Efficiency of single-well geothermal systems with multi-lateral drills

Drilling costs, induced seismicity, scaling and corrosion, emissions, and assessment of the inherent uncertainty associated with the reservoir properties for long-term sustainable provision of thermal energy are the major challenges of deep geothermal systems. In particular, the cost of drilling typically comprises 50-70 % of the total capital investment of a geothermal power plant. As drilling costs are definitive for the accomplishment of deep geothermal systems, the concept to use one single drill hole for extraction and injection is arisen to reduce the overall drilling costs. The aim of this study is to overcome some simplifications made in the previous published studies to investigate such single-well systems in more detail and further resolve the governing physical processes. Focus of the study is on reservoirs with a low natural permeability (around 10(-14) m(2)). Firstly, an analytical solution is applied for justifying the numerical approach to produce consistent results for the presented simplest single-well design. Secondly, the efficiency of different single-well concepts is investigated via sensitivity analysis of variations in reservoir and operational parameters. Lastly, the previous study results are compared with the current approach for the gas-filled porous medium application. The results for the single-well system either with multi-laterals or without laterals demonstrate that the variation of permeability in different directions and the porosity play a crucial role on the overall performance. Single-well systems with multi-laterals are efficient, if the permeability is isotropic and in a range between 1 x 10(-14) and 1 x 10(-12) m(2). The results of gas-filled pore space applications show that the capillary entry pressure has a significant impact on the wetting phase saturation. Simultaneous injection and extraction affect the saturation and heat exchange localized around the wells which decreases the thermal output power.

Automated summary

The major challenges of deep geothermal systems include drilling costs, induced seismicity, scaling and corrosion, emissions, and assessment of the inherent uncertainty associated with the reservoir properties. This study focuses on investigating the concept of using a single drill hole for thermal energy provision in detail, particularly in reservoirs with low natural permeability.