| Abstract |
Hot dry rock (HDR) has high potential of thermal energy. However, caused by its high temperature (150℃~650℃), high strength and abrasiveness, breaking HDR is difficult. The problem of low penetration rate, rapid abrasion of drill bit, long well construction time and high drilling cost seriously restrict the economic exploitation of HDR. Hence, a highly efficient rock breaking method is urgently needed to drill the HDR. In this paper, we propose an innovative cryogenic drilling method in which liquid nitrogen (-195.8 °C) is used as the drilling fluid to assist in PDC bit drilling. To determine the feasibility of this kind of drilling method and the effects of critical parameters such as rocks temperature, drilling speed and liquid nitrogen pressure on the rock breaking efficiency, a series of laboratory experiments were carried out on rock (granite sample at 25℃~400℃) breaking performances by water and liquid nitrogen assisted PDC bit. The experimental results show that liquid nitrogen assisted PDC bit can efficiently break HDR. The weight on bit (WOB) and drilling torque decrease with the rocks temperature increasing. The efficiency of drilling with water and liquid nitrogen are enhanced with the increase of rock temperature and drilling speed. Under this research conditions, the mechanical specific energy (MSE) of drilling with liquid nitrogen is higher than that of drilling with water, which means that the efficiency of drilling with liquid nitrogen is lower than water. It is mainly because that liquid nitrogen could not efficiently clean the bottom hole, leading to repeated cuttings broken, thereby the drilling efficiency is decreased. Besides, it also cannot be ignored that liquid nitrogen has lower capability to lubricate the contact surface of the cutter and granite. Furthermore, it can be found that increasing the liquid nitrogen pressure can obviously improve the drilling efficiency because larger thermal stress could lead to serious damage rocks. Therefore, liquid nitrogen assisted PDC bit breaking HDR is expected to become an effective method for solving HDR drilling challenges if we improve the pressure of liquid nitrogen. The key findings of this study are valuable to provide theoretical guidance and laboratory basis for efficient HDR well drilling. |