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
Depth (m)
2800
2820
2840
2860
2880
2900
2920
0 2 4 6 8 10
(a)
Depth (m)
2540
2560
2580
2600
2620
2640
2660
0 2 4 6 8 10
(b)
Figure 4.7: Histograms of event depths during (a) water injection and (b) CO 2 injection. The
injection depths are marked by the dotted red line. There is a greater spread in event depths
during CO 2 injection.
injection pressures between 16 and 22 minutes during water injection correlates with a drop in event
magnitudes. During CO 2 injection, the initial stepped increase in injection pressures matches the
stepped increase in event magnitudes.
This correlation between injection pressures and event magnitudes can be tested more directly
by computing the correlation coefficients between magnitude and the injection pressure at the time
the event occurs. The statistical significance of the correlations is computed using a t-test, with the
null hypothesis that there is no correlation. I consider the water and CO 2 cases separately. If the
magnitudes are independent of fluid properties, then combining the datasets should not decrease the
correlation. Therefore I also compute correlation for all the data points together. The correlation
coefficients and statistical significance of correlation are given in Table 4.1.
In Figure 4.11 I plot the event magnitudes as a function of the injection pressure. The best linear fit
lines are shown for water and CO 2 separately and for the overall dataset. The data are quite scattered,
and the correlation coefficients are not particularly high. Nevertheless, during water injection, and
for the combined data, the correlation is significant at the 99% level. The correlation coefficient for
CO 2 injection is poorer, and lacks good statistical significance. From Figure 4.11 it appears that the
events during CO 2 injection have slightly higher magnitudes than during water injection at the same
pressures. Although no exact relationship should be inferred from this data, we suggest that larger
event magnitudes should be expected during periods of higher injection pressures, regardless of the
compressibility of the injection fluid.
Above I have noted some differences between water and CO 2 injection. I also note that there is
a greater temporal spread of events during water injection, while the majority of events during CO 2
injection occur during the first 25 minutes. However, the most striking observation is the similarity of
the response from the two fluids. The injection rates and pressures used during both stages are similar,
and the result is that event magnitudes and rates of seismicity are also similar. There is certainly no
evidence to suggest that CO 2 - the softer hammer - has induced less seismicity than water.
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