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

• Main Authors: Xin Tang, Xiaoyi Zhou, Yujie Peng
• Format: Article
• Language: English
• Published: Elsevier 2019-04-01
• Series: Journal of Natural Gas Geoscience
• Online Access: http://www.sciencedirect.com/science/article/pii/S2468256X19300136

In order to investigate the pore structure and the adsorption capacity of illite with respect to the methane in the Longmaxi Formation, isothermal adsorption experiments utilizing mercury intrusion, liquid nitrogen, and low-temperature carbon dioxide techniques were applied to the shale samples from southeastern Chongqing. The adsorption characteristics of illite slit pores varying in diameters were simulated using a Monte Carlo method. The results reveal that the pore volume and the specific surface area of the shale are primarily supplied by pore diameters measuring less than 2 nm. Illite is one of the primary components of the clay mineralogy within the shale that forms parallel or nearly-parallel plate pores. For pore sizes ranging from 0.5 nm to 0.9 nm (at conditions of 303.15 K and 8 MPa), the methane molecules are affected by van der Waals and electrostatic forces that leads to a large excess in the adsorption capacity of methane. Once the pore size becomes less than 0.9 nm, the methane adsorption becomes primarily affected by van der Waals forces. At the said size, the excess adsorption capacity of methane initially decreases, after which it remains constant with an increase in the pore size. The free gas content surges with the growing 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 in the pore diameter. In pore size that exceeds 1.2 nm, the adsorption intensity between the methane molecules and the illite slit becomes stable. In this case, the average adsorption heat is measured to be 6.72 kJ/mol. Meanwhile, the monolayer of methane is adsorbed onto the pore wall and the local density of methane exhibits the characteristics evident of that of a single peak when the pore size is between 0.5 nm and 0.8 nm. The adsorption mode changes from single-layer adsorption to double-layer adsorption when the pore size is between 0.8 nm and 1.2 nm. In addition, the local density curve changes from unimodal to bimodal. In pores sized larger than 1.2 nm, the free volume of methane adsorption can be larger, wherein the local density curve is bimodal. Keywords: Illite, Monte Carlo method, Molecular simulation, Slit pore structure