Investigating and predicting permeability variation in thermally cracked dry rocks

Thermal cracking in reservoir rocks can result in significant changes in transport properties. Siltstone, limestone, and conglomerate samples taken from oil-bearing formations were heated to a range of peak temperatures up to 800 °C at ambient pressure. Porosity and gas permeability were measured. Changes in rock microstructures were observed using scanning electron microscopy (SEM). The results show the strong effect of thermal cracking on gas permeability and porosity. Different rock types have different thermal cracking threshold temperatures. Above the threshold temperature, crack widths and lengths are increased and a crack network is well-developed, resulting in sharp variations in gas permeability and porosity. In order to understand the factors that influence both thermal cracking and gas permeability changes in rock, a mathematical model based on fracture mechanics theory and thermoelasticity theory is proposed to describe how permeability changes with increases in temperature. The proposed permeability model was validated using experimental data. The results from the proposed model indicate that thermal cracking and permeability changes are mainly driven by the difference between mineral thermal expansion coefficients. Below the temperature that causes mineral decomposition, the temperature-driven permeability change obtained from the proposed model is in good agreement with our experimental results.