Microfluidic study of CO2 diffusive leakage through microfractures in saline aquifers for CO2 sequestration

CO₂ diffusive leakage, or diffusive transport, through intrinsic or induced caprock fractures poses a significant concern for the security of CO₂ sequestration in saline aquifers. Although this issue has garnered considerable interest and has been the subject of many numerical analyses, experimental studies remain limited. We present the first experimental investigation of CO₂ diffusive leakage through microfractures in a generalized microfluidic system that represents the key features of the system under realistic CO₂ sequestration conditions. Our findings reveal two-stage depletion kinetics of trapped CO₂ in porous media, driven by dissolution and diffusion through fractures. The first stage is characterized by the rapid dissolution of CO₂ into nearby brine, while the second stage exhibits a steady leakage rate as CO₂ diffuses through the fractures into a water sink, driven by the solubility limit, assuming stable microfracture structures and negligible advection. Between these two stages, there is a transition period during which CO₂ saturation remains stable. Two key parameters are proposed to quantify the diffusive leakage process: the transition time and the steady-state leakage rate. The transition time $0.1\frac{l^2}{D}$ defines the timescale for the onset of a diffusive leakage event, where l represents the fracture length and D the gas diffusivity. The steady-state leakage rate is primarily governed by aquifer conditions and fracture properties, which scales as $\frac{D{C}_1}{l}$​​, where C₁​ is the solubility limit. Our theoretical predictions align well with the experimental results. Additionally, the effects of temperature, pressure, salinity, and storage depth on CO₂ diffusivity and solubility are explored through sensitivity analysis. Despite the simplifications in our experimental design and modeling, our study lays the foundation for future research by progressively incorporating additional complexities. These findings provide broader implications for assessing leakage risks in subsurface geological gas storage, such as H₂ and CH₄.