A diffusion-based compositionally-extended black oil model to investigate produced gas re-injection EOR in Eagle Ford

In this study, an in-house reduced black oil simulation model is used to investigate cyclic produced gas rejection in Eagle Ford shale formation. Molecular diffusion may play a crucial role in mass transfer in tight shale formations. However, the diffusion has not been incorporated in a black-oil type model in previous studies. In this study, the diffusive term is included into the governing equation and its effect on production performance is examined by using a black oil model. Diffusion coefficients are calculated by using Sigmund correlation, which makes them a function of phase compositions, phase properties, pressure, and temperature. Considering that phase compositions are influenced by large gas-oil capillary pressure nanoconfinement effect, diffusion coefficients are also altered based on this affect. This is the first study that incorporates the large gas-oil capillary pressure effect on diffusion coefficients in nanopores, through using a reduced black oil model. The operation parameters including initial injection time, injection rate, number of cycles are examined. We found that larger injection rates and more number of cycles are efficient in improving the oil recovery. Injection rate is the most crucial parameters in cyclic gas injection approach, followed by cycle numbers. Huff-n-puff results in good efficiency at relatively tight matrix, however, it is not always more efficient for very low permeabilities, as for the very tight formation, the injection gas cannot penetrate to the deeper formation to bring out much more oil from the reservoir in the same "huff" and "soaking" time period. The molecular diffusion plays an important role in gas injection process and cannot be ignored. When nano-confinement effect is included, the diffusion coefficient in gas phase is decreased and the diffusion coefficient in oil phase is increased as pore size decreases, but the effect is not strong especially at relatively high pressures.