Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 2.
THE WEYBURN CO 2 INJECTION PROJECT
3
Oil Production rate (m )
Date
Figure 2.11: Comparison of production rates from well 192/09-06 with the rate of microseismicity
in the nearby SE cluster. Microseismic activity occurs when production is temporarily stopped.
Based on Weyburn Microseismic Progress Report, ESG, Canada, April 2004.
The majority of events are located above the reservoir, although some events are located within the
reservoir interval, and some are located below it.
A histogram of event depths during Phase IB is plotted in Figure 2.13. Many events appear to be
located above the reservoir. Although the large depth errors mean that some of these could actually
be located within the reservoir interval, it is clear that at least some activity must be occurring in the
overburden. This is an interesting observation, and without geomechanical modelling it is not clear
whether this could represent fluid migration or merely stress transfer into the overburden.
The event magnitudes are plotted in Figure 2.14 as a function of distance. Event magnitudes range
between -3 to -1. Event magnitudes of -2 are still detectable even at a distance of over 400m. Small
events are still detectable at large distances, which suggests that the small number of events recorded
is not an artifact of high noise levels. Figure 2.14 suggests that surface arrays would have limited use
for microseismic monitoring under these conditions. The largest events recorded have magnitudes less
than -1.0, and many are smaller than -2.0. Dense surface arrays would be required to detect such
events, and their detectability would be strongly influenced by surface noise and the nature of event
focal mechanisms.
2.4.2 Phase II
Recording for Phase II runs from October 2005 to the present. Although there are data recorded on
the geophones, the majority of these represent near surface noise or electrical spikes. Only 39 have
been reliably identified as microseismic events, occurring in two temporal clusters, with 18 events at
the end of October 2005 and 21 in mid-January 2006. The Phase II events were located by ESG
20