(206g) Modeling of the Formation of Carbon Dioxide Hydrate in Porous Media

Carbon dioxide capture and storage (CCS) is a recent and promising technique to reduce the emissions of carbon dioxide (CO₂) in the atmosphere. After its capture the CO₂ can be stored in under-seabed regions. However, there is a risk of CO₂ leakage which may then cause innumerous harm to the environment. The formation of CO₂ hydrate can actually be a natural process of reducing the permeability and may act as a seal and block the leakage path of ascending CO₂.

Gas hydrates are ice-like structures formed by having a guest molecule (gas molecules) enclosed by water molecules in the form of a cage. It is formed at porous regions under a few hundred meters of depth on offshore conditions, where high pressure and low temperature are expected.

The focus of this study is to model the formation of CO₂ hydrate and understanding how the formation affects its morphology within the porous media by considering parameters, such as porosity, water-sand contact angle, and initial water saturation. The CO₂ hydrate growth is simulated by using the phase-field model. The numerical domain will be composed by 4 different phases: water, CO₂ gas, sand and CO₂ hydrate. The first step of the program lies on defining the numerical domain. The sand is created based on real data from Toyoura sand and the initial distribution of water and gas is affected by properties such as the water saturation and water-sand contact angle. The initial stage of gas hydrates are defined as small nuclei and its appearance, which is a stochastic process, will have the effect of simulations having different initial position of gas hydrate.

The temporal change of the CO₂ hydrate growth is shown by the phase change between water (with gas dissolved) turning into CO₂ hydrate. In the phase-field model a parameter Φ_PFM is used to describe the interface between the water and CO₂ hydrate phases. Its value will range from 0 (water) to 1 (CO₂ hydrate) in-between these two phases. The main driving force is related to the difference in fugacity, i.e. the difference of CO₂ concentration of the gas dissolved in water and the equilibrium concentration of CO₂ hydrate. Other important parameter is the mobility, M_PFM, which indicates the speed which the interface between water and CO₂ hydrate moves. Its value was obtained by doing a history matching with experimental data and the obtained value of 1.57x10^-15 was used in the simulations. This mobility value was also tested and validated with a second experimental study.

The numerical domains were created based on the three main parameters chosen: sand porosity, water-sand contact angle and initial water saturation. Different initial position of the CO₂ hydrate nuclei and sand grain configurations are expected to be considered for the CO₂ hydrate growth simulations.

For a fixed value of porosity, different values of initial water saturation and water-sand contact angles were used. For each one of these cases around 45 simulations were done in order to ensure a good estimation of the new average value of permeability.