Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 2.
THE WEYBURN CO 2 INJECTION PROJECT
Figure 2.14: Event magnitudes at Weyburn plotted as a function of distance from the array.
Based on Verdon et al. (2010c). The line marks the limit of detectability as a function of distance
from the array.
using the same method and velocity model as described for Phase IB above. The event locations as
computed by ESG are plotted in Figure 2.15.
The October 2005 events are located close to the observation well at a range of depths, from 900 to
1500m, but usually above reservoir depth. The mechanism causing these events is as yet unidentified.
The majority of the January events cluster to the southeast of the observation well at reservoir depths.
The field operator has attributed these events to completion activities in a nearby borehole. As such,
only the 18 October 2005 events are microseisms, and of these only a minority occur in or close to
the reservoir. Over 3 years of recording this represents a remarkably low rate of seismicity, continuing
the trend already noted in Phase IB. Gaining an understanding of why activity is so low may well be
more informative than the locations of the few events that are available.
2.5 Discussion
The temporal clustering of microseismic events is episodic (Figure 2.6), which raises the question
of what causes these discrete episodes of localised deformation. If the events with a low dominant
frequency are interpreted as fluid movement, why are they seen only occasionally when fluid movement
is occurring continuously Focal mechanism analysis can provide information here. For example
fluid movement would perhaps generate non-double-couple mechanisms. Double-couple mechanisms
describe rock failure in a pure shear mode, where there is no volume change during failure. Where
volume change occurs, perhaps induced by fluid filling a new fracture and propping it open, then the
focal mechanism would indicate not only shear failure but volume increase as well. Focal mechanism
22