Summary of Research Project Results under JSPS FY2001
"Research for the Future Program"

1.Research Institution Tohoku University
2.Research Area Physical and Engineering Sciences
3.Research Field Innovation in Energy Generation, Conversion, Materials and Systems for the Future
4.Term of Project FY 1997 〜 FY 2001
5.Project Number 97P00901
6.Title of Project Investigation on Design Methodology of Supercritical Subsurface Boiler for Next Generation Geothermal Energy Extraction

7.Project Leader
Name Institution,Department Title of Position
Toshiyuki Hashida Tohoku University, Graduate School of Engineering Professor

8.Core Members

Names Institution,Department Title of Position
Kazuo Hayashi Tohoku University, Institute of Fluid Science Professor
Hiroaki Niitsuma Tohoku University, Graduate School of Engineering Professor
Noriyoshi Tsuchiya Tohoku University, Graduate School of Engineering Associate Professor

9.Summary of Research Results

 This project determined the feasibility of creating an artificial geothermal reservoir in a deep-seated rock mass, at conditions exceeding the critical point for water, with hydraulic fracturing/simulation and fluid permeability experiments delineating crack-generation processes. No macroscopic cracks are identified for low injection rates, but microfracturing does occur at 'grain-scale', with a four-order increase in permeability at supercritical conditions compared to the subcritical region. For high flow rates, macroscopic fracturing is characterised by planar shear- and open fractures, and a 'cloud-like' envelope of grain-scale microcracks, that provides an ideal network for thermal extraction. Laboratory tests infer equivalent permeability in the hydraulically-stimulated reservoir, with creation of a fracture-type reservoir in which to establish an energy extraction system. Stress corrosion cracking also increases rock permeability. The durability of crack generation at supercritical conditions, based on the kinetics for microcrack generation and healing, indicates microcrack lifetime of >10 years, which is acceptable for long-term energy utilisation.
 The fracture network and formation mechanism of deep-seated geothermal reservoir's was resolved by geology field surveys, which identified typical petrochemical/tectonic and geofluid characteristics. Water-rock interaction studies clarified mineral dissolution and precipitation behavior in granitoids, using open and closed experimental systems.
 The physical character of deep, supercritical reservoirs were investigated by: (i) measurement of acoustic velocity of granite; (ii) reflection method using AE/microseismicity; and (iii) a drill-bit reflection method. The stress state of a deep-seated rock mass was estimated by: (i) DTF (Drilling-induced Tensile Fracture); (ii) drill core analysis; and (iii) utilization of borehole temperature profile and fracture orientation.
Thermal extraction numerical modeling analysis incorporated fluid flow characteristics and thermal transfer in the surrounding rock mass, considering the effects of temperature and pressure in the supercritical fluid regime, as well as fluid density, viscosity etc. The transition from subcritical to supercritical conditions has a major effect on silica dissolution/precipitation processes. Our work shows there is an optimum injection rate to prevent possible plugging of fluid pathways (dependent on rock mass temperature), and allow long-term extraction of heat energy. Our fluid flow and thermal transfer study also predicts that silica precipitation in parts of the system can create a shell-like "sealed zone", preventing fluid loss from the artificially fractured rock mass - thus improving performance of the system.

10.Key Words

(1)Geothermal Energy、(2)Supercritical Water、(3)Neo-Granitoid
(4)Hydraulic Stimulation、(5)Reservoir、(6)Water-Rock Interaction
(7)Elastic Wave Monitoring、(8)Tectonic Stress Measurement、(9)Thermal Extraction Analysis