Solubility models of CH4, CO2, and noble gases and their geological applications

Solubility models for CH₄, CO₂, and noble gases are widely used in Earth Sciences, playing pivotal roles in the study of homogenization pressure of inclusions, paleoclimate variation, gas migration and accumulation, formation of helium-rich gas plays, and the volumetric ratio of gas to water in reservoirs. This paper reviews solubility models of CH₄, CO₂, and noble gases in pure water and aqueous NaCl solutions. Specifically, the models with high accuracy and wide applicability are introduced in detail: (1) CH₄ solubility model in aqueous solutions within the range of 0–250 °C, 0.1–200 MPa, and 0–6.0 mol/kg NaCl; (2) CO₂ solubility model in aqueous solutions within the range of 0–450 °C, 0.1–150 MPa, and 0–4.5 mol/kg NaCl; (3) Models for calculating the solubility and Henry's constant of atmospheric noble gases within 0–80 °C range; (4) Models for calculating the Henry's constant of noble gases in pure water; (5) Solubility models of noble gases in aqueous solutions within the range of 0–65 °C, 0.1 MPa, and 0–5.8 mol/kg NaCl. The paper also presents some calculated results obtained using these models. The solubility models of CH₄ and CO₂ are complex yet highly accurate, with a broad range of applications. In contrast, the solubility models of noble gases exhibit relatively lower accuracy and a narrower application range, necessitating corrections. In the noble gases-CO₂-H₂O system, low-density CO₂ has little effect on the solubility of noble gases, whereas high-density CO₂ significantly influences their solubilities. Currently, accurately evaluating the solubility of CH₄, CO₂, and noble gases in their mixtures proves challenging, warranting further research into solubility models for gas mixtures.