Factors affecting pore structure of granite under cyclic heating and cooling: A nuclear magnetic resonance investigation

Abstract

High-temperature rocks in the geothermal reservoir are susceptible to damage under periodic extraction and reinjection conditions, and variation of the pore structure threatens the efficient production of geothermal energy. We mainly conducted research on the influence of cyclic heating and cooling on the pore structure of granite by nuclear magnetic resonance technology and magnetic resonance imaging (MRI). The results show the significant increase of porosity and the sharp drop of ultrasonic velocity after cyclic heating and cooling. The total porosity and movable porosity of the sample increase successively with the increasing heating temperature, cooling rate and thermal cycles, while the fractal dimension of the pore structure shows the opposite tendency, this can be attributed to the fact that the dominant permeable pores weaken the pore complexity. The pore structure of granite with lower heating temperature or finer particles can maintain stability for more thermal cycles. MRI observations verified the extended and deepened micro-cracks in granite during thermal cycles, which can be attributed to the comprehensive effects of uneven expansion and shrinkage of the particles, the transformation of the mineral crystal structure, fatigue damage and the cooling-induced thermal shock.