To frac or not to frac: assessing potential damage as related to hydraulic fracture induced seismicity

Concerns about seismic hazards during hydraulic fracturing, and other stimulation and injection practices in the petroleum industry has reached a crossroads in the last few years. Incidents in Blackpool, England, Fox Creek, Alberta, the general increase in seismicity in areas such as Oklahoma and Kansas, to name a few, have shined a light on the potential of hydrocarbon field operations to generate seismicity at levels that are of public concern. In general, to address these concerns, a number of jurisdictions have instituted various ‘traffic-light’ systems to govern the response of the industry to the potential occurrence of significant magnitude events. Generally, these protocols dictate that below a certain magnitude threshold, no response is necessary; once an event is in the ‘amber’ magnitude range, reporting to the regulator may entail more frequency and/or injections that need to be moderated. Finally, once an event is in the ‘red’ magnitude range, the injection in some jurisdictions could be halted. The exact application of these protocols, due to the nature of this approach, varies from jurisdiction to jurisdiction. Further, there has been poor time correlation between injection periods and the occurrence of induced seismic events with many occurring outside time periods we would commonly associate with fluid injection. Magnitude is the most ubiquitously reported parameter about an earthquake. The Richter Scale is probably seismology’s biggest contribution to the public consciousness motivated by a desire to quickly describe how an earthquake ranks with size with respect to others. However, reducing the nuances of an earthquake to a single magnitude number ignores many of the factors that control how the earthquake is perceived. For the example of seismic hazard, the effect of a potential earthquake is not quantified in terms of magnitudes but in probabilities in exceeding certain ground motion thresholds. The utility of this quantification is that it immediately can be related to building codes and the designs for different structures, which are all built to withstand shaking to various thresholds. Determining the magnitude is only part of the equation. Numerous other parameters impact the shaking felt on the surface, including (but not limited to) the depth of the event, the radiation patterns of the seismicity, and the stress release of the events. Considering an approach based on measured ground motion potentially removes the vagueness and ambiguity associated with the traffic light system often suggested by regulatory agents.