Imaging the reopening of a sub-horizontal Hydraulic Fracture (HF) propped with Electrically Active Proppants (EAPs) during pressure injections has been demonstrated by conducting controlled source electromagnetic surveys at UT-Austin's Devine Fracture Pilot Site (DFPS). To detect HF reopening, we collected Bottomhole Pressure (BHP) and salinity, surface tiltmeter, and Distributed Acoustic Sensing (DAS) data. We analyze the DAS slow-strain data to assess fluid arrival time, pore pressure diffusivity, and HF reopening toward each DAS-fiber well. Four observation wells at the DFPS, 75 ft away from the injection well, are equipped with fiber optic installations behind the casing (north, east, and west wells) and a dip-in fiber (south well). In January 2022, 10 injection cycles were carried out at the DFPS, simultaneous with fiber-optic strain data acquisition. Each cycle lasted for 4.5 to 11.5 hours, at an average flow rate of 3.7 gallons per minute. Across these cycles, a total of 13,930 US gallons of fluid were injected into the HF. The low-frequency DAS data in the east, west, and north wells were analyzed alongside BHP profiles from multiple monitoring wells within the HF zone, as well as the cumulative injected volumes.
Consistent with the repetitive nature of the injection cycles, the slow-strain DAS recorded repetitive patterns of strain changes associated with the adequately spaced early injection cycles. These patterns were more complex for the later, larger, less temporally spaced injection cycles due to the superposition of strain changes associated with consecutive cycles. The tensile strains at the observation wells, 75 ft away from the injection well, could be explained by the reopening and closure of a preexisting EAP-filled fracture close to the injection well, pressure diffusion within or ahead of the EAP-filled fracture, and pressure diffusion into a cavity in the cement annulus. The causative mechanism for the compressive strains can be compression of the formation adjacent to the fracture due to pore-pressure diffusion in the EAP pack.
DAS strain data can provide helpful information about the dilated HF area and dip and pore-pressure diffusion. Incorporating this information into EM numerical and tiltmeter models can improve our understanding of rock mechanics and fluid flow behavior during injection cycles. This comprehensive approach will help develop a multi-physics approach for monitoring flow and fracture dynamics, which should be critically useful for various subsurface monitoring applications, including trap integrity in carbon capture and storage.