Water Vapor Adsorption Behavior Evolution of Marine Kerogen across Maturity from Carbonization to Graphitization Stages: New Insights Based on Thermodynamic and Kinetic Characterization

Abstract

The water adsorption behavior of marine overmature kerogen is a theoretical foundation for the shale gas exploration and development and CO2 geological sequestration. However, this issue remains poorly understood, especially regarding graphitization effects. This study investigates water vapor adsorption (WVA) behavior for a suite of Lower Cambrian kerogen samples with a maturity range of 3.23–3.98% EqRo (equivalent vitrinite reflectance) under different temperatures and relative humidity (RH) conditions through thermodynamic and kinetic models. The results indicate that, over the maturity range investigated in this study, the WVA capacity of kerogen is closely related to the maturity-governed structural evolution. With increasing maturity, the adsorption capacity first decreases progressively, then increases markedly, with the turning point of 3.44–3.55% EqRo, corresponding to the graphitization threshold. Thermodynamic analysis reveals that the affinity of kerogen to water vapor and the spontaneity of the WVA process significantly enhance as maturity across this EqRo value. Furthermore, the driving force for secondary adsorption undergoes a fundamental transition from “entropy reduction control” to “enthalpy-driven dominance”. This variation may be associated with the attenuation of residual polar sites in kerogen, the development of graphitic structures, and the restructuring of the pore network. The double-exponential model exhibits the best compatibility with variations in RH, temperature, and maturity. The kinetic behavior of WVA in response to maturity also exhibits a similar inflection as well as RH dependence. At low RH, it is predominantly governed by the evolution of surface chemical properties, whereas at high RH, it is controlled by the evolution of pore structure. This work elucidates the enhancing effect of graphitization on adsorption behavior from both thermodynamic driving forces and kinetic mass-transfer pathways.