In the context of geothermal energy, hot dry rock (HDR) is naturally heated crustal rock with a temperature above 180 ℃ that has little fluid and therefore can provide commercially useful thermal energy. HDR exists everywhere at varying depths. When its temperature not high enough for electricity generation, it often can supply sufficient heat for a variety of direct uses such as space heating and food or chemical processing. If only a small fraction of its thermal reserve can be accessed routinely with existing drilling methods, it would represents an essentially inexhaustible supply of heat, potentially capable of contributing substantially to the world's energy needs. As one of the essential unconventional resources, exploration and production of HDR resource can duplicate the success of shale oil and gas by using mature technologies such as hydraulic fracturing, drilling and completion techniques, reservoir engineering and so on. Although a wide variety of methods can be suggested for extracting energy from hot dry rock, the simplest—probably the most economical—one is to imitate nature by circulating water through it. Usually this will require somehow creating connected pores within the hot rock with enough exposed surface so that heat can be extracted by circulating water at useful high temperatures and rates over long periods of time. Again, a variety of methods can be suggested for producing the required flow passage and heat-transfer area, of which, hydraulic fracturing, i.e., enhanced or engineering geothermal system (EGS) technology, is the chosen one for the initial investigation in HDR program. During exploration of HDR geothermal resources by EGS technology, site screening is one of the most crucial step leading to ultimate success. The overall goal of the EGS program is to demonstrate the commercial feasibility of geothermal energy derived from hot dry rock. Therefore, its principal objectives are to confirm that the potential HDR resource is indeed large and accessible, develop a commercialized technology base for extracting the energy therefrom, and verify that the environmental and social consequences of HDR development are acceptable. On the consideration of resource, engineering and economics of geothermal exploration, middle to deep geothermal resources can be classified into two types: hydrothermal and HDR. The latter constitutes dry quality reservoir, dry inferior reservoir and dry non-reservoir rocks, according to reservoir porosity and permeability. The dry quality and inferior reservoirs are most suitable for ESG technology applications. The amount of natural reserves for the five geothermal resources are pyramid-like and with exploration difficulty increasing from top to bottom. Far more common, at depths of high rock temperatures, the combination of temperature, pressure and mineral deposition reduces any pre-existing permeability to a value too low to permit natural formation of an exploitable hydrothermal reservoir. This is a typical HDR situation. Occasionally, however, because of some geologic barrier, a hot permeable formation is not reached by the ground-water circulation thus unproductive of geothermal fluids. In this situation, permeability may vary widely, but usually is very low. Based on considerations of geological resources including burial depth, temperature, lithology and physical properties of the reservoir, thickness and fault development of caprock, engineering technology such as drilling and completion techniques, reservoir deformation, management and operation, and market requirement and economy, we propose an evaluation method and key indices for EGS site screening by combining tri-factor analysis and multi-tier index calculation. To test and verify this method, we selected 17 candidate regions in China with HDR geothermal resource advantages for HDR EGS site screening. After evaluation and optimization, we determined that the Yangbajing high-temperature geothermal region in Tibet and the buried hill geothermal region of the Jiyang depression in the Bohai Bay basin are among the best successful candidates for the superior zones for EGS testing.
