Thermodynamic Modeling of Chemomechanical Coupling in Porous Media: Distinguishing Solid and Nonsolid Reactions

Abstract Chemical reactions in porous media can proceed either entirely within the pore space (nonsolid reaction) or engage with the skeleton (solid reaction), leading to different chemomechanical coupling mechanisms and constitutive behavior. However, current hydromechanical-chemo models do not explicitly distinguish between these reaction types, and many thermodynamic formulations encounter intrinsic difficulties when handling reactions in closed systems or treating reactions involving changes in the solid skeleton. To address these issues, this paper develops a thermodynamically consistent chemomechanical framework that explicitly distinguishes solid and nonsolid reactions. By using nonequilibrium thermodynamics, the entropy production for both closed and open systems are quantified, and the reaction affinity is decomposed into solid and fluid contributed parts, leading to the treatment of chemical reactions in open and closed systems in a unified way and yielding explicit Helmholtz free energy expressions that distinguish the two reaction types. Following that, chemoelastic models for both closed and open systems are derived, with mass density and chemical potential being the chemical variables. The Biot-type reactive poroelastic model is subsequently recovered and reformulated, in which the chemical reaction in the pore fluid is indirectly coupled with stress through pore pressure, whereas reaction with solid is directly coupled with the mechanical process through reaction-induced stress. The governing equations and a numerical model for the hydromechanical-chemical (HMC) coupling are presented to illustrate the differences between nonsolid and solid reaction.