Experimental investigation of thermal cycling effect on physical and mechanical properties of bedrocks in geothermal fields

High temperature associated with geothermal fields affects the performance of bedrocks. Evaluation of physical and mechanical behavior of rocks in the process of thermal cycling at high temperature is one of the main issue in this application, which is also the main topic of the present study. In this study, microscopic observation and uniaxial compression tests with acoustic emission (AE) monitoring were conducted on two bedrocks (i.e., marble and granite) after treatment with different thermal cycles at high temperature. It is found that the P-wave velocity decreases as the number of thermal cycle increases. The characteristic stress levels and Young's modulus decrease with the increase of the number of thermal cycle in the treatment. The peak strain and the maximum volumetric strain show an increasing trend as the number of thermal cycle increases. After failure, more fragments are observed in specimens treated with more thermal cycles and the integrity is also found to be lower than specimens treated with less thermal cycles. The AE technique is able to capture the failure process and the associated micro-cracking behavior during loading. The degradation of macro-properties of the rocks is to a large extent attributed to the generation of grain boundary and intra-grain micro-cracks inside the rock specimens due to the applied thermal stress. Overall, the thermal cycling weakens the mechanical properties of rocks; however, the weakening effect will become not pronounced with the increase of the number of thermal cycle in the treatment if a high temperature is applied as in this study (i.e., 600 °C).