Chinese scientists have found that burying carbon dioxide in marine sediment in deep sea is "generally safe and permanent," giving support to the way to capture and store the greenhouse gas.

Carbon capture and storage (CCS) is considered a promising way to reduce carbon dioxide and mitigate climate change. Conventional methods include injecting the gas into deep saline aquifers, oil and gas fields, and coal seams.

A team led by Zhang Dongxiao from Peking University explored storing carbon dioxide in deep-sea sediments and found that extreme conditions at the bottom of the ocean help hold the gas in place.

Under great pressure and at low temperature, carbon dioxide and water trapped in the sediment below the sea floor create "hydrate clogs" that can serve as an "impermeable cap" impeding the gas from flowing upward, according to the research.

Carbon dioxide stored in this method is not in direct contact with the water, preventing any environmental impact on the sea.

"Data from our simulation show that storing carbon dioxide in deep-sea sediments is viable," Zhang said.

The equipment used for injecting the carbon dioxide is similar to the semi-submersible offshore platform for drilling combustible ice (aka methane hydrate), and scientists are exploring the possibility of conducting experiments on these platforms, Zhang said.

According to the published paper, “Under a deep-sea setting, the high density and viscosity of CO2 result in a small footprint and, thus, high storage efficiency. This ensures great storage potential due to the wide distribution of deep-sea sediments globally. Compared with terrestrial sequestration, less lateral expansion reduces the possibility of CO2 reaching a potential permeable pathway to the seafloor. The generation of a hydrate cap and the possible negative buoyancy make this option free from reliance on the caprock in terrestrial storage.”

The paper does caution that “faults or fractures may preexist in the sediment or be induced by tectonism or excessive injection overpressure that may create a permeable pathway directly to the seafloor,” though they also state that the formation of hydrate seals could seal the permeable channel and prevents leakage. “Since the whole system is very susceptible to pressure and temperature, changes in the marine environment, such as ocean temperature and sea level, may affect the post-injection fate of CO2 and the efficiency of sequestration.”

Due to these uncertainties, they say, more research is needed. The open source research findings were published in the latest issue of journal Science Advances on 4 July 2018. It can be read for free here.

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