Ogachi HDR reservoir structure evaluated by microearthquake (AE) events

In the Ogachi Hot Dry Rock program, many microearthquake or acoustic emission (AE) events, were observed during hydraulic fracturing and water circulation experiments. The observed AE events were located to evaluate fracture location and dimension as HDR reservoirs. However, the geological condition is so complicated at Ogachi that the errors of the previous AE locations have been thought to be large. In FY 1998, seismic reflection and refraction surveys, and detonation shots at the bottom of the production well were conducted to obtain reliable velocity structure and more accurate AE locations. Then the collapsing method was applied to the new AE locations to evaluate significant structure in the reservoirs. The evaluated structure trending north-northeast consists with the natural joint system which has a dominant strike orientation of north-northeast.