Anisotropic Elastic Properties of Montmorillonite With Different Layer Charge Densities and Layer Charge Distributions Through Molecular Dynamic Simulation

Xueying Wang,Tongcheng Han,Li-Yun Fu

doi:10.3389/feart.2022.854816

Abstract

The knowledge of the anisotropic elastic properties of clay minerals is of crucial importance for the exploration and development of shale oil and gas. Montmorillonite (MMT) is a common natural clay mineral with different layer charge densities and layer charge distributions due to different geological conditions. Therefore, it is important to understand the currently poorly known effect of layer charge density and layer charge distribution on the anisotropic elastic properties of MMTs. This work aims to obtain such knowledge by studying the anisotropic elastic properties of different MMTs under stratigraphic conditions through molecular dynamic simulations. We showed that the in-plane compressional coefficients C11, C22 and C12 decrease with the increasing layer charge density for MMTs with different layer charge distributions, and the MMTs with the layer charges distributed on the two tetrahedral (T) sheets were found to have the smallest C11, C22 and C12. We also showed that the out-of-plane compressional coefficients C33, C13 and C23 of the MMTs with the layer charges distributed in the two T sheets decrease, while those with the layer charges in the octahedral (O) sheet increase and those with layer charges distributed in both the O sheet and the T sheets do not vary much with the increasing layer charge density. The variations of the anisotropic compressional elastic coefficients with different layer charge densities and layer charge distributions were found to be a result of the impact of the density and distribution of layer charges on the molecular interactions within the MMT layer. We further demonstrated that the layer charge density and layer charge distribution do not influence significantly the shear coefficients C44, C55, and C66. The results revealed the mechanisms of how the density and distribution of layer charges affect the anisotropic elastic properties of MMTs and will contribute to the more successful exploration and development of unconventional resources in MMT bearing shale reservoirs.

Highlights

In recent years, shale oil and gas have a contribution to a country’s energy security by lowering the dependence on imported energy (Kobek et al, 2015; Raszewski, 2016), and the shale gas is considered to play a role as a bridging fuel to a low-carbon future (Kirkland, 2010; Few et al, 2017)
Before presenting the simulation results of the anisotropic elastic coefficient with varying layer charge density and layer charge distribution, we need to test the validity of the employed method for the computation of the anisotropic elastic tensors
We have studied the elastic coefficients of MMTs with different charge densities and different charge distributions under stratigraphic conditions using molecular dynamic simulations

Summary

Introduction

Shale oil and gas have a contribution to a country’s energy security by lowering the dependence on imported energy (Kobek et al, 2015; Raszewski, 2016), and the shale gas is considered to play a role as a bridging fuel to a low-carbon future (Kirkland, 2010; Few et al, 2017). As the main mineral type and the matrix of shale, cause shale to be elastically anisotropic because of their layered structure (Bailey, 1966; Weaver and Pollard, 2011). It is of great significance to study the anisotropic elastic properties of clay minerals for the exploration and development of shale oil and gas. To date, have been obtained reporting the anisotropic elastic properties of MMTs under the effect of pressure, temperature, water content and interlayer cation types (Ebrahimi et al, 2012; Carrier et al, 2014; Zhao et al, 2021), which are of significance to understand the elastic anisotropy of clay minerals

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