Geothermal energy is a constant and independent form of renewable energy and plays a key role towards the world's future energy balance. In particular, deep geothermal resources are largely available across continents and can help countries become less dependent on energy imports and build a broader base in their future energy mix. However, despite its significant potential, the total contribution of the geothermal sector to global power generation remains relatively small. The International Energy Agency has recommended devising plans to address technology-specific challenges to achieve faster growth and improving policies tackling pre-development risks for geothermal energy. Reaching considerable depths is a requirement to exploit deep geothermal resources, but experience gained to date from the implementation of complex, engineered deep geothermal projects has unveiled technical and economic challenges, lower-thanexpected performance and poor public image. There is therefore an urgent need for alternative, more sustainable well designs. This paper critically assesses conventional and unconventional deep geothermal well concepts, focusing on the basic Borehole Heat Exchanger (BHE) concept. The discussions are supported by numerical simulations of a BHE design that includes heat conductive fillers to enhance the heat exchange with the surrounding formation, while avoiding direct fluid interaction with the latter.

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Harnessing geothermal energy faces many of the challenges encountered in the exploration and production of oil and gas. Deep drilling is a necessity to attain sufficiently high temperatures for successful recovery of unconventional geothermal energy, such as in Enhanced (or Engineered) Geothermal Systems (EGS). The challenge of deep geothermal exploitation is comparable to that posed by high-pressure, hightemperature oil and gas projects. Accessing such deep targets, and the need for suitable well completions, means operators incur increased capital and operational costs.To date, the implementation of EGS, following the original Hot Dry Rocks (HDR) model, has been dogged by technical problems, lower-than-expected performance and poor public image, as they typically require the ЉengineeringЉ of the reservoir by artificial stimulation in order to create the necessary heat exchange in the subsurface.Here, conventional and unconventional deep geothermal development well concepts are reviewed, breaking down the technologies into three categories: doublets, single well (open loop) solutions and single well (closed loop) heat exchangers.This paper then focuses on the original Borehole Heat Exchanger (BHE) concept for shallow applications, and -based on recent experience with medium-depth BHEs for direct use/district heatingdiscusses variations proposed in the public domain for deep geothermal systems, with the ultimate goal of generating electricity under current economic conditions.