Carbon Dioxide Flow Behavior through Nanopores: Implication for CO2 Sequestration in Unconventional Gas Reservoirs

CO2 sequestration is a promising approach to achieve the net-zero emission across the world, expecting to mitigate the urgent climate change and global warming. In this article, research efforts are dedicated for investigating CO2 sequestration in unconventional gas reservoirs, characterized with nanoscale pore size and complicated mineral compositions. Unlike the majority of previous research studies focusing on CO2 adsorption or competition adsorption in nanoconfined space, this article puts emphasis on the CO2 flow capacity in nanopores under different conditions, including pressure, temperature, pore size, and the wettability effect. CO2 flow capacity plays a prominent role, affecting the efficiency of CO2 sequestration, which currently lacks due attention and entails in-depth elaboration. In this article, the nanoconfined CO2 flow mechanism is first studied, and CO2 physical properties varying from the gaseous state to the supercritical state are incorporated in the proposed model. Furthermore, the adsorption phase thickness and critical pressure/temperature shift induced by CO2 adsorption are coupled. Results show that (a) CO2 flow capacity in the 2 nm pore is greater than that in the 5-20 nm pore as a result of dominant surface diffusion in small pores; (b) CO2 flow capacity at 323 K can reach as much as 1.4 times that at 423 K, and desirable pressure for optimal CO2 flow capacity is approximately 9 MPa in small nanopores; and (c) the wettability effect can cause as much as 96.3% discrepancy for the CO2 flow capacity, while the surface- CO2 contact angle varies from 20 to 160 degrees. This work is able to contribute to CO2 geological sequestration from the perspective of injection efficiency.