The Hijiori Hot Dry Rock test site, Japan: Evaluation and optimization of heat extraction from a two-layered reservoir

The Hijiori hot dry rock (HDR) system, Japan, consists of a shallow and a deep reservoir, both in fractured granitic rocks. During a long-term circulation test (LTCT) lasting approximately 18 months and which tested different fluid production scenarios, large changes were observed in output fluid temperatures, pressures, and flow rates. A multi-reservoir, numerical model of the Hijiori HDR system was developed using the finite element heat and mass (FEHM) transfer code and applied to simulations of the LTCT. The model reproduced the pressure, temperature, and flow data observed during the test. Based on the modeling study, it was concluded that most of the produced fluid came from the shallow reservoir, that the permeability of the deep reservoir changed during the initial part of the LTCT, and that the redistribution of injected water between the two reservoirs had little impact on the relative amounts of deep and shallow fluid production. After validating the model on the LTCT, it was used to optimize injection rates in both reservoirs. The model was also used in simulations of reservoir performance where an additional heat transfer surface area has been created in the subsurface through hydraulic fracturing. © 2007 Elsevier Ltd. All rights reserved