Dynamic viscosity evolution characteristics of ScCO2 and its effects on carbon geological sequestration efficiency

Coupling effects of supercritical CO2 (ScCO2) dynamic viscosity evolution and low-permeability coal reservoirs on CO2 sequestration efficiency sequestering CO2 in coal seams is one of the crucial strategies for reducing carbon emissions. However, the efficiency bottleneck primarily arises from the coupling effects between the dynamic evolution of ScCO2 properties and the low-permeability characteristics of coal reservoirs. The evolution of CO2 dynamic viscosity directly influences its migration within coal seams. Therefore, this study focuses on the quantitative impact mechanism of the spatiotemporal evolution of ScCO2 dynamic viscosity on sequestration efficiency. Based on this, a mathematical model is constructed and numerical simulations are conducted. Using this model, we obtain the transient evolution patterns of ScCO2 physical parameters during the sequestration process and analyze the effects of dynamic viscosity evolution on CO2 storage capacity and sequestration efficiency. The results indicate that under in situ temperature and pressure gradients, ScCO2 dynamic viscosity exhibits nonlinear growth, forming a dynamic viscosity gradient barrier. The low-dynamic viscosity window governs the initial CO2 migration volume, whereas dynamic viscosity surges lead to a decline in sequestration rates in later stages. The heterogeneous distribution of CO2 dynamic viscosity tends to overestimate sequestration efficiency while underestimating the resources and time required to achieve sequestration targets, thereby increasing the risk of cost overruns and project delays. Accordingly, this study proposes a stepwise injection strategy regulating viscosity dynamics and optimizing temperature-pressure coordination to enhance sequestration efficiency, offering theoretical support for accurate efficiency evaluation and risk control.