Oscillation of fluid‐filled cracks triggered by degassing of CO2 due to leakage along wellbores

[1] We present evidence for a seismic source associated with degassing CO2 during leakage along two wellbores instrumented with arrays of downhole seismometers. More than 200 microseismic events were detected in direct vicinity of the monitoring wells. The observed seismic waves are dominantly P waves and tube waves, with no (or extremely weak S) shear waves. The waveforms of these events indicate extremely rapid amplitude decays with distance across the arrays, consistent with the seismometers being in the near field of the seismic source. The frequency characteristics, first‐motion polarities and S to P amplitude ratios suggest a single force source mechanism. Because the seismic arrays were located at the depth where the density of ascending CO2 changes most rapidly, it appears that the transition of CO2 from supercritical fluid to gas triggers an oscillation of fluid‐filled cavities and fractures very close to the wellbores in which the monitoring arrays were deployed. In many aspects, the observed waveforms show a striking similarity to those modeled for degassing processes below volcanoes. We suggest that the single force represents bubble growth and resulting oscillations in cement cavities between the steel casing of the well and the rock adjacent to the wellbores and/or within fractures in the rock just outside the wellbores.