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In order to generate the valid numerical simulation model, the sufficientamount of gathered data from the oil field is required. However, it is notalways possible to acquire such data at the initial stage of projectdevelopment. Buckley and Leverett (1942) developed the analytical solutionallowing to assess the oil displacement efficiency. One of the main assumptionsof this model is incompressibility of oil and injected fluid. For slightlycompressible water and oil such assumption is rational. However, that is notalways the case when the gas is injected. This research aims to identify theconditions at which the usage of the incompressible gas model is appropriate.Likewise, the cases when the model of compressible gas is required are alsoevaluated. To accomplish the goals of this research, the comparative analysisbetween the injection of compressible and incompressible gases was undertakenusing the numerical solution of the correspondent reservoir engineeringproblem. The validation of the numerical model was undertaken showing that itmatches the analytical Buckley-Leverett solution. The findings of this researchindicate that the relative and absolute density change with the pressure of theinjected gas has the profound impact on the convergence between two modelsunder consideration. With the increase in the injection pressure, thediscrepancy between the models of compressible and incompressible gas raisesfor all the considered injection fluids (CO2, CH4 and N2). Due to a steep slopeof 'density-pressure' curve for CO2 at low initial reservoir pressure, theincompressible model cannot accurately predict the oil displacement efficiencyby this gas at any reasonable injection pressure. All 1D results are alsorepresentative for 2D simulation. However, the mismatch between two modelsincreases considerably for 2D simulation scenarios.

Relevance of analytical Buckley-Leverett solution for immiscible oil displacement by various gases