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
Pressure (MPa)
Number of Events
50
40
30
20
10
Pressure
Flow rate
10
0
0 20 40 60 80 100 120 0
15
10
5
0
0 20 40 60 80 100 120
Time (minutes)
40
30
20
Flow rate (bpm)
Figure 4.3: The upper panel shows the injection pressures (at surface) and flow rates during water
injection. The lower panel shows the rate of microseismicity.
injection, microseismic activity was recorded, which continued for the duration of the injection and
a short while after. Figure 4.3 shows in detail the flow rates and injection pressures (at the surface)
during injection, and the rate of microseismic activity. In total, 65 events were reliably identified and
located.
Figure 4.4 shows the locations for events recorded during water injection. The location errors
are generally low - indeed the radial errors are not visible to be seen beyond the dots marking event
location. The events form a trend extending NW-SE at 120 ◦ from the injection well. In cross-section,
the events are highly restricted in vertical extent, occurring only at the depth of the perforation. For
all of the events, the moveout across the array, as shown in Figure 4.5, is such that the first arrivals
are near the centre of the array (geophone 4), with moveout (delayed arrival times) both above and
below this first arrival. This indicates that the waves have arrived sub-horizontally from an event
at reservoir level. The containment of microseismicity to a narrow band at the injection depth may
indicate containment of the elevated pressures caused by injection to a narrow band in the reservoir.
However, without further knowledge of the geology of the reservoir, it is difficult to comment on why
this might be the case.
The observation of events extending in a linear fashion from the injection well almost certainly
represents the growth of fractures with a NW-SE orientation. Assuming that the maximum principal
stress is subvertical, I deduce that the minimum principal stress is horizontal trending NE-SW. From
the lateral extent of microseismic activity I observe the fractures extending at least 125m to the NW
and 80m to the SE of the injection well, a total distance of 205m.
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