Borehole Stability Analysis for an Injection Well in Superhot Rock Drilled with Millimeter Wave Technology
Authors: Porlles J. W. , Houde M. D. , Yao J. , Batir J. , Soroush H. , Madayarov A 
Published: August 2024

ABSTRACT:
Drilling is a primary cost for geothermal energy development; therefore, unsuccessful wells are a significant risk to the growth of the geothermal industry. Utilizing superhot rock (SHR) geothermal resources significantly increases the geothermal market growth rate while reducing investment risk by targeting 20+ MW opportunities for every well drilled. SHR development requires technological advancements in drilling into hard rock (basement) and high-pressure, high-temperature (HPHT) environments and a deeper understanding of how to produce this thermal resource from HPHT settings that are likely limited in producible fluids. Among the other challenges associated with in-situ HPHT environments, the injection of lower-temperature water imposes further complexity on the stress conditions due to dynamic thermomechanical interactions. Understanding its influence on wellbore stability is important to reduce the risks during production and injection operations. In this study, we evaluate the impact of water injection rate on wellbore stability through a sensitivity analysis. The solution to the stress concentrations around the wellbore was obtained analytically using Kirsch formulas for an elastic case with a uniform temperature distribution and numerically using fully coupled thermomechanical elastoplastic 2D finite element analysis.

1. INTRODUCTION
In recent years, there has been an increased interest in geothermal development as a rich source of clean and carbon-neutral renewable energy available at any geographic location for continuous power generation. Super-Hot Rock (SHR) geothermal resources are attractive targets for power generation efficiency. Reaching high subsurface temperatures for efficient geothermal power production requires drilling deep wells into the basement rock, which is a significant cost component.
The geothermal industry has had comparatively minimal growth due to few places with the convenient mix of hot rock, water, and permeability needed to produce high-quality steam. Therefore, there is an incentive for drilling deep wells into the hard rock to extract high-temperature heat and produce steam that could be used to reestablish older plants connected to power grids (Rassenfoss, 2023). However, pure Enhanced Geothermal systems (EGS) have yet to be proven commercial outside of an existing geothermal system mainly because developing an EGS for the given resource is not cost-competitive (Setel et al., 2016). As such, super-hot rock (SHR) geothermal resources may be an exciting target for commercial development (Jian et al., 2021). SHRs are those where temperature and pressure surpass the supercritical point of water, usually more than 700 °F. Also, the supercritical condition increases fluid energy density, and the energy conversion efficiency improves up to 2.5x, enabling a potential 10x increase in heat extraction compared to conventional geothermal (Garrison et al., 2020). Reaching the required depth in hard rock, more than 32,000 ft, requires improving conventional drilling and promoting new technologies. However, there are limitations, such as bits, drilling fluids, and tools (seals and packers), and they are not prepared for these extreme conditions. Thus, Woskov (2009) proposed an innovative technique using a new zone of the electromagnetic spectrum: millimeter wave (MMW) wavelengths at 30-300 gigahertz (GHz) frequency to drill these deep boreholes. This drilling technique was validated in the lab and is being further developed for field trials (Visser et al., 2013).