As a new geothermal power generation, a novel enhanced geothermal system (EGS) concept that uses CO2 instead of water as heat transfer fluid has been proposed. Its advantages may include more heat extraction efficiency than water and storage of unrecovered CO2 in underground geological formations. In Japan it is likely that most geothermal reservoirs are filled with water because of high precipitation weather conditions and derived water from ocean plate. Therefore, when CO2 geothermal power generation is implemented, it is expected in many cases that CO2 is injected into geothermal reservoirs where water (hot water) exists (i.e. hydrothermal reservoir). So, it is important to design with consideration of water-CO2 two phase flow condition in a hydrothermal reservoir. One of the problems to be solved for EGS is to produce as much injected fluid as possible from geothermal reservoirs. In a field test of a water-based EGS targeted on a granitic rock mass in Ogachi, Japan, it was reported that the water recovery rate at the production well was as low as about 20 to 25%. With implementation of CO2 geothermal power generation in Japan in mind, this study focused on the heat extract efficiency and recovery rate of injected fluid when using CO2 as heat transfer fluid for a hydrothermal reservoir through numerical simulations. As a result, it was found that compared with the case when using water as heat transfer fluid, it is important to make CO2 breakthrough as early as possible to ensure a higher heat production from an early stage of production. The higher the reservoir temperature, the earlier breakthrough occurs due to the larger volume expansion of CO2 in the reservoir. In addition, it was indicated that the recovery rate of CO2 at a production well increases to around 50% after CO2 breakthrough, due to the relative permeability effects in the two-phase flow of CO2 and water in hydrothermal reservoirs.

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