四川盆地龙马溪组页岩储层孔隙及伊利石甲烷吸附特征

Molecular simulation of methane adsorption within illite minerals in the shale of the Longmaxi Formation based on a grand canonical Monte Carlo method and pore size distribution

为探究页岩中龙马溪组储层孔隙结构及伊利石对甲烷的吸附能力,基于等温吸附实验、压汞、液氮及低温二氧化碳等实验,研究了龙马溪组页岩孔隙结构及伊利石的分布特征,利用巨正则蒙特卡洛法模拟了不同孔径的伊利石狭缝孔的吸附特征。结果表明:页岩中孔容与比表面积主要由小于2nm的孔隙提供;伊利石为龙马溪组页岩中黏土矿物主要成分之一,常构成平行或近平行板状孔隙;303.15K(30℃),8MPa条件下,孔径在0.5～0.9nm之间时,甲烷分子受范德华力和静电力的共同作用,甲烷过剩吸附量较大;孔径大于0.9nm之后,随着孔径增大孔壁表面电荷的静电力对甲烷分子作用减小,甲烷吸附主要受范德华力控制,甲烷过剩吸附量表现出先减小后基本保持不变的特征,游离气含量表现出随孔径增大而增加的特征;平均等量吸附热反映出伊利石对甲烷的吸附方式属于物理吸附。吸附过程中,孔径介于0.5～1.2nm之间时,随着孔径增大,平均等量吸附热迅速减小;孔径大于1.2nm时,甲烷分子与伊利石狭缝孔间的吸附强度基本稳定,平均等量吸附热为6.72kJ/mol;孔径介于0.5～0.8nm之间时,甲烷分子单层吸附于伊利石晶间处,甲烷局部密度表现出单峰的特征;孔径介于0.8～1.2nm之间时,吸附方式由单层吸附向双层吸附逐渐转变,局部密度曲线由单峰向双峰变化;孔径大于1.2nm时,可供甲烷分子吸附的自由体积较大,局部密度曲线表现为双峰特征。

To investigate the pore structure and adsorption capacity of illite with respect to methane in the Longmaxi Formation,isothermal adsorption experiments utilizing mercury intrusion,liquid nitrogen and low-temperature carbon dioxide techniques were applied to shale samples from Sichuan Basin.The adsorption characteristics of illite slit pores with different diameters were simulated using the Monte Carlo method.The results demonstrate that the pore volume and specific surface area of the shale are primarily supplied by pore diameters of less than 2 nm.Illite is one of the primary components of the clay mineralogy within the shale and forms parallel or nearly-parallel plate pores.For pore sizes ranging from 0.5 nm to 0.9 nm(at 303.15 Kand 8 MPa),the methane molecules are affected by van der Waals and electrostatic forces,and the excess adsorption capacity of methane is big.When the pore size is bigger than 0.9 nm,the methane adsorption is primarily affected by van der Waals forces,and the excessadsorption capacity of methane initially decreases,after which it remains unchanged with anincrease in the pore size.The free gas content increases with increasing pore diameters.The average equivalent adsorption heat reflects that the adsorption of methane onto illite is characterized by physical adsorption.During the adsorption process,when the pore size is between 0.5 nm and 1.2 nm,the average equivalent adsorption heat decreases rapidly with an increase of the pore diameter.When the pore size exceeds 1.2 nm,the adsorption intensity between the methane molecules and the illite slit is essentially stable,and the average adsorption heat is 6.72 kJ/mol.When the pore size is between 0.5 nm and 0.8 nm,the monolayer of methane is adsorbed onto the pore wall,and the local density of methane exhibits the characteristics of a single peak.When the pore size is between 0.8 nm and 1.2 nm,the adsorption mode changes from single-layer adsorption to double-layer adsorption,and the local density curve changes from unimodal to bimodal.When the pore size is bigger than 1.2 nm,the free volume of methane adsorption can be big,and the local density curve is bimodal.