| The main pore throat size of tight porous media is micro and nano scale with higher seepage resistance,larger specific surface area so that the micro scale effect is more highlighted.In this case,it cannot be ignored that the effect of micro force and the rock wall properties on the fluid flow.At present,theories and simulation methods are usually adopted in the research of tight gas reservoir percolation mechanism directly.Therefore,the influence of pore throat scale difference and micro force are not reflected and the micro scale percolation mechanism is still not clear.Based on molecular dynamics,this paper establishes the theoretical model focusing on the tight gas reservoir characteristics by introducing lattice Boltzmann method in macroscopic scale.The theoretical model can be used to carry out the single-phase gas and gas water two-phase flow simulation in micro scale and so as to reveal the tight gas reservoir micro scale flow mechanism.Based on tight gas reservoir geological origin and combined with a variety of test methods,systematic knowledge has been formed about the petrology characteristics,nano scale pore throat structure,porosity,permeability and other physical characteristics in the tight gas reservoir.Carrying on a research about occurrence state of fluid by movable fluid test and the effect of pore throat structure and water in the seepage process on gas phase has been analyzed in tight porous media.Based on the molecular dynamics,this paper carries out a theoretical research and micro simulation of single-phase gas migration rule and mechanism in micro scale flow channel,allowing for non ideal gas dense effect under the condition of high temperature and high pressure in tight gas reservoir.As for the effect of rock surface properties on gas flow in tight porous media,on the one hand,an external force is added to the lattice model after discretization considering wall slippage which can characterize the interaction potential between the gas and the rock surface;on the other hand,the L-fractal theory is proposed to describe the surface roughness of tight porous medium,and the pore throat wall surface model with different fractal dimensions is established;based on the L-fractal theory,a lattice Boltzmann equivalent boundary treatment method is established considering the wall roughness as well.The single-phase gas seepage mechanism in the tight reservoir is revealed by simulating the gas flow in the porous medium with the improved LBM model.Considering the static electricity and solid-liquid intermolecular force,the gas and water two-phase flow LBM model is established to microscopically simulate the gas water two-phase flow.Combined with the thermodynamic equilibrium principle and gas-water potential,the microscopic force of gas-water interface film can be characterized and the water film thickness calculation model is established which is stable adsorbed on the rock surface.Simulation of gas driving water process by three dimensional reconstruction tight gas core,the gas and water two-phase micro seepage mechanism has been revealed.A method for constructing three-dimensional simplified model considering the synergistic effect of compaction and cementation in tight porous medium is proposed.Combined with the established LBM-D3Q19 gas and water micro flow model to simulate the gas driving water process in tight reservoir under high temperature and high pressure and normal temperature and pressure respectively,the similarities and differences of gas-water distribution and occurrence states are compared and analyzed.Research shows that: compaction and authigenic cementation are the main causes of tight gas reservoirs in China.The migration of tight reservoir gas is slip-seepage,and its flow law is the comprehensive effect result of wall slip and gas-solid Van der Waals force.Van der Waals force has the hindrance effect on the gas migration while the smaller the characteristic scale,the more obvious the effect is.The Van der Waals force can be neglected when the flow field characteristic scale is more than 200 nm.The rock surface roughness plays a role in reducing the gas seepage area and increasing the resistance of gas flow.When the relative roughness is more than 2%,the gas flow rate will be decreased obviously.When the fractal dimension of the wall surface is about 1.20,the gas migration resistance is at the maximum value.In the case of higher pore throat coordination numbers,the gas flow gives preference to the larger throat path.Whether the big pores can make a contribution to permeability depends on the throat size which connects them.Allowing for solid-liquid micro force,the gas migration gets more difficult and the leading edge breakthrough time gets longer with gas driving water so that it is difficult to effectively improve the gas migration ability in water-cut reservoirs by increasing the displacement pressure difference.For gas flow in the small pore and throat,the hindering effect of water film on flow is more obvious so that breaking water film is the key to achieve efficient development of tight gas reservoir.Under the condition of high temperature and high pressure,gas and water fluid flow capacity are both better than that under the normal temperature and pressure conditions.This paper proposes that the micro scale effect should be considered in the research on tight gas reservoir seepage mechanism,and establishes the LBM theoretical model under the comprehensive influence,including gas-solid/ liquid-solid intermolecular force,the rock surface roughness and the stable water film adsorbed.By the simulation of the fluid flow process,the micro scale seepage mechanism can be revealed.On this basis,a three dimensional simplified model construction method is proposed considering the synergy of compaction and cementation focusing on tight porous media.The method can be used to simulate the seepage process under the conditions of high temperature and high pressure in tight formations.The results of this paper can provide new ideas and directions for micro flow simulation in tight gas reservoir. |
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