A brief juxtaposition of the effects of combining rules on acid gas geosequestration at a constant temperature

Abstract

Acid gases are hazardous to humanity, industries, the environment, and climate at large and their effects must be blunted as a matter of some urgency in mass-producing fields such as oil and gas for operations to be safely conducted. Diverse means of removal and safe storage exist, for which injecting these acid gases into geological formations (geosequestration) is an attractive one due to its economic viability and environmental friendliness. Some of these geological storage choices include depleted hydrocarbon reservoirs and deep saline aquifers. However, many interactions are in play during geosequestration and the one of interest in this work is the fluid-fluid interaction between the formation fluid (water) and injected fluid (acid gas). Various parameters are key to determining the safety, security, and containment of the injected plume to forestall leakage. Some of these parameters are interfacial tension, absorption, and dissolution of the injected fluid into the bulk water and adsorption of the injected fluid onto the interface of the formation water. Due to the extreme pressure and temperature circumstances, parameters may be hard to secure directly from the reservoir. Therefore, an indirect but reliable method of procuring these important parameters is required. Molecular dynamics offers a mechanism to model reservoir conditions and gain invaluable insight into the nature of interactions between the fluids involved. The modelling of molecular fluids can vary considerably depending upon several factors. Pairwise combining rules is one such factor, and in acid gas geosequestration, however, the influence of the effect of combining rules in the modelling of these fluids has garnered little attention thus far. In this work, we investigated the impact of three of the most common combining rules – Lorentz-Berthelot (arithmetic), geometric and the sixth power – and compared to available experimental data. We found that while some of the parameters compared showed only marginal statistical dissimilarities such as interfacial tension and radial distribution function, on an important front such as density both arithmetic and geometric mixing rules provided results closer to the experimental data. For simple molecules, we conclude, arithmetic and geometric mixing rules should suffice, insofar as the modelling is not for rare gases.