Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 4. A COMPARISON OF MICROSEISMIC MONITORING OF FRACTURE STIMULATION DUE TO WATER
VERSUS CO 2 INJECTION
P-wave arrives first
on 4th geophone
Moveout both
up and down
the array
Injection depth is
between geophones
6 and 7
0.0 0.25 0.5
Time
Figure 4.5: Moveout for a typical event during water injection. P and S-wave picks are marked.
The energy arrives first near the centre of the array, and moveout is similar both above and below.
This indicates that the energy is travelling sub-horizontally from an event at a similar elevation
to the injection depth.
4.2.2 CO 2 injection
CO 2 injection was initiated a month after the water injection stage. The injection well was perforated
between 2617-2637m, and supercritical CO 2 was used as the injected fluid. Again, microseismic
activity was recorded for the duration of injection. The injection rates and pressures are plotted in
Figure 4.6 along with the microseismicity rate.
Figure 4.8 shows the event locations during CO 2 injection. In map view the locations show a
similar pattern to the water injection events, extending to the NW and SE of the injection well at
∼120 ◦ , imaging the formation of vertical fractures with this strike. However, the events migrate
upwards to depths of 2530m, 100m above the injection depth during CO 2 injection. This increased
range of event depths during CO 2 injection can be seen more clearly in Figure 4.7, which shows a
histogram of event depths relative to the injection point.
I confirm that these events are accurately located by considering the moveout for some of the
events recorded near the end of CO 2 injection (see Figure 4.9). It can be seen that energy arrives first
60