Geochemical experimental study on the alteration of granite pore structures under CO2-H2O-rock interactions in CO2-enhanced geothermal systems

Abstract

In CO₂-enhanced geothermal systems (CO₂-EGS), geochemical reactions among CO₂, H₂O and rock dynamically alter pore structures through mineral dissolution/precipitation even with a small amount of water. However, the governing mechanisms remain poorly quantified. This study investigates the CO₂-H₂O-rock interactions of granite under different reaction temperatures and reaction times. The mineral composition and micro-morphology evolution of granite were analyzed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), and the evolution of pore structure and the effects of reaction temperature and reaction time were investigated through mercury intrusion porosimetry (MIP), nitrogen adsorption test, and carbon dioxide adsorption test. Key findings reveal that (1) the content of quartz and biotite decreased, while feldspar and secondary minerals (calcite/kaolinite/dolomite) showed dynamic changes; (2) Mineral precipitation reduced pore diameters and even sealed fracture, whereas dissolution enhanced connectivity by expanding effective pore volume; (3) Mesopores and micropores exhibited the increase of specific surface area but decreased average pore diameter, with pore surfaces homogenizing but the three-dimensional complexity intensifying. Crucially, all changes positively correlated with reaction temperature and/or reaction time, establishing a tunable relationship for CO₂-EGS operation. These results resolve critical uncertainties in pore-structure evolution, offering actionable insights for improving CO₂ storage and geothermal extraction efficiency.