Research on the temperature and stress fields of elliptical laser irradiated sandstone, and drilling with the elliptical laser-assisted mechanical bit

Abstract

Laser drilling is a cutting-edge rock-breaking technology that helps to improve the efficiency of oil and gas resources development in ultra-hard and ultra-deep formations. In this work, an optical lens combination was designed and developed, and comparative tests of circular laser and elliptical laser irradiated sandstone were carried out. The test results show that the elliptical beam irradiates the rock with a higher temperature and temperature gradient. Compared with a circular laser, an elliptical laser can cause a larger area of the rock to melt and vaporize and cause more significant thermal cracking and thermal spalling. Then, a thermodynamic coupling model of the rock irradiated by the elliptical laser was established, which reproduced the whole process of thermal cracking and vaporization of sandstone. The simulations show that the elliptical high-stress area and trapezoidal low-stress area are formed by elliptical laser irradiation. Among them, the rock in the low-stress area is subjected to tensile stress in the Z-direction (i.e. the laser incident direction), which leads to the occurrence and accumulation of damage, and finally causes thermal cracking and spalling. Finally, a method of breaking rock using an elliptical laser-assisted bit was proposed. Compared with the general drilling without laser, the ROP (rate of penetration) increased by 61%, which confirmed the feasibility of the elliptical laser-assisted bit for high-efficiency drilling. This research provides a new idea for realizing efficient drilling of oil and gas resources.