Molecular Dynamics Modeling of Effects of Water and Temperature on Rock Strength

ABSTRACT: The mechanical properties of reservoir rocks are of great significance to the study of rock mechanics, hydraulic fracturing design and reservoir stimulation. The failure mechanism and characteristics of rock under the condition of high temperature hydration have been one of the difficult problems for many scholars. In order to solve this problem, a Molecular dynamics model is used to analyze the mechanism of rock strength degradation. The compressive strength, tensile strength, Young's modulus and Poisson's ratio of quartz crystals with different water content and temperature were quantified using uniaxial tensile and uniaxial compression simulation experiments based on the established single crystal molecular model of quartz. The simulation results show that hydration and high temperature degradation have significant effects on the mechanical strength of rock, and hydration is a major factor leading to the decline of rock strength. In addition, temperature and external loading force make the internal structure of the crystal looser by changing the bond length between Si and O atoms and the angle of Si-O-Si bond. It is also found that the presence of tensile stress will change the distribution of water molecules in the interlayer, making water molecules adsorbed on both sides of the quartz surface. 1. INTRODUCTION With the continuous improvement of oil and gas exploration and development technology, oil and gas exploration and development gradually changed from conventional oil and gas reservoirs to unconventional oil and gas reservoirs[1-2]. As an important unconventional oil and gas resource, tight sandstone reservoir has become a new field of increasing oil and gas reserves and production. Tight sandstone generally has the characteristics of low porosity and low permeability[3], and is more compact than ordinary sandstone, so it needs to be reformed by hydraulic fracturing[4]. Previous studies have shown that the mechanical properties of reservoir rocks are the key factors to determine the fracturing effect besides fracturing parameters and reservoir temperature[5-6]. Therefore, understanding the mechanical properties of reservoir rocks is the key to increase oil production. Previous studies have shown that rock macro-mechanical parameters have an important impact on the study of mechanical properties of rock. However, they can not explain the fracture initiation, fracture propagation and anisotropy in the rock, so it is necessary to further study the characteristics of the rock from microscale or nanoscale[7-8].