Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

2.5. DISCUSSION
500
1000
1100
Northing (m)
0
Depth (m)
1200
1300
1400
1500
−500
−500 0 500
Easting (m)
(a)
1600
−500 0 500
Easting (m)
(b)
Figure 2.15: Map view (a) and EW cross section (b) of microseismic events recorded during
Phase II. Events are broken up into two clusters occurring in October 2005 (red) and January
2006 (blue). The majority of the January 2006 events are located within one cluster to the SE.
Wells are marked as per Figure 2.7.
analysis could also image the triaxial stress tensor in the reservoir. This would provide important
information for guiding injection strategies and groundtruthing geomechanical models. However, focal
mechanism analysis cannot be done with a single well array as at Weyburn.
Another important point is whether or not microseismicity above the reservoir indicates topseal
failure and the migration of CO 2 into the overburden. Stress arching effects, where loading
of the reservoir transfers stress into the overburden, can also lead to failure in the overburden and
sideburden, without any fluid leaving the reservoir. To determine whether or not deformation results
in increased fault permeability it is necessary to consider the rheology of the rock with respect to
the stresses at the time of faulting. This underscores the importance of having a good understanding
of the potential geomechanical behaviour of the storage site - it is likely that fluid migration or a
pore-pressure connection into the overburden will be documented by a different spatial and temporal
pattern in seismicity from those associated with stress arching effects.
A key question is should CCS operations always/sometimes/never employ microseismic monitoring,
and how should this decision be made Downhole monitoring is now a commonly used tool for
monitoring hydraulic fracture stimulation. It presents a low cost option for long term CCS monitoring.
Ideally, such monitoring would record little seismicity, suggesting that the CO 2 plume moves aseismically
through the reservoir, inducing no significant rock failure, as seems to be the case at Weyburn.
In total, over the entire Phase IB and Phase II microseismic monitoring experiment, only 86 reliably
identifiable microseismic events were recorded in 5 years. However, the lack of data has meant that the
microseismic monitoring at Weyburn has provided little information about the reservoir stress state
and injection induced pressure fronts. Microseismic monitoring can be viewed as an early warning
system, where large swarms of events in unexpected locations could be used to indicate that there
is a risk of leakage. Paradoxically then, we should be placing geophones in the ground in the hope
23