A Novel Algorithm for Modelling Gas–Oil Dynamic Interfacial Tension (IFT) and Component Exchange Mechanisms

Interfacial tension (IFT) between two immiscible phases is a key parameter in various oil and gas industries, especially in enhanced oil recovery (EOR) and Carbon dioxide capture and storage (CCS). There are several laboratory methods for measuring IFT, of which the pendant drop method is one of the most commonly used. This method can be used in both thermodynamic equilibrium and dynamic approaches. For a more complete study of IFT, dynamic pendant drop modeling can be used to investigate the process of component exchange between two phases to determine the mechanism of thermodynamic equilibrium. For this purpose, a novel computational algorithm is presented that calculates IFT under dynamic (non-thermodynamic equilibrium) conditions at different time intervals, where each time step is separately considered in equilibrium. Vapor–liquid equilibrium (VLE) calculations were performed using the Peng-Robinson equation of state (PR-EOS), and the IFT was calculated using the Parachor model. The power parameter of the proposed Parachor model was also considered a matching parameter and was calculated using the fit of the model and the experimental data. Over time, the component exchange between oil and gas increases, thereby reducing the IFT. This decreasing process of IFT continues until it reaches a constant (thermodynamic equilibrium) value. In each time step, the exchangeable components between the two phases are calculated, and their transfer directions are determined. The results show that the component exchange rate between the two phases differed at any time. However, the process of intermediate component exchange between the two phases was intense at the beginning of the experiment, but gradually, as time passed and components were exchanged between the two phases, the component exchange rate decreased. This ultimately reduces the average molecular weight and viscosity of oil over time, which is one of the goals of injecting gas into oil reservoirs. Therefore, the proposed algorithm can determine the process of changes in the composition of oil and gas, as well as the properties of oil, to reach two-phase thermodynamic equilibrium. For the oil and gas composition used in this paper, the equilibrium IFT decreased by an average of approximately 31% compared to the first contact due to component exchange. The oil viscosity and molecular mass also decreased by an average of about 39% and 23%, respectively. These results justify the use of rich gas as an injection gas because of the increase in oil mobility during the gas injection process. Thus, the proposed algorithm can be effectively used in gas injection studies into oil reservoirs to accurately identify the mechanisms under different reservoir conditions.