Building a System to Solve the Challenges of Drilling Hot Hard Rock for Geothermal and Oil and Gas

Geothermal energy is gaining attention worldwide as an attractive and vastly underutilized renewable energy source due to its abundance, baseload capability, resiliency, and reliability. While there are many types of geothermal energy concepts, the holy grail of geothermal – that would enable geothermal drilling and production in most places in the world – is hard rock or superhot rock concepts. Developing these systems requires drilling into granitic basement formations, often at temperatures exceeding 300º C. There are two main technological challenges associated with hard, hot rock concepts. Firstly, very hard rock, such as granite or basalt, limits the rate of penetration (ROP). Secondly, the temperature of the drilling system exceeds the operational limits of electronic tools like measurement while drilling (MWD) and Rotary Steerable. This paper discusses the modeling, design, and testing of a drilling system that solves both challenges. Our approach to the ROP problem was to optimize the drilling system for drilling cold hard rock from 0º to 175º C and optimize the system for drilling hot hard rock where temperatures exceed 175º C. We will discuss the design and performance of both PDC drill bits and Hybrid Particle Impact/PDC bits in hard rock formations and the best application of the two methodologies moving forward. Our approach to the temperature problem was to model the entire wellbore and drillstring and investigate the effects of, but not limited to, the starting temperature of the fluid, flow rate of the fluid, type of fluid, impact of the thickness, type of insulation on the inside of the drillpipe, the diameter of the pipe, and continuous circulation. The objective of the modeling was to understand the relative impact of changes to the system on the temperature of the drilling fluid and the most cost-effective way to deliver a 150º C fluid to the bottom of the hole. This paper will discuss the results, observations, and conclusions of testing and running PDC drill bits and Particle Impact Drilling/PDC hybrids in hard formations. The results will derive from lab testing and geothermal drilling projects. The paper will also discuss the field testing and running of components of a drilling system optimized to deliver as cool a fluid as possible to the bottom of the wellbore. The results shown in this paper suggest that we have solved, or are very close to solving, two of the major challenges which prevent geothermal energy from being economically viable worldwide and not just restricted to the small geographic areas where you have very high temperature gradients associated with volcanic activity. The results would also have significant benefits for oil and gas wells where the bottom hole temperatures exceed 175º C.