CO2 fingers. Strong chemical reactions between dissolved carbon dioxide and porous rock (top) may stop CO2 fingers from spreading from the top throughout an aquifer's depth, in contrast to systems with no reaction (bottom).
Even a simple chemical reaction can delay or prevent the spreading of carbon dioxide stored in underground aquifers, according to new research done by the Department's Fluids and Environment Group.
The study, published in the journal Physical Review E on 22 April 2011, shows that distinct regimes of CO2 transport may occur in deep saline rock formations, depending on the strength of the reaction between dissolved CO2 and porous rock.
With strong reactions, the CO2 remains near the top of the reservoir, while with weaker ones, the CO2 will spread from the top throughout the depth of the aquifer.
"If one knows the physical and chemical properties of the aquifer, one can calculate the movement of CO2 across it, and when it will begin to mix with the brine," said Jeanne Therese Andres, a Schlumberger Foundation PhD candidate.
Reaction strengths can vary significantly among deep saline reservoirs - rock formations possess a wide range of chemical reaction rates depending on the mineralogy (e.g. calcite, dolomite, etc) as well as other factors such as temperature and pressure.
"In theory, one can manipulate the strength of reactions," said Andres, "thereby engineering the movement of CO2 - keeping it in one area or moving it to another within the aquifer - to enhance its storage underground."
The researchers established that the basic interaction between fluid flow and chemical kinetics in a deep porous medium is governed by a single dimensionless number, which measures the rate of diffusion and reaction compared to that of natural convection. The work demonstrates how this new parameter controls CO2 flow and mixing in briny porous rock. Through numerical simulations, the researchers found that above this parameter's critical value, reaction stabilizes the CO2 system and convection no longer occurs. Below the parameter's critical value, stronger reactions result in longer delays in the onset of convective mixing throughout the reservoir.
For systems with similar convective mixing strengths, stronger reactions, indicated by rising values of the new parameter, can increase the rate at which pure, lighter CO2 dissolves into the brine, enhancing storage and reducing the risk of leakage.
"Such knowledge will be valuable in guiding future approaches to carbon storage," said project leader Dr Silvana Cardoso, Reader in Fluid Mechanics and the Environment.
Physical Review E is an interdisciplinary journal published monthly by the American Physical Society (APS). The journals of the APS embody its mission "to advance and diffuse the knowledge of Physics."