Competitive Adsorption Mechanism Of Shale Gas And Its Significance

Revealing the competitive adsorption mechanism of shale gas is the basis of clarifying the adsorption rules and transformation conditions of shale gas and establishing the quantitative evaluation model of shale gas adsorption capacity.An in-depth understanding of the adsorption mechanism of shale gas in the pores of different minerals is crucial to the establishment of an ideal shale gas adsorption model and the objective evaluation of the adsorption gas content under geological conditions.Meanwhile,the competitive adsorption mechanism of CH? and CO? or CH? and N? can provide a theoretical basis for gas injection to improve shale gas recovery.In this paper,the Grand Canonical Monte Carlo(GCMC),molecular dynamics simulation method(MD)and isothermal adsorption experiment methods were combined to study the competitive adsorption mechanism of binary mixed gases(CH?,CO?and N?)in the pores of different minerals(kerogen,clay minerals and quartz).The results show that the difference of adsorption mechanism of adsorbate gas on different mineral surfaces is the result of the comprehensive action of various factors,including binding energy(the interaction between gas and mineral surface),adsorption potential,density distribution and other factors,among which the binding energy is the fundamental factor affecting the gas distribution.Due to different adsorbate gas and mineral surface properties,CO? for polar gas molecules,strong interaction with mineral surface(mainly for Coulomb force),cause the CO? adsorption ability than CH? and N?,class quality same as in other not to replace mineral(kerogen,chlorite,quartz)three kinds of gas in the pore of the adsorption capacity of CO?> CH?>N?.The adsorption ability of polar molecules in the pores of clay minerals that are similar to the homogeneity substitution is greater than that of other mineral surfaces,resulting in a small difference in the adsorption ability of CH? and N?.So the difference in adsorption capacity between different adsorbate gases will lead to different competitive adsorption ranges or results between different gases and minerals.The injection of 80% CO? can replace 70%?80% CH?,while injection N?can also improve shale gas recovery,but 80% N? can only replace about 40% of CH?,so the effect of CO? to improve shale gas recovery is obvious better than injection of N? gas.Under water conditions,the H?O molecules on the surface of illite minerals preferentially adsorb the adsorption sites on the mineral surface,and the binding energy to the mineral surface is much larger than that of CH?.Therefore,the presence of water molecules will reduce the adsorption capacity of shale gas.Finally,the molecular simulation method is combined with the reservoir characteristics in the Fuling area.The molecular simulation method reveals that due to the complete hydrophilicity of the illite and kerogen mineral surface,the preferential adsorption of water in the water-bearing formation occupies the adsorption site of CH? gas,resulting in a decrease in the adsorption capacity of CH?.The evaluation of shale gas adsorption capacity by isothermal adsorption experiment shows that organic matter in shale reservoir is the main controlling factor affecting the adsorption capacity of shale gas.Due to the influence of formation water,the decline of adsorption capacity at different depths is mainly controlled by clay.Mineral content,the higher the clay mineral content,the greater the decrease in adsorption capacity.