Experimental and theoretical studies on water adsorption and capillary condensation in nanopores of calcium carbonate

Abstract

Understanding the water vapor sorption in nanopores has significant implications for hydrocarbon extraction and waste storage. This study presents gravimetric water sorption isotherms of nanoporous calcium carbonates. A methodology for quantifying equilibrium water sorption through matrix potential, even as the matrix potential approaches zero is introduced. This method facilitated the determination of adsorption via the Derjaguin‒Landau‒Verwey‒Overbeek (DLVO) theory and of capillary condensation, including the conjugate state, based on Young's Laplace equation. The relative errors between the experimental and calculated values ranged from 0.24% to 10.42%. Quantitative parameters characterizing the water distribution were calculated, and their dependences on water sorption were analyzed. The increase in water sorption over time exhibited a gradual decline, which was better described by a second-order exponential decay function. The stable decline point of the sorption rates ranged from 13.24% to 22.47% of the total time. In most cases, water vapor sorption in the pores is dominated by adsorption, with nanopores showing enhanced adsorption due to weaker inhibition of sidewall effects compared to an increase in adsorption sites. Notably, a substantial increase in equilibrium water sorption occurs when the matrix potential exceeds −106 MPa, corresponding to a relative humidity exceeding 96.6%, indicating strong capillary condensation. Additionally, nanoscale pores exhibit a pronounced propensity for capillary condensation when the pore throat diameter is less than 3.5 nm. These findings enhance our understanding of water sorption in tight rock formations when direct measurements are unavailable, with implications for estimating multiphase fluid transport in unconventional reservoirs.