Field demonstration and integrated productivity-economics assessment of multi-lateral geothermal wells enabled by directional steel shot drilling

Abstract

This study explores the potential of multi-lateral slim-hole drilling to enhance reservoir access and connectivity in fractured geothermal reservoirs. At the core of this work is Directional Steel Shot Drilling (DSSD), a technology that combines conventional mechanical drilling with the erosive action of pressure-accelerated steel shots. This approach enables the cost-effective construction of multi-directional boreholes from a single wellbore. The DSSD technology was piloted at the VersuchsStollen Hagerbach underground research facility in Switzerland. The test site provides access to fractured hard rock formations under realistic stress and pore-fluid pressure conditions. Two horizontal wellbores, each 125 m long, were drilled. Rates of penetration (ROP) increased up to 2–5 times depending on the steel shot concentration in the drilling mud. Controlled building rates under steering (∼ 3.8–6.6°/30 m) and stable trajectories under non-steering conditions (∼ 0.3° /30 m) were achieved. The pilot demonstrated several key aspects: straightforward retrofitting of DSSD on conventional drill rigs, effectiveness of the steering concept, efficient transportation of steel shots in the mud, and the resulting enhanced bit performance in hard rock. To assess field-scale implications, reservoir simulations and a techno-economic assessment were performed using a fractured carbonate reservoir case. A set of 1000 geological realizations was generated to capture uncertainty in permeability distribution. These models were used to evaluate thermal energy production for 20 years, employing single-, dual-, and triple-lateral well architectures. Median results indicated that adding a second lateral boosts production by 45 – 50%, while a third lateral leads to an additional 25 – 30% gain. The techno-economic evaluation shows corresponding reductions in the levelized cost of heat by ∼20% when moving from single- to dual-lateral configurations, and by a further ∼25% when a third lateral is added. Similarly, the specific capital cost declines by ∼24% from single to dual laterals and by an additional 16% for triple laterals. Results remain conditional on assumptions of geology, well design, economics, and scalability to greater depth.