Numerical analysis of mass and heat transfer mechanisms in microscale porous media with varying pore throat sizes.

• Microscopic model for coupling mass transfer and heat transfer. • The scale effect on microscopic mass and heat transfer mechanisms is significant. • The limitations of the continuous medium model become apparent as scale expands. The presence of a tight matrix and various fractures creates multiple scales of environments, complicating mass and heat transfer within porous media. This study presents a microscopic model that describes the coupling of mass and heat transfer mechanisms under local thermal equilibrium. The model incorporates mass transfer driven by differential pressure and diffusion, as well as heat conduction. Numerical solutions using the finite element method investigate these mechanisms across nanopore-throat, micropore-throat, and millimeter-fracture scales. Key findings indicate that at the nanopore-throat scale, mass transfer lags significantly behind heat transfer due to concentration gradients, while heat transfer is primarily facilitated by conduction. At the micropore-throat scale, heat transfer slightly lags behind mass transfer, influenced mainly by pressure differentials. At the millimeter-fracture scale, the rock cools more slowly than the fluid, with mass transfer driven by pressure differences and notable convective heat transfer. Comparisons between microscopic and macroscopic models highlight the importance of scale on transport processes. This research provides a theoretical foundation for microscale numerical simulations. [ABSTRACT FROM AUTHOR]