Thermal Effects on Drilling Performance in Hot Dry Rock

ABSTRACT High-temperature geothermal resources away from tectonic boundaries or geologic hotspots can be accessed by the drilling of deep wells through significant sections of basement rock. This paper evaluates the impact of rapid cooling on rock cutting process, and incorporates this effect as a part of potential drilling performance optimization. Using data from a series of experiments conducted in a high pressure and high temperature (HPHT) drilling laboratory, we demonstrated that rapid cooling enhanced rates of penetration over 60% in the 300+ °C hot dry rock utilizing a PDC bit or a roller cone bit. Drilling was performed in hard igneous rock samples, with the unconfined compressive strength over 200 MPa. Each sample was heated and confined to predetermined conditions, then drilled with controlled parameters. The drilling performance data was acquired for each experiment, complemented with dynamic in-situ temperature measurements, and visual assessment of rock samples before and after drilling. The rapid cooling effect on the rate of penetration was found to be a function of the temperature difference between the drilling fluid at the bit face and the rock sample temperatures. Unlike previous studies on the Drilling Performance, which overlooked thermal effects, our research focuses on the synergy between rapid cooling and mechanical rock cutting process. This study provides a framework to understand and improve drilling performance for HPHT wells, including deep and hot geothermal wells.