Temperature-induced microstructural evolution and fractal characteristics of high-enthalpy Chumathang granite for enhanced geothermal energy

Micro-structural attributes of Chumathang granite from Leh, India, were experimentally determined in the temperature range from 25 to 600 °C for enhanced geothermal systems (EGS). P-wave velocity, thermal crack generation, and pore attributes were analyzed using a combination of pulse ultrasonic velocity study, 3D X-ray tomography and low-pressure gas adsorption experiments, respectively. Results indicate that thermal crack development is driven by mineral composition and differential thermal expansion, with a significant increase in the thermal damage factor between 450 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> <mml:mi>C</mml:mi></mml:mrow> </mml:math> and 600 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> <mml:mi>C</mml:mi></mml:mrow> </mml:math> , accompanied by visible cracks at 600 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> <mml:mi>C</mml:mi></mml:mrow> </mml:math> . Surface area and pore volume decreased up to 300 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> <mml:mi>C</mml:mi></mml:mrow> </mml:math> due to mineral dissolution, then slightly increased up to 600 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mrow></mml:mrow> <mml:mo>∘</mml:mo></mml:mmultiscripts> <mml:mi>C</mml:mi></mml:mrow> </mml:math> due to microfracture formation. Pore size distribution showed a dominance of coarser mesopores, and fractal dimensions decreased with temperature, reflecting simpler pore geometries. These findings enhance the understanding of granite's microstructural changes under thermal stress, informing the optimization of EGS heat extraction efficiency.