The objective of this study is to understand how microseismic events are generated during hydraulic fracturing, as well as the role of geomechanical conditions (i.e., stress and mechanical stratigraphy) in this process. In the industry, microseismic event clouds have been generally used as an "outer-boundary" of the "stimulated reservoir volume" (SRV). However, by comparing with other surveillance data (Low frequency Distributed Acoustic Sensing, or LF-DAS strain) in the Hydraulic Fracturing Test Site (HFTS) 2 experiment, we show that this assumption is fundamentally flawed. The HFTS 2 data has three unique observations that have not been commonly observed in other datasets: 1. Due to influence of offset pad depletion, microseismic data shows that hydraulic fractures from the child well can propagate over 3000 feet into the depleted low stress zone. 2. By comparing microseismic and horizontal fiber LF-DAS strain data, we observe that microseismic event cloud does not necessarily reflect the created hydraulic fracture volume. Particularly, the extent of microseismic event clouds near heel stages are much shorter than what is shown with LF-DAS strain data. 3. Microseismic event magnitudes are larger in the depleted regions. Through geomechanical analysis, we demonstrate that the "bedding-plane-slip" model is likely the mechanism for microseismic generation during hydraulic fracturing. This model successfully explains the above field observations from HFTS 2 experiment. We also provide a quantitative relationship connecting the microseismic event magnitude with fracture width increment and layer mechanical property contrast.
Although microseismic monitoring has been applied to hydraulic fracturing since the beginning of the shale revolution, understanding of the mechanism generating microseismic events remains poor. What exactly causes microseismic events and how they are connected to the hydraulic fracturing propagation process is still debated and consensus among the industry has not been reached (Rutledge et al., 2016; Rutledge, 2019; Teran and Thornton, 2019). Three major theories have been proposed to explain the mechanism of microseismicity generated during hydraulic fracturing: tensile opening, natural fracture slip or bedding plane slip.
