A study published in the Bulletin of the Seismological Society of America suggests that pore pressure diffusion caused by carbon dioxide injection at a carbon storage site in the Illinois Basin led to hundreds of microearthquakes between 2011 and 2012. The analysis indicates that pressure diffusion along existing faults into the basement rock could have destabilized the faults where the microseismicity occurred, ranging from Mw -2 to 1. This highlights the importance of characterizing subsurface faults in locations selected for CO2 sequestration.
The research noted similarities between CO2 injection and wastewater injection from oil and gas operations, although globally, the volumes of injected wastewater exceed injected CO2. Wastewater injection has induced small to moderate-sized earthquakes worldwide, underscoring the need to study how CO2 injection produces seismic activity and if it could cause larger earthquakes. The injection layer at the Illinois Basin-Decatur Project (IBDP) was separated from the basement rock by a less porous and permeable sandstone layer, raising questions about how CO2 reached the basement to trigger seismicity.
Using a modeling approach, the research team led by Ruben Juanes of MIT found that changes in rock pore pressure from the injection traveled along faults connecting the injection layer and the basement. The study showed that the poroelastic effect of fluid injection stabilized the faults hosting the microearthquakes, which were already close to failing before CO2 injection. However, characterizing these small faults and predicting their failure poses a significant challenge to carbon sequestration projects due to limited resolution provided by seismic waves at the depths of interest.
The researchers suggested that starting with small-scale injection could help illuminate smaller faults at a carbon storage site. The study looked at the first year of CO2 injection at the IBDP, which resulted in hundreds of microearthquakes. In contrast, a second period of injection at a shallower depth and further from the faulted basement resulted in minimal seismic activity. Juanes emphasized that injection rates in CO2 projects have been significantly lower than wastewater injection rates, potentially explaining why moderate-sized induced seismicity has not been observed at carbon sequestration sites.
One possible reason for this difference could be better subsurface characterization done for CO2 sequestration projects prior to injection compared to the early days of geologic wastewater disposal, where injections were often made into or close to faulted basement rock. The researchers stressed the importance of understanding how pressure diffusion generated by fluid injection can impact faults and seismic activity in carbon storage projects, highlighting the need for further research in this area to ensure the safety and success of these initiatives.
