Westerly Granite Study of Physical and Transport Properties Monitored by P and S Waves on Spherical Samples: Effect of Temperature and Hydrostatic Pressure
Authors: Lokajícek T. , Rimnácová D. , Petružálek M. , Prikryl R. , Natherová V. , Racek M. , Aminzadeh A. 
Published: August 2024

ABSTRACT:
We investigate the thermal damage of Westerly granite (WG) using ultrasonic methods, mercury intrusion porosimetry, and micrographic observations. WG is of interest due to its relatively high strength and low permeability in intact conditions. Our study focuses on the impact of pressure and temperature on simultaneous heating of two spherical specimens to mitigate the effects of heterogeneity. The spheres were heated to temperatures of 100°C, 200°C, 350°C, 500°C, 650°C, and 800°C. One sphere was designated for a comparative study to measure changes in weight and size, while the second sphere underwent measurements of both P and S wave velocities in 132 independent directions under hydrostatic pressures ranging from 0.1 to 120 MPa at six different values of acting pressure. The data obtained enables us to calculate the full stiffness tensor of WG without presumptions and invert it to derive all elastic parameters in the principal material directions. Additionally, the specific shape of the specimen facilitates easy measurement of porosity, strain, and bulk modulus of WG, allowing us to investigate the isotropic orientation and uniformity of microcracks throughout the specimen's volume. We will compare these findings with results from common methods such as mercury intrusion porosimetry, thin section analysis, and P and S wave measurements. Furthermore, we will discuss the appropriateness of using thermal stressing to induce isotropic microcracking in the specimen. We will present the 3D dependencies of P and S wave velocities, along with their corresponding elastic constants determined in principal materials directions such as Young's modulus, shear modulus, and Poisson's ratio, as functions of temperature and pressure

1. INTRODUCTION
Granite and mica schist are widely acknowledged as suitable host rocks for numerous geothermal reservoirs worldwide, exemplified by sites like Rittershoffen in France (Baujard et al., 2017), Grimsel in Switzerland (Sakha et al., 2022), and Litoměřice in the Czech Republic (Šafanda et al., 2020). These rocks, serving as geothermal reservoirs, experience temperature increases that can induce thermal microcracking. Understanding their behavior under such conditions is crucial for realistic assessment and management of geothermal reservoirs, necessitating laboratory testing. Our investigation into the mechanical behavior was conducted on Westerly granite, a site of pioneering studies by Brace et al. (1966). Building upon this early work, numerous researchers have employed advanced experimental techniques and numerical simulations to discover the complexities of Westerly granite's mechanical response, like Tullis and Yund, 1977, Wong, 1982, Lockner, 1998, Dwivedi et al., 2008, Nasseri et al., 2009, Blake and and Faulkner, 2020, Lokajíček et al., 2021. In this study, we investigate the influence of pressure and temperature on a single spherical specimen of Westerly granite (WG). By measuring both P and S wave velocities in 132 independent directions, we determine the full stiffness tensor of WG without any prior assumptions, enabling us to obtain elastic parameters in their most general form for the first time. Moreover, the specific geometry of the specimen facilitates easy measurement of porosity, strain, and bulk modulus of WG, allowing investigation into whether microcracks are uniformly and isotropically oriented throughout the specimen's volume. Additionally, we aim to compare our findings with results obtained from conventional methods such as mercury intrusion porosimetry and thin section analysis. Furthermore, we will discuss the appropriateness of thermal degradation as a method to induce isotropic microcracking in the specimen. Through this study, our goal is to provide a comprehensive understanding of the behavior of Westerly granite under varying pressure and temperature conditions. This will contribute to the broader knowledge base of geothermal reservoir dynamics and aid in the development of effective reservoir management strategies.