This study is aimed to discuss the feasibility of a new utility of the combination of mini-frac test and acoustic emission (AE) monitoring for the identification and delineation of pre-existing fault/fracture with exceptionally high permeability in the vicinity of the well. In particular, geothermal field exploration often targets such a natural fault/fracture zone, because these zones are favorable for a large amount of steam production. However, the drilling success rate is only around 30 percent, which could be due to the limited spatial resolution of conventional geophysical exploration methods. A more accurate exploration method for permeable fault/fracture delineation is required.
Mini-frac test has been widely conducted in the oil, gas, and geothermal fields to measure some initial states in the target formation, such as in-situ stress and pore pressure, usually before the massive hydraulic fracturing operation. Furthermore, if AE occurs during the mini-frac test, its spatio-temporal hypocenter distribution could help to estimate the dynamic behaviors of fracture extension or pressure diffusion. However, a relationship between the spatio-temporal hypocenter distribution and the existence of highly permeable fault zones is not clearly understood, because complex factors such as pumping rate, pumping volume, the fluid-flow capacity of the path from well to the permeable fault zones, and fluid-flow capacity of the permeable faults themselves are mutually affected.
Hence in this study, we conducted the sensitivity test on some sets of pumping parameters and geological parameters by using the numerical fracturing simulator "SHIFT". The simulation model was constructed based on actual field data acquired at the Hijiori Hot Dry Rock (HDR) site, Japan, and also some uncertain parameters are carefully tuned by matching the simulated fracturing pressure to the actual pressure response observed at the site. Consequently, the numerical simulation test suggested that the pressure-diffusion arrival to the target fault can be detected by drastic pressure drop as well as a series of AE events occurrence along the fault, if the fault has a certain range of permeability in its hydraulic property and an orientation likely to slip under the in-situ stress state. These findings make it possible to identify the pre-existing exceptionally permeable fault/fracture structures.