| Carbon dioxide (CO2) capture and storage is considered as one of the most promising options for reducing emissions of CO2, which has received extensive attention from both the governments and the industrial sectors. Among the various geological reservoirs for CO2storage, saline aquifers are the most attractive as a potential option for long-term storage for several reasons:the saline aquifers are widely distributed; the volume of pore space available is enormous; and more importantly, the technology is feasible that can be carried out without major technical difficulties.For CO2storage in saline aquifers, the targeted injection formations are below800m. Under the pressure and temperature of storage conditions, CO2is a supercritical fluid. The supercritical CO2has the characteristics of high density, low viscosity and high mobility, which are favorable for CO2storage in saline aquifers. At the typical storage conditions, CO2is less dense than brine. Accordingly, strong buoyant forces will drive CO2upward within the formation, and a plume of CO2will form. Over time, four main storage mechanisms start operating, namely, static, residual, solubility and mineral trapping. The storage mechanisms act together, and their individual contribution and relative importance evolve with time. These mechanisms do not necessarily increase the CO2storage capacity, but they increase the storage security. The storage mechanisms are a focused point of investigation in this study.CO2storage in saline aquifers involves complex physical and chemical processes, such as the migration of injected CO2and native brine, advection and diffusion, convective mixing, phase appearance/disappearance, dissolution and precipitation of minerals, and geo-chemical reactions. Meanwhile, a series of specific flow phenomena will happen in the saline aquifers due to the injection of a large amount of CO2, such as pressure buildup, the displacement of brine by the injected CO2immiscibly and the changes of porosity and permeability of rocks. Among these phenomena, some are beneficial for CO2storage in saline aquifers, for example, the convective mixing due to the density differences can increase the dissolution rate of CO2into brine, while others may impact on the environment, such as the potential hazardous effects on groundwater of CO2or formation fluid leakage. The main aim of this study is to investigate the coupled physical and chemical processes as well as the specific flow phenomena. Multiphase fluid dynamics based on the theory of flow in porous media and computational fluid dynamics have been used to study CO2storage in saline aquifers in this study. Using numerical simulations, the storage behaviours and the flow and transport characteristics are obtained. The relations between storage and related geological parameters such as permeability, porosity, compressibility are studied. Large scale numerical simulations are carried out using parallel computations. Some insights on CO2storage in saline aquifers have been obtained. The detailed research carried out is outlined as follows:In the research of storage mechanisms, the fluid dynamic behaviours and influencing parameters of solubility trapping are studied. Dissolution of CO2into the underlying brine increases the density of brine, leading to a gravitational instability. This instability could give rise to the fingering of CO2-rich brine, which would trigger fluid convection and greatly enhance the dissolution rate of CO2. Both two-dimensional and three-dimensional simulations are carried out to investigate the differences, aiming at selecting the proper dimensions for the simulations of density-driven convection. Results indicate that there are four distinctive time periods in the temporal evolution of CO2dissolution process, i.e., the diffusive period, the modulated convective period, the constant convective period and the decay convective period. It was also found that the number of fingers decreases and the onset time of convection delays with increasing angle when the sealing caprock has an inclined angle.In the research of flow phenomena, multiscale behaviours and related flow characteristics are analyzed, which involve pressure buildup in a large scale, CO2plume evolution in a medium scale and salt precipitation in a small scale. The different behaviours of these phenomena in saline aquifers with different boundaries are also investigated. Three-dimensional simulations have been performed to investigate the propagation of pressure and the impact of salt precipitation on the process of large scale CO2injection into the saline aquifers. Apart from the different scales of the process, the numerical results show clearly different behaviours of the pressure changes in saline aquifers with different boundaries. The increase and propagation of pressure largely depend on the boundary conditions. Different types of salt precipitation occur adjacent to the injection well, presenting different impacts on the fluid flow. Affected by the precipitation, the porosity and permeability could decrease, leading to declined transportation and degraded injectivity under different boundary conditions. The interplay between pressure buildup and solid saturation is compared in saline aquifers with different boundary conditions.In the research of large scale phenomena, with regard to the potential hazards to the shallow groundwater resources, the propagation of pressure and transportation of brine are used to evaluate the potential environmental impacts. An idealized multilayered groundwater system is used to study the regions of influence of pressure buildup and brine migration and to evaluate the potential implications for the shallow groundwater resources, with a sequence of aquifers and aquitards (sealing units) extending from the deep saline storage formation to the upper most fresh-water aquifer. With the focus on the multilayer impact of CO2injection, seal permeability over a wide range is compared. Numerical simulations results indicate that the regions of influence of pressure buildup and brine migration are significantly larger than CO2plume region; the variation trend of pressure buildup depends on both the permeability of seals and location; brine migration into shallow groundwater aquifer is extremely unlikely. |
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