Tapping hot rocks

The economic exploitation of petrothermal systems is considered the “holy grail” of deep geothermal energy, as hot and dense rocks are found in many geological settings all over the world. However, the original approach of artificially creating new flow paths in a rock and heating controlled amounts of water in them (hot dry rock—HDR) has not proven successful. The technological breakthrough came from another method developed in the Soultz-sous-Forêts European research project, called the enhanced geothermal system (EGS). In EGS, existing fracture systems in otherwise low-permeability rock are used to improve hydraulic permeability through stimulation measures. In the Upper Rhine Graben, this technology has been used in several projects to achieve consistent and economic production rates, which are used for electricity generation and heat supply. Other international projects have been less successful and have had to be abandoned due to low flow-rates or significant induced seismicity. So what does EGS depend on and how does it differ from HDR or natural gas fracking? This review gives a brief overview of the efforts made in the field of petrothermal systems and shows that, despite research efforts dating back to the early 1970s, the question of a scalable technology for its economic exploitation remains open. Research efforts need to be made in particular with regard to maintaining sustainable hydraulic performance in demonstration plants with well explored sites.