# Compositional Modeling of Dimethyl Ether-CO2 Mixed Solvent for Enhanced Oil Recovery
## 1 Introduction  
As the discovery of new oil fields becomes increasingly difficult, most oil companies are focusing on maintaining economic oil prices through enhanced oil recovery (EOR) technology to maximize the recovery from oil fields [1]. Solvent injection is a mature EOR method [2]. CO2 and hydrocarbon gas are widely used as injection fluids [3], wherein their contact with reservoir fluid causes viscosity reduction and oil swelling. The effect is greater under miscible conditions than immiscible conditions [4]. Research using hydrocarbon gas or CO2 as a solvent has been conducted, from the laboratory to the field, for decades [3, 5, 6]. Additionally, research is being conducted to improve oil recovery and economic efficiency using a mixed solvent of CO2 and hydrocarbon gas or a surfactant [7, 8, 9, 10]. In particular, according to previous studies that added liquefied petroleum gas (LPG) to CO2 flooding, LPG increases the displacement efficiency by reducing the minimum miscible pressure (MMP), which accelerates the effects of oil viscosity reduction, interfacial tension (IFT) reduction, and oil swelling, resulting in an increase in oil recovery [6, 11, 12]. Dimethyl ether (DME), an industrial chemical used as a fuel additive and aerosol propellant, is being investigated as a solvent for EOR. Recently, DME was mixed with LPG and used as fuel for transportation as well as for cooking and heating fuel [13, 14]. As a solvent, DME has the advantage of being soluble in both water and oil. DME can be mixed nearly ideally in the oleic phase and is partially soluble in water because of its slight polarity [15]. These properties enable a unique and improved oil recovery mechanism. DME remaining near the injection well is extracted by the chasing water, which acts as a DME carrier, improving the oil mobility near the production well [13]. Additionally, DME can be extracted fromthe produced fluid and reused through distillation and adsorption processes [14]. DME was first introduced as a solvent for EOR by Shell [15; 16; 17; 18], and this method has proven effectiveness in improving oil recovery [14; 15; 16; 19; 20; 21]. Recent research on DME in the field of EOR has focused on modeling and experiments of the DME-water-oil phase behavior or studies modeling the core scale [15; 16; 21; 22; 23; 24; 25]. Additionally, most of the EOR using DME has been studied for the DME-enhanced water flood method, and to the best of our knowledge, no studies have been reported on a CO\({}_{2}\)-DME mixed solvent. Though DME has been proven as an effective solvent in improving oil recovery, it is an expensive solvent around $565/m\({}^{3}\)[26]. Therefore, the optimum amount of DME into the CO\({}_{2}\) flood should be determined through extensive compositional simulations to improve the economics of the project. Therefore, this study evaluated the use of a mixed solvent to reduce DME and increase the oil recovery rate by adding DME to the CO\({}_{2}\) flooding. In addition, the amount of DME that was recovered by chased water was examined with compositional analysis of the produced fluid. In this study, a compositional model was developed to investigate oil recovery when injecting a CO\({}_{2}\)-DME mixed solvent under the WAG scenario. Based on the results from simulations, the sweep efficiency and displacement efficiency by WAG using a CO\({}_{2}\)-DME mixed solvent were analyzed in terms of oil viscosity, viscous gravity number, and MMP.