Unconventional Completion Design for Deep Geothermal Wells

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, 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.

Casing structural integrity and failure modes in a range of well types - A review

“…In addition, for geothermal wells that have low permeability, hydraulic fractures are created to increase productivity. Temperature difference between the injected water and reservoir induce significant structural damage of both casing and cement (Okech et al, 2015.) Also, indicated that the main corrosion agent in almost every geothermal well is CO2 -which when it encounters water or steam, forms carbonic acid that leads to metal corrosion. This can be divided into two principal forms of CO2 corrosion: mesa attack (localised CO2 corrosion) and pitting.…”

Assessment of deep geothermal energy exploitation methods: The need for novel single-well solutions

“…While solutions to enhance the heat exchange between the heat-carrying fluid and the ground in shallow BHE systems have been extensively presented in the literature [39], less is known regarding the improvement of downhole heat exchange characteristics in thermally-boosted, deep BHE systems. Following preliminary results [40], a numerical model was therefore built as part of this study to assess the bulk thermal potential associated with the deep BHE concept of Fig. 17 when extrapolated to the most challenging environment, which -for some regions in the world -may represent the only target for temperatures high enough to generate electrical power.…”