Detection of transient time lapse seismic signatures associated with CO₂ injection

Kristopher A. Innanen, Donald C. Lawton, Kevin W. Hall, Malcolm B. Bertram, Kevin L. Bertram

In 2018 CREWES researchers reported on a collaboration with JOGMEC in interpreting an unusual time-lapse data set, in which cross-well waveforms transiting a plume of injected microbubble water were seen to undergo several difficult-to-explain alterations. In addition to attempting to explain these phenomena, we also undertook to seek similar variations in seismic waveforms propagated through the plume of CO₂ being injected into the 300m formation at the CaMI-FRS in Newell County AB. In 2019 we expanded on that experiment, carrying out a 160hr set of repeated seismic shots taken during pressuring up, injection, and then pressuring down. We observe remarkable repeatability in the wave-forms generated with the Vibroseis source, which was fixed with pad down throughout, and sensed with permanent 3C geophones cemented behind casing in the CaMI geophysics well. On this backdrop, with variability due to any source other than the pressure and fluid changes caused by the injection minimized, we examined the data. We identified several spectral variations which appear and then vanish over the course of the experiment. Because at each "cluster" of shots we repeated the sweep anywhere from 5 to 29 times, we can attach error bars to the spectral measurements; the changes we identified generally exceed these error bars significantly, often by roughly an order of magnitude. The changes are consistent those leading to our 2018 elastic bracing explanation, in that lower frequencies tend to decay (and then relax) and higher frequencies tend to boost (and then relax). They are however more subtle, though this may be because on the scale determined by the dominant wavelengths and injection depths of this experiment, the CO₂ plume is much smaller than was the microbubble injection zone.