Unconventional Completion Design for Deep Geothermal Wells

Abstract

Abstract 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, high- temperature 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 "engineering" 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 heating - discusses variations proposed in the public domain for deep geothermal systems, with the ultimate goal of generating electricity under current economic conditions.