On the mechanism of thermally induced micro-cracking assisted rock cutting in hard formation

Abstract

Low rate of penetration (ROP) and high drilling costs are the severe problems encountered in hard formation drilling, which heavily limit the efficient exploitation of oil and gas resources. In present study, a rock cutting model of granite using PFC2D-GBM, which considers the confining pressure, hydrostatic pressure, temperature, inter-grain and intra-grain contact failures, rock heterogeneities, is carried out to explore the mechanism of thermally induced cracking assisted rock cutting in hard formation. The research results show that, with the temperature increasing, the micro-cracks, mainly the inter-grain cracks, will be induced in intact rock due to the unequal expansion of grains. The thermally induced crack number increases linearly with increasing the treated temperature until a certain temperature, then it increases sharply deviating from the linear relationship, at last it almost keeps a constant due to the rock fully lose its strength. The cutting force decreases with increasing treated temperature, with exception of 200 °C without considering hydrostatic pressure and 300 °C with considering hydrostatic pressure. The effects of treated temperature and hydrostatic pressure on cutting force are more obviously than the confining pressure. This study leads to an enhanced understanding of thermally induced cracking assisted rock cutting mechanisms in hard formation, and provides the basis for developing the thermally induced cracking assisted rock cutting machines.