Molecular Simulations Of Gas Adsorption?Desorption-Diffusion In The Aqueous Coal

Coalbed methane (CBM) reservoirs are saturated with water, and water can affect adsorption and diffusion of gas. Hydraulic fracturing is the main stimulation for improving the CBM production, and one of the key factors is the applicability of fracturing fluid which decides that the fracturing effect is good or bad. Therefore, aiming at the characteristics of gas adsorption-diffusion in the aqueous coal and the effect of active water and foam fracturing fluid on the CH4 diffusion, molecular simulation and experiment are used to carry out the following researches:(1) CBM reservoirs are heterogeneous, so the Fuchs coal chemical structure model is used to construct slit pore models. In the research of gas adsorption on the dry coal, the adsorption characteristics of H2O, CO2 and CH4 under different temperatures and apertures are simulated by the monte carlo method, while the adsorption features of CO2, CH4 and CH4/CO2 in the aqueous coal under different temperatures, apertures, moistures and mole ratios are studied.(2) Parameters such as diffusion coefficient, mean free path of molecules and equipotential surface of CO2, CH4 and CH4/CO2 under different temperatures, pressures, apertures, moistures and mole ratios are got by the molecular dynamic method, which reveals the diffusion regualtions of gas in the aqueous coal.(3) Molecular structure models of foaming agents (sodium dodecyl sulfate, sodium dodecyl sulfate, sodium dodecyl benzene sulfonic acid and betaine) and stabilizing agents (PAM, PVA, PEG and CMC) are established based on the molecular simulation method. Interaction energy between foaming agent or stabilizing agent and coal, water or aqueous coal and the diffusion coefficient of CH4 influenced by the foam fracturing fluid are obtained respectively from the molecular dynamic method. Compared with simulation and experiment results of the diffusion coefficients of CH4, the effect of the foam fracturing fluid on CH4 diffusion in coal seams can be revealed.(4) The effect of KCl on water molecules and cleanup additives (sodium dodecyl sulfate and sodium dodecyl benzene sulfonic acid) adsorption on coal surface is studied by the molecular simulation method. The diffusion coefficient of CH4 influenced by the active water is obtained from the molecular dynamic method. Compared with simulation and experiment results of the diffusion coefficients of CH4, the effect of the active water on CH4 diffusion in coal seams can be revealed.Based on the above researches, the laws of adsorption and diffusion are summed up in the following aspects:(1) The gas adsorption strength in the dry coal is H2O?CO2?CH4. The effect of the temperature increasing on the H2O adsorption is greater than the influence of CO2 and CH4 adsorption. The aperture increasing makes the H2O adsorption capacity gradually increase, while CO2 and CH4 show the opposite trend. The adsorption heat of H2O induced by hydrogen bond and van der waals potential is higher than that of CO2 and CH4. The density of H2O in the slit pore is equally distributed, while the density distributions of CO2 and CH4 are focused on the wall.(2) The increase of moisture, temperature and aperture in the aqueous coal can make the adsorption capacity of CO2 and CH4 gradually decrease. The conclusions of the bicomponent CH4/CO2 are similar to that of CO2 and CH4, but when temperature and fugacity are increasing gradually, absolute adsorption and excess adsorption curves of CH4/CO2 have the overlap. The aperture increasing is beneficial to the increase of the CH4 absolute adsorption capacity. However, no matter what apertures, the excess adsorption capacity of CO2 is always greater than that of CH4. The increase of the CO2 molar content makes the excess adsorption curve having peak, open area and overlap. The aperture increasing makes the excess adsorption capacity of the high molar content of CO2 increase gradually. When the molar ration of CH4/CO2 is less than 1, the CO2 adsorption under the big aperture is at a disadvantage in the competition.(3) In the aqueous coal, pressure and moisture decreasing, aperture increasing and temperature rising can promote CO2 and CH4 diffusion in the slit pore, which results in the diffusion coefficient increasing. The bicomponent CH4/CO2 also has the similar conclusions. When the molar ration of CH4/CO2 is gradually reducing, the diffusion coefficient of CO2 is gradually increasing, but the diffusion coefficient of CH4 is only changing around the 0.35 X 10-8m2/s. From that we can see, whether the molar content of CO2 has an advantage, the CH4 diffusion is always at a disadvantage. Although the influencing factors are always changing, molecular diffusion and Knudsen diffusion are always the main diffusion ways.(4) Although the molecules of foaming agent and stabilizing agent can adsorb on the coal surface, the adsorption behaviors of them have big differences. Compared with the diffusion coefficients of CH4 without foaming agents, adding foaming agents can make the diffusion coefficeints of CH4 reduce significantly, which makes the CH4 diffusion from the coal matrix to the fracutre block. Compared with the diffusion coefficients of CH4 only having foaming agents, adding stabilizing agents greatly reduces the diffusion coefficients of CH4, which describes that the macromolecular polymers can bring great harm to the coal seam so that the diffusion rates of CH4 reduce significantly.(5) KC1 in the active water can reduce the molecules of water and cleanup additive to adsorbing on the coal surface. K+ effectively disturbs the water molecules occupying the adsorption sites of the coal surface, which mitigates the coal swelling initiated by the adsorption of the water molecules, so the diffusion coefficient of CH4 influenced by the 2% KCl solution is greater than that of CH4 affected by water. The diffusion coefficicent of CH4 influenced by the active water is minimal because of the adsorption between clean additive and coal surface.In conclusion, molecular simulations of gas adsorption and diffusion in the aqueous coal is completed in this paper, which reveals adsorption and diffusion laws of monocomponent and bicomponent gas and the CH4 diffusion regulations under the influence of active water and foam fracturing fluid. The breakthroughs of research methods and understanding conclusions are obtained.