Creation of a mixed-mode fracture network at meso-scale through
hydraulic fracturing and shear stimulation
Abstract
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.