Abby L. Harvey
GHG Monitor
10/3/2014
An international team of scientists has found that environmental damage that would be caused by a small-scale sub-seabed carbon dioxide leak would be minimal, according to the results of a study published late last week. The study was conducted off the shore of Scotland using a controlled CO2 leak to mimic what could potentially happen if CO2 stored offshore leaked into the seabed. “Our work demonstrates that biological effects from a small short term leak are detectable, but not catastrophic and that recovery is measurable in days to weeks,” the study says. However the authors do “caution that impacts are likely to increase step-wise if a greater proportion of CO2 is emitted in the gaseous phase, either through fractures or as pore waters become super-saturated, or if the carbonate buffering capacity of the sediments is limited or becomes exhausted. Without operational evidence, realistic leakage scenarios can only be approximated.” The study was led by Plymouth Marine Laboratory (PML) and funded by the Research Councils UK, the Natural Environment Research Council and the Scottish and Japanese governments.
The study was designed to address questions regarding what might happen if carbon from a carbon capture and storage-type project stored in a reservoir offshore began to leak into the water. “Although debated, a number of mechanisms potentially facilitating leakage have been proposed, including abandoned exploratory boreholes, geological discontinuities (for example, fractures) and operational malfunction (blowout scenario) Here we do not address storage integrity, but focus on the likely environmental consequence of leakage, and how best to detect leakage if it were to reach the marine environment,” the study says. To address this question, the team drilled a borehole 11 meters deep below the sea floor and under 12 feet of water 350 meters off shore. “A total of 4.2 tonnes of CO2 was injected into the overlying unconsolidated sediments, over a 37 day period, during which flow was increased from 10 to 210 kg. The temporal and spatial migration and impact of this CO2 release were assessed using a variety of acoustic, chemical and biological techniques, before, during and after release at both control and exposed sites,” the report says.
According to the report, impacts on the biological systems near the leak were minimal. “Impact is indicated not by change per se, but by deviations from well-established normal cycles. No biological effect was detected during the initial stages of the release, consistent with the lack of a chemical signal in the superficial sediments. However, towards the end of the release and in the initial days of the recovery period, the change in benthic macrofauna community structure at the leak epicenter was significantly different from that observed at the other, un-impacted sites,” the report says. However, following the leak, recovery began quickly and was complete within weeks, the report says.
The study also suggests monitoring practices for any offshore CO2 storage. “We suggest that the optimal monitoring strategy for storage locations should use mobile autonomous underwater vehicles equipped with chemical (for dissolved phase) and acoustic (for gas bubbles) sensors with a horizontal spatial sampling resolution approaching 10 m, deployed close to the sea floor. Such a multisensory approach, supported by analysis against a well-constrained baseline will maximize the chance of detecting the preliminary stages of a small leakage,” the report says.
