Forecasting microseismic cloud shape as a proxy of stimulated rock volume is essential for the design of an energy extraction system. The microseismic cloud created during hydraulic stimulation is known empirically to extend in the maximum principal stress direction. However, this empirical relationship is often inconsistent with reported results, and the cloud growth process remains poorly understood. This study investigates microseismic cloud growth using data obtained from a hydraulic stimulation project in Basel, Switzerland, and explores its correlation with measured in-situ stress. We applied principal component analysis to a time series of microseismic distribution for macroscopic characterization of microseismic cloud growth in two- and three-dimensional space. The microseismic cloud in addition to extending in the direction of maximum principal stress expanded to the direction of intermediate principal stress too. The orientation of the least microseismic cloud growth was stable and almost identical to the minimum principal stress direction. Following the stimulation, the orientation of microseismic cloud growth was consistent with the in-situ stress direction. Further, microseismic cloud shape ratios showed good agreement when compared with in-situ stress magnitude. The permeability tensor estimated from microseismicity also presented good correlation in terms of direction and magnitude with microseismic cloud growth. We show that in-situ stress plays a dominant role by controlling the permeability of each existing fracture in the reservoir fracture system. Consequently, microseismic cloud growth can be scaled by in-situ stress if there is sufficient variation in the existing faults.

Monitoring the hydraulic properties within subsurface fractures is vitally important in the contexts of geoengineering developments and earthquakes. Geophysical observations are promising tools for remote determination of subsurface hydraulic properties; however, quantitative interpretations are hampered by the paucity of relevant geophysical data for fractured rock masses. This study explored simultaneous changes in hydraulic and geophysical properties of natural rock fractures with increasing normal stress and correlated these property changes through coupling experiments and digital fracture simulations. We show that electrical resistivity is linked with permeability and flow area regardless of fracture roughness, whereas elastic wave velocity is roughness dependent. We also are able to categorize fracture flow patterns as aperture-dependent, aperture-independent, or disconnected flows, with transitions at specific stress levels. Elastic wave velocity offers potential for detecting the transition between aperture-dependent flow and aperture-independent flow, and resistivity is sensitive to detect the connection/disconnection of the fracture flow.

Geothermal systems consisting of fractures in impermeable rocks are difficult to characterize by in situ methods. In an effort to link characteristics of small-scale and large-scale fractures, this study investigated possible relations between their geophysical parameters. We upscaled the relationship between fracture permeability and formation factor in a laboratory specimen to larger fracture dimensions. Microscopic flow characteristics indicate that this relationship is related to the tortuosity of flow paths. We derived an empirical formula that directly predicts changes in fracture permeability from changes in formation factor. This relation may make it possible to monitor subsurface hydraulic activities through resistivity observations.