Behavior of an Artificial Reservoir Created in a Formation with a Natural Fracture Network for HDR Geothermal Heat Extraction

Abstract

Growth behavior of an artificially created reservoir system for HDR geothermal heat extraction in a formation with a natural fracture network, is analyzed for the stage of the creation of the reservoir system by hydraulic fracturing and also for the stage of fluid circulation through the reservoir system. It is revealed that the reservoir system grows up with an elliptical shape and its growth rate rapidly decreases with time after the circulation of fluid through the reservoir system starts. The fluid loss during the circulation has been found to be fairly small and the most amount of the injected fluid can be recovered; more than 80% of the injected fluid, for examples, can be recovered when the permeability of the natural fractures is less than 2 x 10/-10m2. This implies that, even if a natural fracture network exists in a formation, it is possible to extract heat from such a formation by using an artificially created reservoir system.