The role of thermal cycling in micro-cracking development and flow alteration: Advanced insights from CT and lab studies

Abstract

The recent interest in geothermal energy, nuclear waste management, and coal gasification has led to an in-depth examination of how granite rock behaves under subterranean reservoir conditions. Our research delves into the intricate microstructural and fluid dynamics of the Australian Harcourt granite. We emphasize its reactions to repeated temperature changes, cycling between 25 and 900 °C over 1–15 cooling episodes. To achieve this, we utilised X-ray computed tomography (CT) to visualize and reconstruct the heat-treated samples. Subsequently, we measured the porosity, identifying both total and directly interconnected spaces within these samples post-thermal treatment. Using avizo 2021.2, we created a pore network model (PNM) based on the connected porosity data to shed light on how fluids move through these heat-affected samples.The data highlighted that total porosity stayed below 0.5% up to 500 °C, with no directly connected porous regions detected. However, beyond this temperature, interconnected porosity began to appear, intensifying with increasing temperatures and more cooling cycles. The PNM allowed us to visualize and quantify specific details like pore and throat dimensions, their volumes, their interconnectedness, and the lengths of their connecting channels. Notably, these properties became more pronounced with escalating temperatures and cooling episodes, largely because of the fractures caused by the heat. Furthermore, we used AVIZO to analyse the absolute permeability of the heat-treated samples, providing a detailed understanding of their permeation characteristics. We also ran a set of rigorous lab tests to assess the impact of both confining pressure (ranging from 10 MPa to 40 MPa) and temperature (from 22 to 250 °C) on the fluid movement in the heat-treated rock. The experiments confirmed that increasing confining pressure non-linearly decreased Darcy's permeability. A rise in temperature further reduced permeability, mainly due to the narrowing of flow channels from mechanical and thermal stresses.Overall, the study provides a holistic understanding of porosity evolution and the subsequent fluid movement within thermally quenched rock samples under various reservoir conditions, providing valuable insights for energy and waste management applications.