| dc.description.abstract | Storage of CO2 into geological formations is a reasonable technical choice for
decreasing carbon dioxide emissions to the atmosphere. Brine aquifers are
considered as 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, 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. 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.
The main objective of this study is to increase scientific knowledge mainly about
the density-driven natural convection mechanism in dissolution of CO2 in
isotropic/anisotropic and homogeneous/heterogeneous underground brine
aquifers using experimental and numerical tools and explaining this mechanism
and its likely impacts to the general public. For this purpose and at the first step,
density-driven natural convection mechanism is investigated using a black-oil
numerical solver to study the effects of different properties on this process and
the amount of dissolved CO2 into brine in homogeneous and heterogeneous brine
aquifer models. The onset times for convection triggered by numerical round off
errors found in numerical solvers of isotropic homogeneous models are
significantly later than predicted by stability analysis theory that is based on real
physics of the problem. So for more accurate predictions of convective mixing
behaviour in isotropic and anisotropic brine aquifer models, a new approach for
initializing the numerical models is developed. In this approach the gravitational
instability is triggered by a weak wavy perturbation with dimensionless
wavenumber of K that is consistent with the perturbation in stability analysis
theory. This numerical solution is used for prediction of the critical time for the
onset of convection and the critical unstable wavelength in semi-infinite
anisotropic homogeneous brine aquifer models. Moreover after validating this
methodology, it is applied in heterogeneous barrier type numerical models for prediction of the related critical times for the onset of convection for different
barrier patterns and geometries. In next step we focus on upscaling of properties
in simulation models and its effect on performance of convective mixing process
in heterogeneous brine aquifers with variation of permeability and barrier type
aquifers. Effect of upscaling on onset time for convection and dissolution rate of
CO2 in brine is investigated there.
After the numerical and theoretical studies of convective mixing process, we start
experimental studies of density-driven natural convection mechanism in different
Hele-Shaw cell geometries using two sets of fluids; water/brine and water/CO2.
At first step we present the results of preliminary experiments about densitydriven
natural convection mechanism performed in a homogeneous Hele-Shaw
cell using two fluids with different densities; water and brine. With this analysis,
the effects of density-driven natural convection on accelerating the rate of
dissolution are investigated. Also the growth and development of convection
fingers and the changes in their geometries with depth and time are studied. After
these initial and preliminary experimental studies and understanding the real
concept of density-driven naturally convection mechanism, we focus on
performing a series of experiments about density-driven natural convection
mechanism in different Hele-Shaw cell geometries using water and CO2. In this
part after introducing our precise experimental set-up and the suitable procedure
for performing of the experiments in different Hele-Shaw cell models, the results
of several experiments are presented. 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. Onset time
for convection, critical wavelength of convection fingers and CO2 dissolution rate
into water are objective parameters here for study. The important point in the
analyses of the experiments is that there are several specific dimensionless
numbers that can be related to each experiment and it can be said that the results
of the experiments can be scaled to other systems like real brine aquifers with the
same dimensionless numbers. Moreover the experimental results are studied by
scaling them to dimensionless forms and also compared with numerical
simulation results for investigating the effectiveness of numerical simulators for
describing convective mixing process. Furthermore the prepared continuous
movies from the whole period of these experiments can be helpful in improving
the public knowledge about CO2 storage and one of its trapping mechanisms in
underground brine aquifers. | nb_NO |
