Abstract
Thermally induced microfracturing has emerged as an effective method to enhance permeability of low-porosity crystalline rocks in Enhanced Geothermal Systems (EGS). Most existing studies focus on high-temperature treatments (>500 °C), which limits their applicability under field conditions, where moderate temperatures and controlled thermal shocks are preferred. This study examines the evolution of porosity and pore structure in granite subjected to repeated heating-cooling cycles at relatively low temperatures to assess the feasibility of inducing microfracture development in geothermal reservoir stimulation. Granite samples were heated to 200, 300, and 400 °C and then cooled by immersion in water to generate a steep thermal gradient. Different groups of samples were exposed to 1 to 5 thermal cycles. Total and open porosity, density, and ultrasonic wave velocity were determined to assess progressive thermal damage. In addition, mercury intrusion porosimetry was performed to characterize variations in pore volume, pore size distribution, and pore connectivity. X-ray diffraction (XRD) analysis was conducted to detect potential mineralogical changes resulting from repeated thermal exposure. The results show a progressive decrease in bulk density and ultrasonic velocities with increasing temperature and cycle count, indicating the accumulation of microstructural damage. Open porosity shows a significant increase, particularly at 300 and 400 °C, while total porosity shows more limited changes, suggesting that increasing the cycle count improves pore connectivity. The porosimetry results confirm a shift toward larger pore sizes and improved connectivity after five cycles, especially at higher temperatures, reflecting microcrack coalescence. XRD analyses show no peak displacements or new mineral phases, confirming that the observed changes are not associated with mineralogical transformations. Overall, this study demonstrates that thermal treatment at moderate temperatures effectively improves porosity and microfracture connectivity in these rocks, offering a practical and energy-efficient method for geothermal reservoir stimulation.
