Integrating microseismicity and low frequency DAS to characterize hydraulic fracturing.

Hydraulic fracturing is a widely used technique for developing unconventional reservoirs. Thus, knowledge of the fracture geometry is essential to evaluate the success of a hydraulic fracturing operation. Traditionally, the spatio-temporal distribution of the microseismicity has served as a proxy for the stimulated reservoir volume (SRV), based on the assumption that seismic events are triggered near the fractures. Recently, distributed acoustic sensors (DAS) have been utilized to measure strain changes resulting from hydraulic fracturing, allowing to measure fracture extends directly to some degree. By integrating DAS with microseismic data, we can validate parameters obtained from each dataset and improve our understanding of seismicity induced by hydraulic fracturing. We characterize hydraulic fracture growth across 26 stages of HFTS-2 in three different directions with microseismic data. For this we apply a formulation of the triggering-front based on non-linear fluid-rock interactions, assuming negligible fluid leakage and fluid conservation within the fractured volumes, which effectively describes the spatio-temporal distributions of microseismic events. Additionally, fracture extents obtained from microseismic data are compared with those measured directly with DAS. This comparison reveals that the assumption that microseismic events are triggered in the immediate vicinity of hydraulic fractures is not always accurate. Observed are both extremes: microseismic clouds being significantly larger than the fractures, or vice versa. An analysis of the spatio-temporal distributions of the microseismicity, combined with numerical modeling of the fiber responses, demonstrates that seismic events associated with the triggering-fronts are related to the destabilization of rocks surrounding fractures through elastic stress transfer. Hence, fracture growth can be tracked with varying precision using only microseismic data, with the extreme case of completely aseismic growth. The combination of microseismic monitoring with DAS yield the most valuable insights.