| Authors |
Takashi OKABE, Tatsuya SATO, Kazumi OSATO, Kazuhiro SAEKI, Keiichi SAKAGUCHI, Kenji FUJIMOTO, Tadaaki SHIMADA, Toshiyuki TOSHA |
| Keywords |
EGS, R&D, JOGMEC, Water recharge, Recharge well, Superheated reservoir, Simulation, TOUGH2, MINC model, Yanaizu-Nishiyama, Okuaizu, Fukushima |
| Abstract |
An R&D project funded by Japan Oil, Gas and Metals National Corporation (hereafter referred to JOGMEC) on technology development for geothermal reservoir evaluation and management is under way. The purpose of the project is to propose guidelines for a technical manual, based on numerical simulation and model verification, for better understanding artificial water recharge effects to geothermal reservoirs and/or hot spring aquifers. Relevant examples include steam shortages resulting from the imbalance between the steam production rates and the natural water recharge; corrosion of surface facilities by superheated steam; and production of highly acidic fluid generated by superheating within the geothermal reservoir. These problems are widespread, and occur not only in existing geothermal power plants, but also in new geothermal power plants and in newly developing areas. Our aim is to develop new and general countermeasures to such problems, which are both technically effective and cost effective. We will then collate these comprehensive measures into a new set of guidelines to ensure a stable supply of geothermal energy. Artificial water recharge is one of the EGS (Enhanced Geothermal Systems) technologies which has been successfully applied and shown to increase steam supply in the Geysers and Larderello geothermal fields. We will develop and verify our artificial water recharge technology through R&D set in the Okuaizu geothermal field in Fukushima prefecture, whose installed capacity is 65MW and has been running since 1995. The utilization factor of this power plant has reduced today to 43.6%, mainly because of depletion of steam, the superheating effect, acidification and decline of productivity and/or injectivity. The R&D project consists of project planning, design & management, survey and modeling, design and construction of test facility, drilling of a recharge well, well test and logging, operation of recharge test, numerical reservoir simulation, monitoring, and preparation of a technical operation manual. In order to locate a recharge well, extensive reservoir simulations in superheated conditions by using TOUGH2 with the MINC model, are underway. Recharge simulation shows shallow depth injection is more effective than deep injection, because it recovers heat from subsurface rock mass where geothermal fluid has been almost depleted, and acts similarly to a Hot Dry Rock system. In contrast, deep injection tends to block fluid flow supplied from the deeper two phase zone, and depends on the injection rate. The risk to production from cold sweep will be evaluated by tracer analysis and past field injection tests, separately. The location of the recharge well will be decided by the comprehensive analysis of the simulations and the risk evaluation. |