Quantifying the dynamic density driven convection in high permeability packed beds

Two-dimensional micromodels for studying the convective dissolution of carbon dioxide in 2D water-saturated porous media

Convective dissolution is a perennial trapping mechanism of carbon dioxide in geological formations saturated with an aqueous phase. This process, which couples dissolution of supercritical CO₂, convection of the liquid containing the dissolved CO₂, and mixing of the latter within the liquid, has so far not been studied in two-dimensional porous media. In order to do so, two-dimensional (2D) porous micromodels (patterned Hele-Shaw cells) have been fabricated from UV-curable NOA63 glue. NOA63 is used instead of PDMS, which is permeable to CO₂ and does not allow for a controlled no flux boundary condition at the walls. The novel fabrication protocol proposed here, based on the bonding of a patterned photo-lithographed NOA63 layer on a flat NOA63 base, shows good reproducibility regardless of the patterns' typical size, and allows for easy filling of the cell despite the small value of the gap. A pressure chamber allows pressurizing the CO₂ and outside of the flow cell up to 10 bars. Experiments were performed in 11 different porous media geometries. As expected, a gravitational fingering instability is observed upon injection of gaseous carbon dioxide in the cell, resulting in the downwards migration of dissolved CO₂ plumes through the 2D porous structure. The initial wavelength of the fingers is larger in the presence of a hexagonal lattice of pillars. This effect can be correctly predicted from the theory for the gravitational instability in a Hele-Shaw cell devoid of pillars, provided that the permeability of the hexagonal porous medium is considered in the theory instead of that of the Hele-Shaw cell. Fluctuations around the theoretical prediction observed in the data are mostly attributed to a hitherto unknown weak locking of the wavelength on the distance between closest pillars.

New Spectrophotometric Method for Quantitative Characterization of Density-Driven Convective Instability

CO₂ convective dissolution has been regarded as one of the fundamental mechanisms to accelerate the mass transfer of CO₂ into brine. We present a new spectrophotometric method to characterize the convective instability and measure the dissolved CO₂ mass, which enables the real-time quantitative visualization of CO₂/brine transport mechanisms. Successive images were captured to identify the finger development regimes, and the convection morphologies were analyzed by the fingers length and affected area. CO₂ solubility was experimentally studied, and the results are in agreement with the theoretical calculations. CO₂ mass transfer flux was investigated as the Sherwood number changed. The increase in salinity and temperature has a negative effect on CO₂ dissolution; here, numerical simulation and experimental phenomena are qualitatively consistent. In general, these findings confirm the feasibility of the method and improve the understanding of the physical process of CO₂ convective dissolution, which can help assess the CO₂ solubility trapping mass.

Change in Convection Mixing Properties with Salinity and Temperature: CO2 Storage Application

In this study, we visualised CO₂-brine, density-driven convection in a Hele-Shaw cell. Several experiments were conducted to analyse the effects of the salinity and temperature. The salinity and temperature of fluids were selected according to the storage site. By using charge coupled device (CCD) technology, convection finger formation and development were obtained through direct imaging and processing. The process can be divided into three stages: diffusion-dominated, convection-dominated and shutdown stages. Fingers were formed along the boundary at the onset time, reflecting the startup of convection mixing. Fingers formed, moved and aggregated with adjacent fingers during the convection-dominated stage. The relative migration of brine-saturated CO₂ and brine enhanced the mass transfer. The effects of salinity and temperature on finger formation, number, and migration were analysed. Increasing the salinity accelerated finger formation but suppressed finger movement, and the onset time was inversely related to the salinity. However, the effect of temperature on convection is complex. The dissolved CO₂ mass was investigated by calculating the CO₂ mass fraction in brine during convection mixing. The results show that convection mixing greatly enhanced mass transfer. The study has implications for predicting the CO₂ dissolution trapping time and accumulation for the geological storage of CO₂.

Experimental Study on the Density-Driven Carbon Dioxide Convective Diffusion in Formation Water at Reservoir Conditions

Density-driven convection, which can accelerate the dissolution rate of carbon dioxide (CO₂) in resident brine, is critical for the long-term fate of the injected CO₂ permanence and security of CO₂ geological storage. Visualization experiments and pressure-volume-temperature (PVT) testing were conducted to investigate the influence from gravitational convection. For investigate gravitational instabilities and convective diffusion, we designed a Hele-Shaw cell rated to 70 MPa and Rayleigh number of 346 to conduct visualization experiments with the micro-schlieren technique. The average diffusion coefficient and time-dependent values were measured in the PVT experiments. We also calculated the convection parameters, including Rayleigh number and critical onset time, with a series of PVT testing at the temperature ranging 293.15-423.15 K and pressure ranging 14-24 MPa by using the constant-pressure method. Through visualization experiments, we observed convective currents triggered by the density gradient in the gas-liquid interface, which noticeably enhanced the CO₂ dissolution rate. The PVT testing confirmed that the diffusion coefficient increased sharply under the influence of the gravitational convection at the early stage and then decreased toward the average diffusion coefficient with time. The PVT testing also demonstrated the Rayleigh number increasing with temperature or pressure increasing under the reservoir conditions. The gravitational convection will be more likely to occur and more rapid with a greater pressure or higher temperature.