Abstract
<p><strong>Abstract.</strong> 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.</p>
Highlights
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)
Over the past 40 years, more and more geothermal system classifications such as hot dry rock, enhanced or engineered geothermal systems, hot wet rock, hot fractured rock, and hot sedimentary aquifer (HSA) systems have been defined in order to better characterise geothermal projects
Some of these definitions are deceptive, such as that for deep heat mining, which suggests that the geothermal heat is mined and not available anymore after the geothermal production
Summary
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. Stimulated geothermal systems, deep heat mining (Häring, 2007), and deep earth geothermal were introduced to describe deep geothermal systems that are typically created in crystalline rocks and are independent from water-bearing structures. All these definitions are related to PSs. K. The new definition of enhanced or engineered geothermal systems (EGSs) was introduced for deep geothermal systems, which required technical enhancement such as stimulation to create an artificial reservoir or the supply of water (MIT, 2006a; AGRCC, 2010; Williams et al, 2011; BMU, 2011). The term EGS is excluded from our new classification as it carries a vague definition and provides insufficient information about the system, e.g. if natural water is available in the underground heat exchanger and if the permeability is high enough to produce heat or electricity
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