The Numerical Simulation Study On CO₂-Water Two Phase Flow In Porous Media Based On CFD

Climate anomaly and natural disasters caused by global warming has seriously threatened survival and sustainable development of human society. The increase of emissions of greenhouse gases （GHGs） sourced from human activities is an important reason for causing the global warming. Among different kinds of GHGs, carbon dioxide （CO2） accounts for the largest portion, about 76.7%, thus becoming the major target for mitigating GHG emissions. CO2 sequestration in saline aquifers is regarded to be one of the most promising technologies for reducing anthropogenic CO2 emissions to the atmosphere and has got great attentions from governments and research institutes worldwide.The deep understanding of CO₂-brine two-phase migration in porous media can provide a theoretical foundation for reliably predicting the efficiency and security of CO2 saline aquifer sequestration projects. Pore-scale modeling is especially helpful in understanding the multiphase migration processes and describing macroscopic migration phenomena. Furthermore, it is also an important tool for providing constitutive relations and computing parameters for continuum-scale models. Therefore, such modeling study has become an important constitutional part of the approaches to research on CO2 sequestration in saline aquifers.Based on the microstructural information of reservoir cores obtained from CT imaging technology, a computer program is especially developed for extracting and rebuilding effective flow paths in porous media. Then, in combination with computational fluid dynamics （CFD） method, a reliable method is proposed to calculate the local permeability and relative permeability of the cores. In addition, this study uses phase field model to effectively track the interface between CO2 and water in the CFD simulations, which lays a basis for the numerical analysis of the influencing factors of CO2-water unstable displacement in porous media. Numerical simulation results show:1) capillary number and viscosity ratio have a significant influence on the displacement process. In case of high capillary number （>0.002） and high viscosity ratio （≥1）, fingerings do not occur during the whole displacement process and the CO2-water displacement can be regarded as stable process. Assuming low viscosity ratios （<0.02）, viscous fingering take places and viscosity force dominates the displacement; if capillary number is small （e.g.<0.00002）, there exist capillary fingering during the displacement and the displacement is dominated by capillary force; 2) the effect of density ratio on the displacement can be neglected when the studied model is small; 3) surface wettability of matrix grains can also affect the stability of the displacement process. In case of water wet, the displacement is unstable and fingering can form, whereas obvious fingering do not happen and the displacement is stable under the condition of hydrophobicity; 4) the effect of CO2 injection direction cannot be ignored. If being horizontally injected, significant flow paths will appear significant disconnection. By contrast, when CO2 is vertically injected, the disconnected flow weakens, but the difference is small compared with the horizontal injection; 5) if the heterogeneity is considered, the displacement is an approximately stable process when porous media are homogeneous. Comparably, in case of the heterogeneity, significant disconnected flow paths appear during the displacement. It requires some essential conditions that CO2 accumulate before the barrier.