Molecular Simulation of the Hydration of Montmorillonite under High-temperature and Highpressure

Abstract

The hydration expansion of clay minerals in shale is an important factor affecting the sidewall stability of shale gas well. The montmorillonite is the main constituent of clay minerals, and its hydration mechanism is very important to solve the problem of borehole wall instability in shale gas well. In order to further explore the hydration mechanism of the montmorillonite under high temperature and high pressure, molecular simulation software was used to study the inter-layer spacing, inter-layer material movement, ionic hydration properties and mechanical parameters of the montmorillonite under different temperature and pressure conditions from a microscopic perspective. The results show that when the temperature increases and the pressure decreases, the inter-layer spacing of montmorillonite augments, and relatively the temperature has a great influence on the inter-layer spacing, which ranges from 1.823-2.042 nm. The diffusion rate of inter-layer water molecules and sodium ions increases with the increase of temperature and decreases with the increase of pressure. The hydration coordination number of sodium ion is large at low temperature and high pressure, and the coordination number varies from 2.35 to 4.35. When the temperature goes up, the volume modulus, shear modulus and elastic modulus of montmorillonite crystal goes down and poisson's ratio increases, while the effect of pressure on the mechanical parameters of montmorillonite crystal is the opposite. It can be seen that the research results of this paper have important theoretical guiding significance for the research of shale formation hydration mechanism under high temperature and high pressure formation environment.