Enhanced (or engineered) geothermal systems (EGS) have evolved from the hot dry rock concept, implemented for the first time at Fenton Hill in 1977. This paper systematically reviews all of the EGS projects worldwide, based on the information available in the public domain. The projects are classified by country, reservoir type, depth, reservoir temperature, stimulation methods, associated seismicity, plant capacity and current status. Thirty five years on from the first EGS implementation, the geothermal community can benefit from the lessons learnt and take a more objective approach to the pros and cons of 'conventional' EGS systems.

Abstract. The geothermal community lacks a universal definition of deep geothermal systems. A minimum depth of 400 m is often assumed, with a further sub-classification into middle-deep geothermal systems for reservoirs found between 400 and 1000 m. Yet, the simplistic use of a depth cut-off is insufficient to uniquely determine the type of resource and its associated potential. Different definitions and criteria have been proposed in the past to frame deep geothermal systems. However, although they have valid assumptions, these frameworks lack systematic integration of correlated factors. To further complicate matters, new definitions such as hot dry rock (HDR), enhanced or engineered geothermal systems (EGSs) or deep heat mining have been introduced over the years. A clear and transparent approach is needed to estimate the potential of deep geothermal systems and be capable of distinguishing between resources of a different nature. In order to overcome the ambiguity associated with some past definitions such as EGS, this paper proposes the return to a more rigorous petrothermal versus hydrothermal classification. This would be superimposed with numerical criteria for the following: depth and temperature; predominance of conduction, convection or advection; formation type; rock properties; heat source type; requirement for formation stimulation and corresponding efficiency; requirement to provide the carrier fluid; well productivity (or injectivity); production (or circulation) flow rate; and heat recharge mode. Using the results from data mining of past and present deep geothermal projects worldwide, a classification of the same, according to the aforementioned criteria is proposed. ReviewIn the past, definitions such as hydrothermal and petrothermal have been created to categorize deep geothermal systems, i.e. systems with a depth greater than 400 m, into two groups. The first group includes geothermal reservoirs that provide a heat source, a natural reservoir with high enough permeability, and a water recharge. The second group comprises geothermal systems where only a natural heat source exists, while the underground heat exchanger must be created artificially and water must be supplied for water circulation within. Hydrothermal systems (HSs) are clearly dominant in comparison to petrothermal systems (PSs) with regards to number of occurrences worldwide and megawatts of electricity generated.In 1970, the hot dry rock (HDR) concept was introduced to describe a system which uses hot and dry rock as a heat source and where an artificial underground heat exchanger had to be created (Cummings and Morris;1979;Tester et al., 1989;Potter et al., 1974). However, during the history of deep drilling, it was found that most rocks are actually not completely dry, but contain at least some naturally occurring water. This finding led to the development of a definition of hot wet rock (Duchane, 1998). In addition, the category of hot fractured rock was created to describe geothermal reservoirs that consist of hot rocks, ty...