Optimizing Geothermal Wellbore Design: Evaluating the Trade-Offs Between Smaller and Larger Wellbore Designs

Abstract This paper presents a techno-economic analysis of geothermal wellbore design, comparing the performance and cost-effectiveness of different well architectures (5 ½ in.to 9 5/8 in. tubing/casing size) across two Enhanced Geothermal System (EGS) configurations: a traditional EGS injector–producer pair and a single-well alternating injection/production system. The objective is to identify optimal wellbore designs that balance capital cost, thermal output, parasitic load, and mechanical integrity under specific reservoir conditions. A comprehensive simulation-based approach was used to evaluate key performance metrics, including surface production temperatures, parasitic loads, casing safety factors, and net power at resource temperatures of 200 °C and 300 °C. The analysis incorporated advanced well design software, thermodynamic modeling, and mechanical load assessments, including cyclic fatigue analysis. This paper does not serve as a prescriptive guide for wellbore selection design. Instead, it outlines a methodology to support the evaluation of wellbore architecture options. Although the levelized cost of electricity (LCOE) is not addressed, drilling and casing cost estimates are provided to inform early-stage decisions when considering larger wellbore diameters. In the traditional EGS with the injector-producer configuration, the 9 5/8-inch tubing delivered the highest power output at both 200 °C (5.20 MWe) and 300 °C (19.16 MWe). However, at 200 °C, the 7-inch tubing—producing 4.17 MWe—may be a viable alternative when drilling costs are considered. Drilling a 9 5/8-inch wellbore was approximately 1.5 times more expensive than a 7-inch wellbore. Additionally, the 9 5/8-inch casing required significantly heavier material (47 pound/foot (lb/ft)) compared to the 7-inch casing (26 lb/ft), increasing material costs. It also necessitated a 13 3/8-inch casing weighing 72 lb/ft, further adding to expenses. At 300 °C, the performance gap between the 9 5/8-inch (19.16 MWe) and 7-inch (14.06 MWe) configurations is substantial, making the larger casing the more favorable option despite the higher cost. This work provides novel insights into the interplay between mechanical design, thermal efficiency, and economic viability in geothermal well construction. By integrating fatigue analysis, cyclic stress modeling, and considerations for drilling costs, the study offers a practical framework for optimizing wellbore architecture in next-generation geothermal systems. The findings support the development of scalable, cost-effective EGS projects and contribute to the broader goal of accelerating the global energy transition.