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 l2 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 DC1 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.