Fluid flow through fractured media is typically governed by the distribution of fracture apertures, which are in turn governed by stress. Consequently, understanding subsurface stress is critical for understanding and predicting subsurface fluid flow. Although laboratory-scale studies have established a sensitive relationship between effective stress and bulk electrical conductivity in crystalline rock, that relationship has not been extensively leveraged to monitor stress evolution at the field scale using electrical or electromagnetic geophysical monitoring approaches. In this paper we demonstrate the use time-lapse 3-dimensional (4D) electrical resistivity tomography to image perturbations in the stress field generated by pressurized borehole packers deployed during shear-stimulation attempts in a 1.25 km deep metamorphic crystalline rock formation.

Enhanced Geothermal Systems could provide a substantial contribution to the global energy demand if their implementation could overcome inherent challenges. Examples are insufficient created permeability, early thermal breakthrough, and unacceptable induced seismicity. Here we report on the seismic response of a meso-scale hydraulic fracturing experiment performed at 1.5 km depth at the Sanford Underground Research Facility. We have measured the seismic activity by utilizing a novel 100 kHz, continuous seismic monitoring system deployed in six 60 m-length monitoring boreholes surrounding the experimental domain in 3-D. The achieved location uncertainty was on the order of 1 m, and limited by the signal-to-noise ratio of detected events. These uncertainties were corroborated by detections of fracture intersections at the monitoring boreholes. Three intervals of the dedicated injection borehole were hydraulically stimulated by water injection at pressures up to 33 MPa and flow rates up to 5 L/min. We located 1933 seismic events during several injection periods. The recorded seismicity delineates a complex fracture network comprised of multi-strand hydraulic fractures and shear-reactivated, pre-existing planes of weakness that grew unilaterally from the point of initiation. We find that heterogeneity of stress dictates the outcome of hydraulic stimulations, even when relying on theoretically well-behaved hydraulic fractures. Once hydraulic fractures intersected boreholes, the boreholes acted as a pressure relief and fracture propagation ceased. In order to create an efficient sub-surface heat exchanger, production boreholes should not be drilled before the end of hydraulic stimulations.

Seismic sensors and seismic imaging have been widely used to monitor the geophysical properties of the subsurface. As subsurface engineering techniques advance, more precise monitoring systems are required. Seismic event catalogs and seismic velocity structures are two of the major outputs of seismic monitoring systems. Although seismic event catalogs and velocity structure are often studied separately, published reports suggest constraining them simultaneously can lead to better results. We conducted a double-difference seismic tomography analysis to constrain both the seismic event locations and the 3D seismic velocity structure. Passive seismic data collected from a geothermal research project in Lead, South Dakota were used to image a 3D volume on the scale of tens of meters. Specifically, around 18,500 P-wave and 8,900 S-wave arrival times from 1,874 seismic events were used. Checkerboard tests showed that the observed data can image the seismically active region well. We compared tomography results with fixed seismic event locations against those with updated event locations. Tomography results with updated event locations showed better fits to the observations and improved the seismic event catalog, showing sharper patterns compared to the original one. These patterns helped us monitor the seismically active fractures since the seismic events were mostly due to hydraulic stimulations. Two parallel fractures revealed by the updated seismic event catalog spatially correlated with independent borehole temperature observations. The average seismic velocity values of the well-constrained volume agreed to the first order with core sample measurements and active-source seismic surveys.