Abstract

Substantial progress has been made in carrying out small-scale field experiments to investigate the scientific basis for disposal of CO2 that results from burning of fossil fuels in the deep ocean. A remotely operated vehicle (ROV) was used to carry liter quantities of CO2 to depths from 250 to 3650 m in the ocean and to release the CO2 in a controlled manner. A video imaging system allowed observation of the behavior of the CO2 released. CO2 released at intermediate depths forms bubbles that rise. Below about 2750 m liquid CO2 is more dense than sea water, and CO2 released at greater depths descends further. Hydrate formation was observed for depths below about 350 m. We show through video imagery, direct measurement, and model calculations that the dissolution rate of liquid CO2 in the deep ocean is 3 µmoles/cm2/sec. The slow dissolution rate limits the local concentration of CO2. Limited observations of the approach of marine life to CO2 released on the sea floor showed no apparent avoidance reactions.

Introduction

CO2 released to the atmosphere via burning of fossil fuels and deforestation have led to an increase of atmospheric concentration of CO2 from a preindustrial level of about 270 ppm to a current level of about 370 ppm. While there is no doubt that CO2 is a greenhouse gas that increases radiative heat transfer to the atmosphere, there is considerable debate concerning the magnitude of the long-term temperature response of the planet's highly nonlinear climate system to the increased heat load. Because fossil fuels are and will continue to be a critical component of world energy supplies in this century (in addition, of course, to conservation and substitution of other energy sources), methods for disposal of CO2 that do not involve long residence times in the atmosphere are now being investigated. Options being considered include injection of CO2 into geologic formations (oil and gas reservoirs, aquifers, and coal beds), and direct disposal in the ocean1.

Recent projections of future CO2 emissions suggest that stabilization of atmospheric concentrations at 550 ppm would require sequestration of 3.7 billion metric tons of CO2 per year by 2025 and about 4 times that by 20502.Worldwide emissions of CO2 are expected to be 43 billion tons in 2025, so the amount to be sequestered represents about 9% of total emissions. To put these numbers in perspective, a billion tons of CO2 per year is the equivalent of 25–35 million barrels per day for CO2 densities in the range 0.5–0.7 g/cm3, typical values for injection into oil and gas reservoirs at reservoir conditions. World wide production of crude oil is about 72 million barrels per day. Volumes for disposal in the ocean would be somewhat lower because CO2 is denser at typical ocean temperatures. At densities of 0.9–1.0 g/cm3, the daily volume of one billion tons/year would be 2.7–3 million m3/day. Thus, if CO2 sequestration of quantities of CO2 sufficient to have an impact on future atmospheric concentrations is undertaken, the volumes of CO2 that will have to be handled will be quite large, and it is likely that more than one sequestration method will be required to handle them. Thus, it is important to investigate the full range of sequestration options.

This content is only available via PDF.
You can access this article if you purchase or spend a download.