Behaviour of Density-Driven Natural Convection Mechanism

Storage of CO2 into geological formations is a reasonable technical choice for decreasing carbon dioxide emissions to the atmosphere. Brine aquifers are considered one of the most favorable options for this purpose and leakage of CO2 from these storage sites is one of the main concerns about it. To decrease the risk of leakage, the trapping mechanisms of CO2 into brine should be fully understood. More contribution of trapping mechanisms of CO2 reduces the time available for leakage and is therefore crucial to storage security. The dissolution of supercritical CO2 in formation water is one of the main long-term trapping mechanisms of CO2 into brine aquifers. Density-driven natural convection mechanism is predicted to occur, which accelerates the dissolution of CO2 in the brine formation water. This unusual phenomenon arises from the increase in the density of brine when saturated with CO2.

In this work we aim at contributing increased scientific knowledge mainly about the density-driven natural convection mechanism in dissolution of CO2 in heterogeneous and anisotropic underground brine aquifers using experimental tools. The timing of the onset of this instability and the dissolution rate across the phase contact are important operational issues when assessing the feasibility of a potential storage site. Another aspect is the issue of public acceptance of underground CO2 storage. CO2 storage is poorly understood by the general public and as a result, there is a general lack of public support for CO2 storage. So the main objectives of this research work are improving our understanding, knowledge and experimental ability about dissolution of CO2 into the homogeneous and heterogeneous brine aquifers, and explaining this mechanism and its likely impacts to the general public.

We performed a series of experiments about density-driven natural convection mechanism in Hele-Shaw cell geometries using CO2 gas phase and brine. In these experiments the behaviour of density-driven natural convection mechanism in different geometries like homogeneous models with different permeabilities and dips, heterogeneous models with barriers and layered permeability models is investigated. The objective parameters during analyzing of the experiments are onset time for convection, critical wavelength of convection fingers and CO2 dissolution rate into water after onset time for convection. The used Hele-Shaw cell in this work has dimensions of 50×50 cm, the cell was filled by fresh water and gas phase CO2 was injected on top of the water. We used pH indicator method with a solution of 0.025 wt. % bromocresol green for the experiment and imaging. With this approach the fresh water and the CO2-saturated water have blue and yellow colors.

Our experimental results consist of quantitative results or amounts of dissolved CO2 into water and qualitative results or captured images from the Hele-Shaw cell during the tests. We captured images every 10 sec, 20 sec, 1 min and 2 min using a Canon EOS-1Ds Mark II camera connected to a PC to have a continuous movie from movement of convection fingers in the cell. The created movies were speeded up 1/1600 and 1/3200 and have been uploaded in the following URL address:

http://www.youtube.com/playlist?list=PLfLgKEdPkuBPIlO1JDrmUJ0XnSCJOIBJD

The prepared continuous movies from the whole period of these experiments can help us in improving the public knowledge about CO2 storage in brine aquifers and one of its trapping mechanisms in underground brine aquifers.

Amir_Taheri2

PhD student Amir Taheri