Abstract
When drilling in geothermal and deep formations, the rock-breaking mechanism of high-temperature formations is not clear. In this work, mechanical tests of rocks subjected to high temperatures were carried out, and rock-breaking models of bottom hole thermal stress dispersion and polycrystalline diamond compact (PDC) cutter were established. Aiming at efficient rock breaking pursued by drilling in high-temperature formation, rock-breaking simulations of PDC cutters with different front rake angles under the condition of temperature and confining pressure changes were carried out based on critical penetration depth. The mechanism of rock breaking is analyzed from the point of view of stress variation in the process of brittle rock breaking. The study shows that rock plasticity is enhanced after high temperature, and the temperature difference between the drilling fluid and bottom hole will make the outer part of the bottom hole shrink obviously. Under the conditions of this study, the optimal rock-breaking angle of the PDC cutter is 20 deg. The confining pressure of deep high-temperature formation will hinder rock breaking at a lower value range, and rocks under high confining pressure are more prone to brittle fracture. The increase of rotational speed has an obvious promotion stage for efficient rock breaking, and too large rotational speed will result in low brittle rock-breaking efficiency. These works are helpful in understanding the efficient brittle rock-breaking mechanism in high-temperature drilling, and can provide references for tooth design and rotational speed optimization of PDC bits.
