Abstract
Abstract Thermally induced changes in mesocrack and the physical properties of fine-grained granite may influence their stability, transport characteristics, and performance related to various deep subsurface energy projects. In this study, granite was heat-treated at different temperatures (20°C, 100°C, 200°C, 300°C, 400°C, 500°C, and 600°C). The propagation and evolution of different types of cracks and the physical properties of the granite were quantitatively investigated, using optical observations of petrographic thin sections, P-wave velocity measurements, and permeability tests. The results show that as the temperature increased, the number and length of cracks increased, and the cracks were randomly distributed in all directions. This led to an increase in rock damage (λn) and an increase in permeability (K). In particular, when the temperature was ≥400°C, the damage rate significantly increased, and the number and length of intragranular cracks significantly exceeded the number and length of intergranular cracks. This led to changes in the permeation path, causing it to mainly travel through the interior of mineral particles. Using the inverse of P-wave velocity (VP), the dimensionless crack density (ρ) of granite was found to increase as the temperature increased, and this result was similar to the change of optical crack density (Pl). These analyses laid a reference for understanding the correlation between microcrack characteristics and macrophysical properties of granite.
Highlights
In the process of geological evolution, a variety of fossil energy sources have been formed, which are exist in different depths [1]
More than 90% of the cracks were generated along the periphery of the particles, and approximately 10% of the thermal cracks passed through the mineral particles
Assuming that the elastic properties of rocks are related to their damage, according to Effective medium theories (EMTs), we can obtain the dimensionless crack density of rocks with cracks [30, 31]
Summary
In the process of geological evolution, a variety of fossil energy sources (coal, oil, shale gas, etc.) have been formed, which are exist in different depths [1]. The mechanical properties of rock decrease in the heat-cooling cycles, because of the increase in the density of tensile microcracks caused by thermal action. Chen et al [16] studied the thermal damage and permeability evolution of Beishan granite at different heating rates (1-5°C/min) They found that the physical and mechanical properties of the granite weakened as the temperature increased. Studying the propagation and evolution of different types of mesocracks and the changes of rock physical properties helps establish a meso-macro correlation. This enables a deeper understanding of the effects of high temperatures on rocks and further informs engineering practices. Microstructure of fine-grained granite and its macrophysical properties under the influence of different temperature gradients
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