Thermal Kaiser Effect and Thermo-mechanical Response of Utah FORGE Granitoid

Abstract

Understanding the rock response to temperature variations is essential to EGS and superhot resource development. In particular, the impact of heating and cooling on the rock porosity and permeability is pivotal for advancing various core-based techniques used for in-situ stress and poroelastic properties determination, and log data calibration at Utah FORGE. The effects of mechanical cyclic loading and related microcrack development have been relatively well-studied, however, the nature of microcrack development in relation to the heating and cooling processes have not been clearly established. The extent of damage and operating mechanisms induced by thermal cycling remain poorly understood, particularly under stress and pressure conditions. In this paper we present the result of thermal cycling test on Utah FORGE granitoid samples. Granitoid core plugs, from FORGE were subjected to multiple heating and cooling cycles at varying peak temperatures and under a constant stress state resembling their in-situ stress conditions. Wave propagation velocities and acoustic emissions were recorded continuously throughout the thermal cycling process and the former were used to determine dynamic elastic properties. The manner and timing of thermal cracking, AE, and the variation of elastic properties of the rocks demonstrate interesting new phenomena with practical consequences for laboratory investigation of thermal stimulation, stress measurement and log interpretation.