Title: MRI visualization of nanoparticle enhanced convective mixing associated with CO2 storage

Biography:

2017-present, PhD student at Dalian University of Technology, China. 2022.01-2023.01, visiting PhD student at Imperial Colloge London, England. CO2 capture, utilisation and storage (CCUS) is an important technology option for achieving the goal of carbon neutrality as a technology that enables large-scale fossil energy reduction and low carbon use. Sijia Wang has interests on thermophysical measurements, and fluid flow in porous media related to CCUS during his PhD.

Abstract

Global warming, which is the main manifestation of climate change, has been caused by greenhouse effects, and the primary anthropogenically emitted greenhouse gas is carbon dioxide (CO2). CO2 capture and sequestration (CCS) is an effective method of reducing the amount of CO2 emitted into the atmosphere. As sites of CO2 geological storage, deep saline aquifers have demonstrated great potential because of their large storage spaces and existence in many locations around the world. Dissolution trapping in saline aquifers is considered one of the most effective and safe methods of CO2 geological storage. Under ambient temperature and pressure conditions in a typical aquifer, CO2 dissolve into the aqueous phase (brine) and create a dense interfacial layer The CO2 -saturated brine in a saline aquifer is 0.1% to 1% denser than natively formed brine, depending on the pressure, temperature, and salinity. Gravitational instability or unstable stratification trigger the convection of fluids, which is commonly referred to as density-driven convection in the literature. As the CO2 dissolves into the underlying brine, the CO2-saturated brine becomes denser than the native brine, and the density difference causes instability and triggers convection. Dense CO2-saturated brine migrates downward, while light native brine rises, and the natural convection of these two fluids generates convective fingers. The downward migration of the CO2-saturated brine significantly enhances the mass transfer of CO2 and reduces the time required for total dissolution. More importantly, relative to the native brine that fills the pore spaces, the negative buoyancy of the CO2-saturated brine may reduce the risk of leakage along geological faults, which is of considerable concern relative to long-term sequestration security. Recently, a method has been proposed to accelerate the dissolution of CO2 and brine and reducing the risk of leakage with co-injection of CO2 with nanoparticles (NPs). Low-level nuclear waste containing NPs was suggested to be an economical method through a fully mixing with CO2 before injection. In this study, we visualized and quantitatively measured the density-driven convection in porous media by using magnetic resonance imaging (MRI). An analogue fluid pair system of 80% glycerite (dense fluid)/MnCl2 (light fluid) was used to simulate the brine with dissolved ScCO2-formation brine system. The impact of NPs fluids added into CO2 on fluid-fluid interfacial instability was analysis. Superhydrophilic SiO2 NPs with different mass fraction (0 wt%, 0.1 wt% and 1 wt%) was added to the dense fluid to investigate the optimum mixing ratio. And several dimensionless parameters were calculated to characterize the convection. The onset time of convection is the time at which the signal intensity shows a significant downward trend and obvious convection fingers can be observed. The power function relationship between the Rayleigh number and onset time can be determined. The earlier onset time reflects the better sequestration safety. The convective finger development process consists of the following stages: finger appearance, finger propagation, new finger generation, and finger coalescence. The largest finger number appears in the finger appearance stage, and there is a linear downward trend of the finger number after this point. Convection occurring in porous media depends not only on whether the mixture has reached the critical Rayleigh number but also on the domain aspect ratio. The interfacial behavior of the fluid during convection mixing process is continuously captured. It shows that, dense fluid with NPs leads to more stable interface behavior. Furthermore, experimental results confirm that reduction in surface tension between NPs and CO2 would enhance fluid miscibility and the mass transfer during convection. The dimensionless mass transfer coefficient, which is characterized by the Sherwood number, can be directly measured via the finger growth velocity. It shows that the Sherwood number follows a power law relationship with the Rayleigh number. Compared with traditional injection method, CO2 contains high concentration of nuclear waste may have higher sweeping area and higher sequestration efficiency. These results challenge our view of carbon sequestration and dissolution efficiency in the subsurface, suggesting that co-injection of ScCO2 and low-level nuclear waste containing NPs perhaps a feasible approach which can enhance sequestration potential.