Abstract
Abstract Drilling through hard crystalline rocks like granites is challenging and taxing on the overall performance due to reduced rate of penetration (ROP). While efforts have been made in improving polycrystalline diamond compact (PDC) bits to increase ROP in hard rocks, the evidence of their field performance is currently restricted to only a few sites. There exists scope for alternative drilling technology where a significant fraction of hard rocks are present, such as in deep geothermal wells for electricity generation. In the ORCHYD project, two mature technologies – high pressure water jetting (HPWJ) and percussion drilling - were combined. A combination of bit profile and peripherical groove (slotted by HPWJ) creates a stress-relief effect releasing the rock from surrounding geological stresses, requiring lower energy to break the rock using a mud hammer. Furthermore, pressure waves due to percussion are reflected by free surfaces at the groove aiding in rock breakage. In this project, an experimental study on the influence of operating conditions such as HPWJ pressure, bottom hole pressure and surrounding geological stresses on the drilling performance was conducted. Several tests were performed at a dedicated drilling laboratory where the operational parameters can be varied to emulate drilling conditions for depths up to 4 km. As compared to tricone roller bits, ORCHYD technology guaranteed at least 4 times increase in the drilling performance. The performance of HPWJ in slotting a peripheral groove and mud hammer in rock breakage were strongly influenced by the operational conditions, e.g., for a given jet pressure the groove depth decreased significantly with increased bottom hole pressure. In this work, effects of such operating conditions on drilling performance were tested for different types of rocks such as Sidobre, Kuru Grey and Red Bohus. A sensitivity analysis of the influencing parameters on drilling performance of this technology is presented in this work. With increasing geological stresses, the proposed drilling technique is more effective in increasing ROP due to the stress relief effect. A novel technique combining HPWJ and percussion drilling using a mud hammer prototype was developed to show improved drilling performance in deep, hard rocks as compared to conventional drilling technique. Through this work, the performance of this method under different realistic drilling conditions was studied to optimize ROP, especially when drilling hard abrasive formations in deep oil and gas or geothermal wells.