Earthquakes in Alberta and their possible relationship to subsurface operations such as hydraulic fracturing and waste disposal have made a lot of news over the past few years. The figure below shows the location and time distribution of 2,225 AB earthquakes from 2006 to early 2019 (AGS, 2019). There are changes from 2014 when the number of earthquakes, along with magnitudes and depths increased significantly.
Hydraulic fracturing has been practiced for decades in AB, but the intensity increased significantly with the advent of modern horizontal drilling and fraccing technologies, and especially since 2013. The figure below shows a side-by-side display of fracced wells and earthquake locations and their frequencies since 2006.
Analyses based on the temporal and spatial proximity of the seismic events and frac jobs shows that fraccing may be responsible for some of the change in the trend of seismicity. These analyses show that most of the identified induced earthquakes are concentrated in the Fox Creek and Red Deer as demonstrated in the animated figure below. These earthquakes seem to be associated with specific sets of faults that may have been triggered more than once. While the Duvernay Formation is the main target of wells potentially responsible for induced seismicity, the Montney, Cardium, Wilrich and a number of other zones seem to contribute as well. The depths of these events have become more scattered (mostly deeper) in the later years probably because farther segments of the faults have gradually awakened through a geomechanical mechanism called progressive failure.
Hazard Mapping and Mitigation
Investigations by Canadian Discovery show the significance of geological and geomechanical characteristics of the rock in creating a highly sensitive environment for triggering earthquakes. In the figure below, the values of two geomechanical properties (instantaneous shut in pressure (ISIP) gradient and Poisson’s ratio) are graphed with respect to whether or not seismic activity happened when the wells were fracced. These graphs show statistically distinct differences between these two cases. Such observations can be used to map the risk of induced seismicity in an area.
The investigations have also shown that differences in completion design such as well orientation, frac fluid and its pressure, stage interval spacing, fraccing technology, flowback, etc. can influence the potential for induced seismicity. In the figure below, the values of two completion design parameters— pumped fluid normalized by the length of the fracced section of the well and the number of frac stages—are graphed with respect to whether or not seismic activity happened when the wells were fracced. Such observations demonstrate the potential for mitigating the hazard of induced seismicity by optimizing well placement and fraccing.
The full article was published in Canadian Discovery’s Discovery Digest.