Induced seismicity associated with hydraulic fracturing has become a significant regulatory issue in Western Canada, after the occurrence of felt events in three specific reservoirs. Seismicity results from elevated pressure of the stimulated fracture network reducing the effective clamping force and triggering slip of tectonically stressed faults. Several published examples indicate activation over progressively larger fault regions, as multiple stages interact with critically stressed faults in various relative orientations to the treatment well. Geomechanical modeling is used to examine progressive fault slip from multi-stage fracturing and the associated seismicity. The modeling explores different operational scenarios to examine progressive fault activation as hydraulic fracture stages sequentially pressurizes more of the fault. Testing these mitigation strategies on faults in different orientations provides potential operational guidelines to support seismic traffic light systems, typically used to mitigate injection induced seismicity.
Simulations show that the amount of injected fluid interacting with the fault plane controls the intensity of observed seismicity for a specific fault. Stages farther away from the fault can have an impact on fault slippage but with a delayed effect. Sequence of propagation of the hydraulic fracture stages compared to fault orientation is important. If the first stage is closest to the fault, more of the injected fluid will interact with the fault, triggering a large slipping patch on the fault plane. Successive stages will have a lesser effect due to stress shadowing. However if the first stage is the most distant from the fault, slippage on the fault plane will be gradual, thus reducing the amount of seismic moment release. The sequence in which wells on a multi-well pad are stimulated could also impact the associated seismicity.
There has been increasing occurrences of recent seismicity associated with fault activation during hydraulic fracturing, resulting from elevating pore pressure on optimally-oriented, pre-existing faults leading to triggered release of stored tectonic energy (Maxwell, 2013). Anomalous seismicity is similar to microseismicity, although larger magnitude fault seismicity corresponds to inelastic slip over a larger area. However, triggered fault slip has in certain conditions lead to felt ground shaking. For example, three Western Canadian reservoirs located close to the tectonically-active trust belt have experienced anomalous activity: specifically at localized regions of the Horn River Basin, Montney and Duvernay Shales (Atkinson et al., 2016). Operators and regulators in Canada have proactively engaged the issue, establishing operational practices including seismic monitoring for a traffic light system to guide seismic hazard mitigations strategies. Clearly the topic continues to be of concern and establishing mitigation best practices is increasingly important.