Sorption of Deep Shale Gas on Minerals and Organic Matter from Molecular Simulation

Abstract

Deep shale gas is one of the key targets of China's natural gas exploitation in the future. Abnormally high reservoir pressures cause an unclear occurrence state of shale gas. In this paper, molecular modeling techniques are first used to establish slit-pore models of illite, quartz, and kerogen with different widths. Monte Carlo and molecular dynamics methods are second applied to simulate the sorption of methane under the geological conditions of deep reservoirs. Then, the gas distributions in the pores and changes of energies in sorption are analyzed. The applicability of the Langmuir model is also verified. The results show that (1) shale gas is approximately adsorbed as strong monolayers in a pore; the density of the adsorbed gas in mesopores is not sensitive to the pore size; (2) the Langmuir equation is applicable for the sorption of shale gas over a wide range of pressures; the surface excesses and maximum adsorbed amounts for the mesopores follow the order of illite > quartz > kerogen, which results from the different actual surface areas depending on the roughness and holes on the surfaces; (3) in a mesopore, total gas energy reduces more in adsorption under higher pressure; van der Waals force dominates gas adsorption, while electrostatic force affects weakly; under high pressure, the total gas energy for kerogen matrix decreases less when pressure increases, which is related to the extremely dense absorbed gas and nonmonotonic variation of van der Waals force with the distance between molecules; the interactions between shale gas and different types of pores follow the order of kerogen matrix > kerogen mesopore > illite > quartz. This study clarifies the sorbing behaviors of deep shale gas as well as the applicability of the Langmuir model, which can be a reference for developing the deep shale gas.

A Review of Molecular Models for Gas Adsorption in Shale Nanopores and Experimental Characterization of Shale Properties

“…In addition, quartz is a major component of the mineralogical composition of shale . In recent years, there has been an increasing number of studies focusing on the structure and properties of shale inorganic minerals and using molecular simulations to study the gas adsorption behaviors in inorganic minerals. − Therefore, two main shale inorganic mineral models are presented in this section: one is the clay mineral model, which consists of aluminum phyllosilicates, and the other is a quartz nanopore composed of silicon and oxygen atoms.…”

“…The K-illite model is typically used in molecular simulations, which is represented by dioctahedral illite with the general unit cell formula of K x [Si a Al 8– a ]­[Al b Mg 4– b ]­O 20 (OH) 4 . Using the GCMC method, Chen et al simulated the adsorption behavior of CH 4 and CO 2 in K-illite slit pores and revealed the key gas adsorption mechanism. , CH 4 molecules without polarity are adsorbed in the center of the six-membered oxygen ring on the silicon oxygen tetrahedron surface, while CO 2 molecules with an electric quadrupole moment are closer to the polar oxygen atoms in the ring, so the electric quadrupole moment makes the adsorption capacity of CO 2 in the K-illite pores much greater than that of CH 4 . The clay pores of shale formations are expressed as basal surfaces and edge surfaces, where in illite the edge surface is dominated by the A and C chain surface and the B chain surface .…”

Methane–Water Interfacial Tension in Nanopores: A Dissipative Particle Dynamics Study

“…Deep shale gas is referred to as the shale gas buried over 3500 m under the surface, which is in a high-temperature, high-pressure environment. In the Sichuan Basin, deep shale gas accounts for over 80% of the total shale gas resources, 1 which is a promising growth in gas energy production in China. Effective development of deep shale gas is of great importance.…”