Mathematical modelling of permeability reduction in porous media caused by CO_2 hydrate formation using microscopic phase-field model simulations

摘要

Carbon capture and storage is a promising technique for reducing significant amount of carbon dioxide (CO_2), which is stored in aquifers under the seabed, although there is a risk of leakage that may pose some influence on the marine environment. Gas hydrate has ice-like structures formed by the enclosure of gas molecules by water molecule cages. CO_2 hydrate can be formed under the conditions of high pressure and low temperature, such as in sub-seabed shallow formations in the deep sea. If CO_2 leaks from sub-seabed aquifers and rises in the geological formation upto a depth where hydrate is formed, the hydrate can suppress or even block the outflow. To use gas hydrate more positively, another approach has been advocated in which CO_2 is injected directly into the hydrate stability zone and is stored in the form of gas hydrate. This study provides a mathematical model of the permeability reduction caused by hydrate formation, using microscopic numerical simulations of hydrate formation on the pore scale. Virtual sand sediments were created numerically using the CT scan data of the Toyoura sand and the particle growth method. The phase-field model was used to simulate hydrate growth, and the lattice Boltzmann method was used to calculate the permeability reduction. Various values of porosity and initial water saturation were considered to investigate their influences on the hydrate growth and the subsequent permeability reduction. The proposed mathematiical model for the permeability reduction was able to capture the overall behaviour of the simulated results, suggesting its applicability as a submodel of reservoir-scale simulations.