Two-phase flow of CO2/brine in porous media is critical to the capacity and safety of carbon sequestration into the brine aquifer. In order to provide valuable information and important theoretical basis for site selection and CO2 injection, the microscopic visualization technology was employed in this study to conduct displacement experiments of CO2/brine at the pore scale. Four micromodels with different sizes and structures, five injection rates of CO2 and six salinities of brine were used to study the effects of micromodel’s structure and displacement pattern on two-phase flow. Several parameters including the differential pressure, contact angle, permeability, velocity field and force field were obtained by experimental measurement, image post-processing and theoretical analysis, and then these parameters’ variation was investigated. Phenomena such as thin film, corner flow and Haines jump were also found during the displacement. Although brine could be completely displaced by CO2 in the capillary duct, the backflow of wetting phase would occur at low injection rate. Phenomena different from the theoretical analysis also occurred in pore doublet models: some brine was residual in the homogeneous pore doublet model at low injection rate, while the heterogeneous pore doublet model was fully occupied by CO2 at high injection rate. These phenomena are very useful for two-phase flow, and multiple factors need to be comprehensively considered to determine the operating conditions of CO2 storage into the brine aquifer.

High–pressure methane gas generally exists stably under methane hydrate stability zone at several hundred meters cutting through the marine sedimentary strata. The usually employed bottom simulating reflector (BSR) for hydrate recognition represents the interface between hydrate and fluid areas in typical natural methane hydrate reservoir system with hydrate, water and gas layers. In this study, the gas–seawater migration in hydrate reservoir was simulated through gas–seawater injection, and the existence of hydrate–containing sealing layer was experimentally confirmed. The hydrate reformation was observed by MRI during the gas–water injection process above the methane hydrate phase equilibrium pressure and it is the fundamental reason that hydrate reservoir has sealing effect on free gas. As the decrease of pore spaces in sediments, the interaction of seawater and hydrate in the reservoir products capillary sealing in the narrow space, thus the free gas and seawater migration are inhibited and the free gas exited stably underlying the hydrate layer. However, low methane concentration in seawater caused by high gas–water flow rate (4–1 ml/min) resulted in the hydrate dissociation, the hydrate–bearing sediments can’t produce the sealing effect. Hydrate further forms in the sealing layer and leads to seawater depletion until it is too salty to form hydrate. Finally, the gas layer, water layer and hydrate layer coexist under the seabed. In addition, the hydrate–containing sealing layer could be broken through, and the breakthrough pressure is a significant parameter for hydrate reservoir.

The intricacy of the thermo-hydro-chemically coupled process of hydrate phase transition requires real-time in-situ observations, therefore, thermometry maps are of particular value to reveal the heat transfer process during crystal growth and dissociation. By using the temperature dependence of water proton chemical shift, the temporally- and spatially-resolved thermometry of tetrahydrofuran hydrate growths is presented in this study. Images of temperature changes were synchronously obtained by a 9.4 T 1H Magnetic Resonance Imaging (MRI) system, in order to predict the saturation of aqueous solution, solid hydrate phases, and the positive temperature anomaly of exothermic reaction. Variations of MRI signal decrease and histories of temperature rise differ significantly in space and time, which have a great use for analyzing the physical micro-mechanism and the heat transfer process of hydrate growth. The extension of these predicted results could have important implications for optimizing the phase transition process of gas hydrates.