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

The presence of water in narrow pore spaces affects theoccurrenceand flow of methane, which in turn affects shale gas production. Therefore,studying the occurrence and distribution characteristics of wateris of great significance to predict gas production. Based on moleculardynamics simulations, this study investigated the occurrence characteristicsand influencing variables of liquid water in kaolinite nanopores insitu. Owing to its widespread distribution, kaolinite is the mostprevalent clay mineral with two surfaces with different characteristics.Three systems of pure water, a CaCl2 solution, and a H2O/CH4 mixed phase were created at varied temperatures(80-120 degrees C) and pressures (70-120 MPa). The presenceof gas and water in the nanopores was investigated thoroughly. Theresults showed that the adsorption of water on the Al-O octahedralsurface of kaolinite was not affected by external conditions underin situ conditions, whereas the adsorption of water on the Si-Otetrahedral surface decreased with increasing temperature, but thechange was small. When ions were present in the system, the watercapacity decreased. Based on the aforementioned results, externalconditions, such as temperature and pressure do not affect the basicstate of water. However, if there are more than two fluid types inthe system, the adsorption of water on the mineral surface is reducedowing to competitive adsorption. In addition, a CH4-H2O mixed system was simulated, in which methane molecules weredistributed in clusters. There are two types of adsorptions in pores:gas-solid interactions and solid-liquid-gas interactions.CH4 molecules are thought to be clustered in water moleculesbecause of the strong hydrogen bonding interactions among the water.

Automated summary

This study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores using molecular dynamics simulations. The adsorption of water on the Al-O octahedral surface of kaolinite was not affected by external conditions, while the adsorption on the Si-O tetrahedral surface decreased slightly with increasing temperature. The presence of ions in the system decreased the water capacity. In a CH4-H2O mixed system, methane molecules clustered with water molecules due to strong hydrogen bonding interactions.