Miscibility Study of Flue Gases for a Coupled CCS-EOR Project

Abstract

Abstract Miscible hydrocarbon (HC) rich gas injection is a common EOR technique included in the development plan of many Abu Dhabi oil fields. This technology, that provides a very high oil recovery, requires the supply of a gas that is very valuable for other uses or its commercialization. Utilization and storage of flue gases rich in CO2 and N2 can play a key role in lowering the overall reservoir emissions by coupling carbon capture and storage (CCS) with EOR techniques. Thus, the substitution of hydrocarbon gas by these gases is of great environmental and industrial interest. Nevertheless, the change of the injection gas is not straightforward and requires a previous study to ensure the gas miscibility. The aim of this piece of research is to estimate the minimum miscibility pressure (MMP) of the studied gas, in order to clarify if miscibility conditions are achieved at reservoir pressure. Thus, a comprehensive study was performed applying three different approaches to predict/estimate the MMP: Empirical correlations from the literature based on experimental results. Experimental measurements with Slim Tube tests. Computer modeling based on equations of state for phase-behavior calculations. Miscibility of CO2 and N2 was successfully investigated through the three approaches. In the case of CO2, miscibility at reservoir pressure was totally ensured at 120°C since MMP was estimated to be the bubble pressure of the studied oil, more than 100 bar below reservoir pressure. Otherwise, the conclusion of the nitrogen miscibility studies is that N2 gas flooding is not a feasible EOR technique since MMP is far above reservoir pressure. Before completely discarding the use of nitrogen, miscibility of mixtures of HC rich gas and N2 was first studied by PVT simulations. Thermodynamic modelling was compared against experimental Slim Tube tests evaluating HC+N2 mixtures. Laboratory work led to conclude that mixtures with 42% of N2 could be injected at miscible conditions and revealed that simulations overestimate the minimum miscibility pressure of the studied gas mixtures. Finally, miscibility of CO2 and N2 mixtures with typical compositions of flue gases from post-combustion processes were studied using the thermodynamic model retuned with previous experimental measurements. This work presents a thorough study of minimum miscibility pressure of CO2 and N2 in oil from Abu Dhabi, which is the first step of any EOR project coupled with CCS. The developed methodology covered the three different approaches and the results provide a broad comparison amongst correlated, measured and simulated MMP.