Interval Patterns of the AE Sequences and Subsurface Fracture Extension in Ogachi HDR Geothermal Field

Abstract

This paper describes characteristics of subsurface fracture extension examining acoustic emission (AE) event sequences detected during hydraulic fracturing at Ogachi Hot Dry Rock geothermal fields in Akita Prefecture, Japan. We detected two types of AE signals: i.e., P-wave-dominant high frequency signals (Type I) and low frequency signals with both P-and S-waves (Type II). Impulse series which represented the AE onsets were analyzed to characterize the time intervals of their occurrences. Power spectra of the impulse series for the high frequency AE sequences were flat, which signifies that the high frequency AE events occurred independently of each other. On the other hand, the power spectra of the low frequency AE sequences could be represented in the low frequency range below 0.007 Hz by S(f) ∝f^-n. This power law index n varied during the hydraulic fracturing experiment. Moreover, periodic intervals, about 0.02 Hz, whose intensity varied in relation to the water injection, were found. The subsurface fracture extension process during the hydraulic fracturing was studied relating the results of the AE sequences analysis to the other AE properties: AE activity and AE source location. This analysis indicates that the subsurface fracture extended in three stages during the hydraulic fracturing experiment.