Occurrence Characteristics of Water in Nano-Slit Pores under Different Solution Conditions: A Case Study on Kaolinite

Abstract

The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl 2 solution, and a H 2 O/CH 4 mixed phase were created at varied temperatures (80−120 °C) and pressures (70−120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al−O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si−O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH 4 −H 2 O mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas−solid interactions and solid−liquid−gas interactions. CH 4 molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.

Molecular Simulation of H2/CH4 Mixture Storage and Adsorption in Kaolinite Nanopores for Underground Hydrogen Storage

“…The H 2 and CH 4 molecules are located in the pore center nearby the water layer, and the water molecules in the second layer reside toward the pore ends due to the competitive adsorption of H 2 /CH 4 and water, thus generating a thicker water film at the pore ends. This is in accordance with previous simulation work. , In the middle of the pore, the local density of H 2 and CH 4 remains relatively constant and is not affected by the water molecules near the gibbsite surfaces. The H 2 molecules are more concentrated in the middle of the pore.…”

“…This is in accordance with previous simulation work. 68,69 In the middle of the pore, the local density of H 2 and CH 4 remains relatively constant and is not affected by the water molecules near the gibbsite surfaces. The H 2 molecules are more concentrated in the middle of the pore.…”

Storage Capacity and Microdistribution of Pore Water in Gas-Producing Shales: A Collaborative Evaluation by Centrifugation and Nuclear Magnetic Resonance

“…This result is different from the result in this study maybe because of the difference of the pore surface property between drilling cores and outcrop shales. According to the molecular simulation, there are two adsorption layers of water on the pore surface, including a strong adsorption and an extremely weak adsorption, and the single layer thickness of adsorbed water is about 0.3 nm . The obtained thickness is between 0.3 and 0.5 nm, which indicates that one or two adsorption layers occur on the pore surface as a result of heterogeneity of organic and inorganic pores.…”

“…Further, the amounts of adsorbed and free water in situ can be obtained. Molecular simulation provides a feasible method for determining adsorption parameters at different temperatures and pressures …”