Measurement of CO₂ diffusion coefficients in both bulk liquids and carven filling porous media of fractured-vuggy carbonate reservoirs at 50 MPa and 393 K

Diffusion coefficients are necessary to describe the mass transfer and adsorption rate of CO₂ in formation fluids. However, data is scarcely reported for actual reservoir conditions of high pressure and temperature, which are normal in most scenarios of the CO₂-enhanced oil recovery process in China's fractured-vuggy reservoirs and carbon storage process. Accordingly, this work employed the pressure decay method (PD) and relevant mathematical models to determine the CO₂ diffusion coefficient in both liquids and cavern filling porous media at 50 MPa and 393 K. The effects of the type of reservoir fluids, the properties of carven filling porous media, and water saturation on CO₂ diffusion coefficients were investigated. Results in bulk reservoir liquids showed that the CO₂ diffusion coefficient in the oil sample was 4.1243 × 10^-8 m² s^-1, much higher than those in the pure alkane phase, pure water and brine sample from reservoirs. Results of CO₂ diffusion in carven filling porous media saturated with oil demonstrated a significant dependence on properties such as porosity and permeability, and a correlation in the CO₂ diffusion coefficients between the bulk oil phase and cavern filling porous media in the form of touristy was documented. CO₂ diffusion in the fractured cavern porous media was much higher than that without fracture. An increase in water saturation reduced CO₂ diffusion coefficients in the carven filling porous medium studied, herein. Thus, the CO₂ diffusion coefficient is essentially related to the type of liquid and properties of the filling media.