Research On Shale Gas Adsorption Model

Shale gas,with its considerable resource reserves and huge development potential,has gradually become a focus in the petroleum industry.However,shale reservoirs are different from traditional sandstone and carbonate reservoirs,because they have unique rock properties,ultra-low formation permeability(na darcy scale)and rich nano-scale throat system.Adsorption characteristics of shale gas in these nano-scale pores are important for the evaluation of reservoirs in shale gas reservoirs and their development and exploitation.Based on the research results of confined fluids in the field of adsorption in recent years,this paper has further analyzed and verified the existing state equations of restricted fluids and chemical potential expressions,and made appropriate corrections.Combined with the discriminating conditions of adsorption phase equilibrium,the mathematical model of adsorption phase equilibrium was constructed,the solution method of the model was analyzed,and the calculation software was compiled.The comparison was made with the experimental data and the classic isothermal adsorption model.Research indicates:(1)The Lenonardo method and method of this article were used to predict the molecular diameters of various materials,and compared with the molecular diameters calculated by the viscosity method.It was found that the molecular diameter predicted by the method of this article is closer to the molecular diameter calculated by the viscosity method.(2)The effect of the pore size rp/? on the close packing density ?max was analyzed and it was found that as rp/? increases,?max increases and then becomes constant.(3)A solution method for single component adsorption equilibrium was established.Newton's iterative method is used to solve the discriminating conditions of adsorption phase equilibrium,and genetic algorithm is used to fit the parameters of single component adsorption phase equilibrium,and a calculation software was compiled.(4)The experimental data of adsorption of methane,hydrogen,nitrogen and carbon dioxide in activated carbon were fitted,using the prepared adsorption equilibrium fitting and prediction software,and a good fitting effect was obtained.The average fitting relative error of the four gases was respectively 2.22%(methane),4.19%(hydrogen),1.67%(nitrogen),6.28%(carbon dioxide).(5)Comparing the fitting effect of the model and six classical isothermal adsorption models to the experimental data of carbon dioxide adsorption,the results show that the fitting effect of the low pressure section of the model is close to the classical adsorption model,and the fitting effect of the high pressure section is better than that of the low pressure section.This model has better applicability when describing higher pressure adsorption.(6)The isothermal adsorption experimental data of methane gas in shale core was measured by gravimetric method.Based on the experimental data under low pressure conditions,the model parameters were obtained by fitting the experimental data with adsorption equilibrium fitting and prediction software.The relative error of fitting was only 6.79%at the highest.This shows that the model can be used to describe the adsorption of shale gas.According to the model parameters obtained from the fitting of low-pressure adsorption data,the adsorption data of the high-pressure segment is predicted.The relative error of prediction is only 20.79%at the highest,indicating that the model is very effective in predicting the adsorption of shale gas.