A thermally sensitive permeability model for brittle mineral-cracking and fractal microstructure evolution

The excessively high temperature of hot dry rock can lead to the development of secondary fractures in geothermal caprock. Nevertheless, the evolution mechanisms of heat treatment on the microphysical behaviors and permeability of mud shale caprock have not been thoroughly investigated. In this study, laboratory tests and theoretical analysis were comprehensively carried out to explore these evolution mechanisms. First, mud shale samples from the Upper Sinian were heated at seven temperatures ranging from room temperature (20 °C) to high temperature (300 °C). Second, the evolution of mineral composition, microstructures, and pore size distributions at different high temperatures was experimentally observed through x-ray diffraction, scanning electron microscopy, and high-pressure mercury intrusion tests. Third, the permeabilities of shale were examined using the transient pulse technology. Furthermore, a brittle-fractal permeability model was proposed to describe the cracking process of fractured rocks induced by changes in temperature. This model focused on the effect of brittle mineral-cracking ability and fractal microstructure cracking on the evolution of permeability. Finally, this model was applied to fit the experimental data obtained from published literatures and our permeability test. It was found that the thermal expansion of minerals induced a significant increase in micropores, and thermal fracturing caused the development and interconnection of mesopores. The permeability increased initially, then decreased slightly, and increased sharply after treatment at 200 °C. The feldspar content changed consistently with the permeability behavior, and quartz was more sensitive to high temperature. This brittle-fractal permeability model could effectively describe these microphysical behaviors during the fracturing process.