynamics,micro percolation mechanics,rock mechanics and reservoir numerical simulation.This paper studies the microscopic gas flow mechanism emphatically at multiple scales under reservoir conditions,establishes the production prediction model of multi-stage horizontal wells,and further analyzes the production performance of shale gas reservoir.The key methods and corresponding conclusions are as follows:(1)This paper analyzes the characteristics of shale reservoir and its microscopic pore structures,and further reveals the evolution characteristics and distribution of gaswater in organic-inorganic pores further.The results show that,the shale in the study area is dominated by quartz(?43.6%)+ clay(?38.7%)+feldspar(?15%)with good brittleness.The kerogen type is mainly?with higher maturity.There are mainly four types of pores in shale: organic matter pore,inorganic intergranular pore,inorganic intragranular pore and microfracture.In the process of hydrocarbon generation,the pores of organic matter are gradually transformed from “saturated water” to “continuous gas phase”,while the pores of inorganic matrix are transformed from “saturated water” to “gas core with water film”.Moreover,the shale generally with the characteristic of “ultra-low water saturation”.(2)Based on the N-S equation and modified slip boundary conditions,gas transport models of organic matter and inorganic matrix at the nanoscale are established respectively.The results show that,considering the interaction between the adsorbed gas and bulk gas,the gas flow in the organic pore consists of three parts: pure free gas flow,surface diffusion and free gas flow induced by the adsorbed gas.Under the condition of high pressure and in micropores,the adsorption contributes to the percolation of gas flow in organic pores,up to 80%.In the inorganic pore,the water film decreases the gas conductivity seriously,while the nanoscale effect weakens the influence of the water film to some extent with the increase of the Knudsen number.(3)Based on the fractal distribution of micro fractures,the models of single-phase gas flow and gas-water two-phase flow in micro fractures are established.The results show that,the variation of gas apparent permeability is different at various boundary conditions under the influence of effective stress,adsorption deformation and coupling of microscale effect.Under the reservoir conditions(with a certain confining pressure),the apparent permeability changes in a similar V-shape.For gas-water two-phase flow,the reservoir pressure has great influence on the relative permeability of gas phase,but little influence on the relative permeability of water.If the viscosity difference and water slippage are ignored,the flow capacity of gas phase will be overestimated obviously.(4)A multi-scale horizontal well productivity prediction model of organic matter-inorganic matrix-natural fractures-hydraulic fractures is established and the production dynamic of shale gas reservoir is analyzed.The results show that,the parameters with the greatest influence on the productivity are fracture number,fracture aperture and fracture compressibility coefficient,followed by fracture spacing and half length.The more natural fractures developed in the reservoir(the more the number of fractures),the higher gas production.The higher the water saturation,the lower the cumulative gas production is.In this paper,the multi-scale coupling productivity prediction model of horizontal wells of fractured shale is established based on the study of microscopic gas flow mechanism in shale reservoirs.The research results in this paper further improve the theory of shale gas reservoir development in our country and provide theoretical guidance for efficient shale gas reservoir development. |
PDF Full Text Request