Simulation Of Solution/Adsorption And Diffusion To The Exploitation By Injecting Non-Hydrocarbon Gases Using LBM Method

Methane is a kind of natural gas which is accompanied by the process of coal forming,most of it is adsorbed in the small,micro porous surface of coal,and is recognized as a new type of clean energy in recent decades,China’s methane has rich and huge reserves,the development of methane has become one of our country energy strategic plans.The study found that the injection of non-hydrocarbon gases in the coal can effectively increase the rate of methane,the commonly used injection gas is mainly CO₂,N₂ and the mixture of the two gases,at present the gas injection technology in China is still in the research stage.In fact,the method of methane extraction is the process of multi-gas interaction,competitive adsorption and mutual substitution,in order to fully understand its mechanism,it is necessary to describe the interaction mechanism between different injection gas and coal and methane from microcosmic level and study different effects of factors on the exploitation by injecting gases.In this paper,the equivalent structure model of three-dimensional coal is reconstructed by Quartet Structure Generation Set method（QSGS）.Using the multi-molecule Langmuir isothermal adsorption equation,the first Fick law and Lattice Boltzmann method to simulate the solution/adsorption and diffusion process of the exploitation of methane by injecting CO₂,N₂ and mixed gas,and comparing the mechanism and difference of different gases,to provide certain guidance and reference of gas source selection for the exploitation by injecting gases.The effects of porosity,initial reservoir pressure,gas injection pressure and temperature on the exploitation by injecting gases are discussed,which can provide qualitative conclusions for the actual gas injection technology,this paper draws the following results through simulation study:（1）The equivalent model of the porous medium generated by the Quartet Structure Generation Set method is controlled by the porosity,the initial growth nucleus distribution probability and the growth probability of each direction.Porosity determines the pore size and pore connectivity of the equivalent model of the porous medium;the initial growth nucleus distribution probability control the size of the pores in porous media equivalent model and quantity;and the pore morphology and connectivity probability related to the growth of the different directions.By properly adjusting these parameters,we can generate any equivalent porous media model.（2）The injection of CO₂,N₂ and the mixed gas can increase the recovery rate of methane,and methane recovery rate from large to small is: CO₂ > mixed gas > N₂.The main mechanism of methane exploitation by injecting CO₂ is the inter gas displacement;mechanism of injecting N₂ is the dilution and diffusion of gas and the carrying effect of gas;the effect of the mixed gas mixed the advantage of the N₂ and CO₂ gas,and the higher proportion of N₂ components in the mixed gas,the less mining time,and the higher initial CH₄ unit time production.Injecting CO₂ to exploitation is more suitable for the first injection of gas mining mode,and injecting N₂ to exploitation can be used to select the side of the gas mining mode.（3）The greater porosity of coal,the higher methane recovery rate and the storage gas volume and the replacement rate,so increasing porosity has positive effect on the methane exploitation;the greater pressure of the reservoir is conducive to the initial methane mining,but has little effect on the final production;the bigger injection pressure has a good role in promoting the exploitation effect of methane,but the gas injection pressure can’t be too large,should be chosen according to the actual situation;when injecting gases to mine methane,we can use some methods to increase the coal reservoir temperature for improving the effect of gas injection exploitation.