<p>We investigate coupled fluid flow, heat transfer, species transport, and surface reaction in a single reactive pore embedded within a low-permeability matrix layer separating adjacent aquifer strata, motivated by aquifer thermal energy storage (ATES) applications. Building on fracture-scale reactive-transport simulations, we develop a pore-scale model that resolves reaction-driven pore evolution, and assess whether such evolution can compromise the impermeability assumption employed in homogenized ATES models. Analysis of a spatially independent reduction reveals a robust thermochemical structure in which the dominant reaction mode is governed by thermal forcing, while reaction and matrix-fluid heat exchange control transient relaxation toward equilibrium. Numerical simulations of the full pore-scale system of partial differential equations (PDE) show how axial transport redistributes these local dynamics without altering their underlying structure. Using time-dependent thermal and chemical forcing extracted from a fracture-scale model, we identify parameter regimes in which reaction-driven pore evolution remains insufficient to exceed experimentally reported permeability thresholds. These results delineate a regime of validity for the impermeability assumption in ATES modeling and provide a mechanistic link between fracture-scale forcing and pore-scale transport processes.</p>