Lattice Boltzmann Simulations About Shale Gas Flow In Contracting Nanochannels

With the decrease of oil reserves and the arising of serious environmental problems,shale gas have received increasing attention by people.Shale gas reservoirs are characterized by very fine-grained,low-porosity,and ultrapow-permeability formations.The shale pore structure mainly comprises nano-to micropores and fractures.At equilibrium,gas molecules are stored in the fine pores and fractures at three different states:free gas in the pores,adsorbed gas on the surface of pore walls,and minor dissolved gas in pore water and kerogen.Shale gas is one type of unconventional gas and the main component of it is methane.Compared with conventional natural gas,the development of shale gas has the advantages of long working life and long production cycle.Shale gas has multi-scale seepage mode,such as adsorption,desorption,diffusion,non-Darcy flow,Darcy flow,etc.,which involves multi-scale,multi-disciplinary and other scientific problems.In recent years,with the development of advanced experimental equipment and technology,the internal structures of shale formations can be observed in detail.Relevant experimental studies have shown that the pore size of shale is much smaller than that of conventional natural gas reservoirs.The shale matrix contains a large number of interconnected nanoscale pores and the size of slit pores in shale formations is mainly within 4 nm to 200 nm range.Therefore,when shale gas transports in shale formations,it is not difficult to imagine that it will flow among a large amount of cross-sections.Thus,a systematic study of shale gas transports in channels with variable cross-section is needed to provide valuable insight into the flow characteristic.In this thesis,we perform multi-relaxation-time LBM simulations to investigate the microscale flow behavior of shale gas in sudden and gradual contraction channels.Firstly,the flow characteristics of gas in two different contracting channels are studied.The influences of Knudsen number and cross-section shrinking coefficient on the shale gas transport are discussed in detail.The results show that the relationship between the gas centerline velocity and Knudsen number is dependent on the gas flow regime.On this basis,the pressure distribution versus Knudsen number is observed.The nonlinear pressure deviation shows a negative correlation with the Knudsen number.That is,the nonlinear pressure deviation decreases with the decrease of the Knudsen number.Finally,The paper focus on the effect of cross-section shrinking coefficient and cross-section types on the mass flow,permeability and Knudsen minimum effect of shale gas.The slip mass flux and second-order permeability correlation factor are calculated based on different cross-section shrinking coefficients.The slip mass flux and the second-order permeability correlation factor are found to have positive correlations with the cross-section shrinking coefficient,demonstrating that the shale gas apparent permeability would be underestimated without considering the influence of cross-section contraction.Besides,Knudsen minimum effect in shale gas flow through variable cross-section is also investigated in this thesis.The Knudsen minimum effect is an important feature in the gas microscale flow mechanism.The results reveal that the shrinking of pore channel has a great influence on Knudsen minimum effect.A high cross-section shrinking coefficient and dramatically varied cross-section type will lead to a low critical Knudsen number of Knudsen minimum effect.Based on the above analysis,it is shown that the cross-section shrinking coefficient and the cross-section type are two important influencing factors in the microscale flow of shale gas.By investigating the flow behavior of shale gas in the variable cross-section channels,we further deepen our understanding of the microscale flow mechanism of shale gas.