Molecular dynamics of CH₄/CO₂ on calcite for enhancing gas recovery

The effect of temperature and pressure on the adsorption of CO₂ and CH₄ gases on calcite (104) has been studied by means of classical molecular dynamics. The results show that carbon dioxide greatly improves methane desorption in the 323–373 K range, even at low CO₂ concentrations. However, this effect is less pronounced for very high temperatures (423 K), where most of the methane is desorbed and CO₂ tends to desorb in large quantities. Radial distribution function (RDF) analysis reveals two distinct peaks for CO₂ (0.36 and 0.47 nm) and two for methane (0.87 and 0.57 nm) and the intensities of these peaks tend to decrease with increasing temperature. Such peaks are always clearly visible for CO_2, while the methane profile gets very broad already for mild conditions of temperature and CO₂ concentration. These results highlight how the CO₂ geometry of adsorption is well defined and characterized by strong interaction, while methane adsorption is quite loose and depicts a very dynamic picture. Focusing on the effect of pressure, RDF peaks intensities increase, although this effect is limited to the 1–5 MPa range. Moreover, the high CO₂ presence further decreases the effect of pressure on methane adsorption. In fact, from pure methane to 20/80 CO₂/CH₄, methane adsorption increases linearly with pressure. For gas mixtures with a CO₂ concentration higher than 40%, higher pressure has less impact on methane adsorption. Overall, the results obtained yield important details to tune the gas composition and conditions for efficient and enhanced natural gas recovery and sequestration of CO₂.