Limited CO2 resources greatly narrow down the field application of CO2 EOR for improving oil recovery in tight formation. CH4, as a by-product of produced oil, is a relatively cost-efficient agent to be injected into the tight formation with CO2, especially, taking into account the similar EOR mechanisms, compared with CO2. In this work, an experimental and mathematical method is proposed aiming to probe the effect of CH4 composition on the mass transfer between a CO2-CH4 gas mixture and crude oil collected from a tight oil reservoir. Experimentally, the pressure-decay tests for different CH4-CO2-light oil systems are conducted at a constant temperature in a pressure / volume / temperature (PVT) setup. Also, the gas mixtures composition before and after the experiments are analyzed with gas chromatography in order to investigate the mass transfer of different components. Theoretically, mathematical formulations are developed to describe the mass transfer between the gas mixture and light oil based on translated Peng Robinson equation of state (PR-EOS), a real gas equation, and one-dimensional convection-diffusion equations. The individual diffusion coefficients of CH4 and CO2 as well as the concentrations distribution can be obtained with minimizing the deviation between the calculated pressure and the measured ones. The results indicate that a higher the content of CO2 in the initial gas phase, a faster the pressure drops are and less time it takes for the oil and gas phases to reach a stable pressure, which implies a high mass transfer rate with an increase in CO2 composition. In particularly, the diffusion coefficient of CO2 is found to be 4 times larger than that of CH4 under condition of same composition. However, it is noted that the individual diffusion coefficients of CH4 or CO2 are not constants. They will decrease with an increase in CH4 or CO2 composition in CH4-CO2-light oil systems, although a high concentration gradient can be achieved on the gas-liquid surface with a large composition. Accordingly, the optimal compositions of CH4-CO2 mixture are eventually gained and recommended.

The Effects of Gas Mixture Composition on the Mass Transfer and Individual Diffusion Coefficient in Ch4-Co2-Light Oil Systems

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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