Abstract
It has been widely accepted worldwide that geothermal energy is stable, environmentally friendly to the earth and huge amount of resources are available for both power generation and direct thermal use with practically acceptable costs. The typical drill target in development of geothermal power station with conventional single/double flush cycle is naturally created hydrothermal reservoirs, which consist of fluid filled fractures within high enthalpy rock mass (200 ℃). However, such hydrothermal systems distribute very locally even in countries in volcanic belts including Japan because fractures with appropriate permeability and liquids are indispensable for hydrothermal systems. This localization of the hydrothermal systems brings economical and technological risks to geothermal development, and became one of the indirect reasons of higher power generation cost relative to the power generation by fossil fuels and hydro. The geothermal reservoir can be created or enhanced by human operation to the rock mass, which include fracturing and stimulation by various means. These artificially created/enhanced geothermal systems are referred to as engineered geothermal systems (EGS). The EGS has been considered as one of the most promising development ways of non-volcanic geothermal resources in US, Europe and Australia, and a number of projects are underway in these areas. The EGS is also of importance for Japanese geothermal industries to enhance productivity and maintain sustainability by stimulating dry-wells and injection wells with lower injectivity. Geophysical exploration and various kinds of monitoring technologies are indispensable for the EGS to identify high temperature (HT) geothermal resources, monitor reservoir creation process and long term reservoir behavior. It is required in EGS that location, orientation, extension and permeability of individual fracture are identified with practically acceptable resolution. Other tectonic information, such as stress state and temperature distribution, is also required to design technical creation process of EGS. Many of the currently existing geophysical technologies, however, do not have sufficient ability for this purpose, and technology development is necessary for EGS.
