Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

2.4. EVENT TIMING AND LOCATIONS
Northing (m)
500
250
0
−250
−500
−500 −250 0 250 500
Easting (m)
(a)
Depth (m)
1000
1100
1200
1300
1400
1500
1600
−500 −250 0 250 500
Easting (m)
(b)
Figure 2.10: Map (a) and EW cross-section (b) views of reliably located Phase IB microseismic
events. Wells are marked as per Figure 2.7. The events are colour-coded by time of occurrence:
yellow = pre-injection (Aug - Dec 2003), magenta = initial injection period (Jan - Apr 2004), red
= during elevated injection rate period (Jul - Nov 2004). The events are found near the producing
wells to the NW and SE.
Having removed the events that are associated with drilling and completion activities, and those
that cannot be reliably identifiable as microseismic events, I am left with 68 microseismic events for
the period August 2003 to November 2004. This is a very low rate of seismicity in comparison to
the 100s or even 1000s of events recorded per month at other producing carbonate reservoirs such
as Ekofisk, Valhall (North Sea) and Yibal (Oman) (Dyer et al., 1999; Jones et al., 2010; Al-Harrasi
et al., 2010). Figure 2.10 shows the locations for the remaining events that are reliably identified. The
events can be divided into 2 clusters, one to the northwest of the injection well towards production
well 191/11-08, and one to the southeast of the injection well, around production well 192/09-06.
The first cluster of events is located to the southeast of the injection well, around the horizontal
production well 192/09-06. These events are all located in and just above the reservoir. These events
occur throughout the monitoring period, including the period before injection. Comparison with
production data for well 192/09-06 (Figure 2.11) indicates that the timing of the events correlates
with periods where production is temporarily stopped. It is likely that these events are being generated
by pressure increases around the well that result from the temporary cessation of pumping. Therefore
these events are probably not directly related to CO 2 injection.
The second cluster of events is located between the injection well and the horizontal production
well 191/11-08 to the NW. The first of these events occur on January 21st, coincident with the
initiation of CO 2 injection. Microseismicity occurs at the onset of injection, and also appears to
be correlated with periods of increased injection (Figure 2.12), although unfortunately the recording
system was locked out during the period with maximum injection rate. The event locations mark
a cloud of microseismicity which centres on the production well. Some events are located between
the production and injection wells, while some events are located to the NW of the production well.
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