Newswise — A recent study published in Seismological Research Letters highlights the pioneering use of fiber optic distributed acoustic sensing (DAS) by researchers at a field site in Victoria, Australia. The DAS technology was employed for precise monitoring of induced seismicity resulting from a small-scale carbon dioxide (CO2) injection.
The CO2CRC Otway Project in Victoria serves as a research test site aimed at exploring subsurface carbon dioxide storage as a potential method to mitigate the impact of carbon emissions on climate change. However, a concern arises due to the possibility of induced earthquakes when gigatons of CO2 are injected into the same geological basin through multiple storage projects over several decades. Thus, scientists seek a deeper understanding of the triggers and evolution of such seismic activity.
One intriguing finding from the DAS deployment at Otway is that the minor earthquakes that accompanied two injection phases at the site appear to align with the movement of the CO2 plume's saturation front within the rock, rather than being associated with the pressure front resulting from injection.
Study lead author, Stanislav Glubokovskikh of Lawrence Berkeley National Laboratory, emphasizes that to their knowledge, the Otway Project stands as the sole CO2 storage initiative where induced seismicity was, at the very least, coincident with the movement of the saturation front, rather than the pressure front.
"We relied on frequent snapshots of the storage formation to establish a connection between the CO2 plume evolution and induced seismicity," stated the researcher. "When considering practical monitoring systems, the multi-well DAS vertical seismic profiling stands out as the most suitable option, offering exceptional temporal and spatial resolution for tracking a small CO2 plume."
The seismic monitoring system was developed by a team of geophysicists from Curtin University in Perth, Australia, led by Roman Pevzner and Boris Gurevich. Their approach involved using five deep boreholes equipped with sensitive fiber optic cables to monitor the injection of 15,000 metric tons of CO2, referred to as Stage 3, at the Otway site over a 610-day period. Throughout this duration, they successfully detected 17 minor seismic events, with the highest magnitude recorded at 0.1.
In a previous endeavor labeled "Stage 2C," a similar-sized CO2 injection was also monitored at the site using geophones buried below the surface, which identified several microseismic events.
One of the primary objectives of the Stage 3 injection was to explore cost-effective, long-term monitoring solutions for geological carbon storage, as emphasized by Glubokovskikh. "To achieve continuous, long-term monitoring, the permanent downhole installation of seismic sensors was essential. This approach mitigated the need for repetitive deployment and demobilization of the array during each active seismic survey, which would have been economically impractical and disruptive to the landowners. DAS technology proved to be the optimal choice for such circumstances."
The DAS observations yielded a significant discovery, exposing a seismogenic fault below the surface that had eluded earlier seismic imaging efforts.
According to Glubokovskikh, the exact mechanisms behind the small earthquakes at the site remain uncertain. However, the intriguing observation that the seismic activity coincides with CO2 saturation may offer valuable clues.
One plausible explanation put forward by Glubokovskikh is the "geochemical weakening" of the reservoir faults caused by CO2. This hypothesis gains support from laboratory experiments where core samples from the injection interval broke down during CO2 core-flooding tests.
Yet, there are still critical unknowns, such as the mineralogical composition of the fault gauge and the specific flow and pore fluid composition at the site. These uncertainties make it challenging to conclusively confirm the phenomenon of geochemical weakening.
Besides the seismic events directly linked to the Stage 2C CO2 plume, another group of events occurred outside of any CO2 accumulation areas. Interestingly, these events only took place during the injection operations, but they showed no clear relationship with either the injection pressure or the movement of the saturation plume.
Looking ahead, the Otway project is advancing toward a Stage 4 injection, set to occur close to the locations of the previous two CO2 plumes. This development is expected to generate another series of induced seismic events, offering further insights into the triggering mechanism. Glubokovskikh points out that even if the new injection does not produce detectable events, this observation could still be perceived as additional evidence supporting the flow-related nature of the Otway seismicity.
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