Application of focal-time analysis for improved induced seismicity depth control: A case study from the Montney Formation, British Columbia, Canada

Abstract

Characterization of induced seismicity and associated microseismicity is an important challenge for enhanced oil recovery and development of tight hydrocarbon reservoirs. In particular, accurately correlating hypocenters of induced events to stratigraphic layers plays an important role in understanding the mechanisms of fault activation. Existing methods for estimating focal depth, however, are prone to a high degree of uncertainty. A comprehensive analysis of inferred focal depths is applied to induced events that occurred during completions of horizontal wells targeting the Montney Formation in British Columbia, Canada. Our workflow includes a probabilistic, nonlinear global-search algorithm (NonLinLoc), a hierarchical clustering algorithm for relative relocation (GrowClust) and depth refinement using the recently developed focal-time method. The focal-time method leverages stratigraphic correlations between P-P and P-S reflections to eliminate the need for an explicit velocity model developed specifically for hypocenter depth estimation. We show that this approach is robust in the presence of noisy picks and location errors from epicenters obtained using a global-search algorithm, but it is limited to areas where multicomponent 3-D seismic data are available. A novel method to determine statics corrections is developed here, to ensure that both passive seismic observations and 3-D seismic data share a common datum in areas of moderate to high topography. Our results highlight the importance of transverse faults, which appear to provide permeable pathways for activation of other faults at distances of up to 2 km from hydraulic fracturing operations.

Anatomy of a buried thrust belt activated during hydraulic fracturing

“…1). The seismometers were buried to the depth of 1 m and comprised of a mix of five broadband seismometers and twenty-two 4.5 Hz threecomponent geophones (Riazi et al, 2020).…”

“…Reliable and accurate seismic event locations can provide important clues regarding fault interaction and triggering mechanisms of induced earthquakes. However, event locations are subject to various sources of uncertainty (Riazi et al, 2020, Jones et al, 2014. For example, most earthquake location methods rely on an explicit model for wave velocities (Trugman and Shearer, 2017, Lomax, 2005, Zhang and Thurber, 2003, Waldhauser and Ellsworth, 2000, which may be insufficiently constrained by independent data.…”

“…For example, most earthquake location methods rely on an explicit model for wave velocities (Trugman and Shearer, 2017, Lomax, 2005, Zhang and Thurber, 2003, Waldhauser and Ellsworth, 2000, which may be insufficiently constrained by independent data. In the case of sensor networks deployed at the surface, focal depth is the least well resolved location parameter (Riazi et al, 2020); yet, accurate depth constraints are essential to quantify risk and to elucidate fault activation processes.…”

Machine Learning‐Based Analysis of Geological Susceptibility to Induced Seismicity in the Montney Formation, Canada

“… Distance from the Debolt Formation . The Debolt Formation is a massive carbonate unit that is known to host larger induced events (Riazi et al, 2020). In addition, it contains fault networks that extend upward to at least as shallow as the Middle Montney (Richards et al, 1990).…”

“…In addition, there is evidence that SAP is depth dependant, possibly because induced seismic events are typically associated with faults within the underlying Debolt Formation (Rogers, 2018). A recent analysis of focal depths for induced events, based on a dense local seismograph array, indicates that event hypocenters are largely confined to this formation (Riazi et al, 2020).…”