Modeling of laboratory gas flooding in tight chalk with different non-equilibrium treatments

This paper focuses on proper modeling of bypassed oil in tight chalk during gas injection, caused partlyby the small-scale heterogeneity and the non-equilibrium contact especially in low permeable chalk.Conventional compositional simulators using the local equilibrium assumption tend to predict excessivevaporization of the residual oil. We present the laboratory gas flooding results in tight chalk and discusshow different non-equilibrium treatments can provide more realistic simulation results. Composite core flooding experiments with low-permeable tight chalk and natural gas were conducted atdifferent pressures below the minimum miscibility pressure of the live oil used. The ECLIPSE compositionalsimulator E300, using an EoS model tuned with the swelling data, was used to history match the results. Itwas found that the simulation without considering non-equilibrium effects over-predicted the oil productionin the late stage. Two methods were tested to avoid the excessive vaporization of oil: the Sorm method(excluding the residual oil from flash calculations) and the transport coefficients (alpha factors) methodtogether with pseudo-relative permeability curves. Our results show that the sub-grid non-equilibrium effect is significant in tight chalk. Compositionalsimulation without considering this effect leads to unrestricted vaporization and over-prediction of theoil recovery in gas injection into tight chalk even for laboratory experiments. Both methods tested hereare suitable for reproducing the flooding results, in particular, the residual oil in the late stage. For theexperiments studied here, the Sorm method seems to show a better performance in maintaining no furthermass transfer between the residual oil and gas after the ultimate recovery is reached, since it excludes thebypassed oil fraction from flash calculations and models the immobile saturation explicitly. For the alphafactors method, oil production keeps a slow increase at the late stage as long as gas is being injected. Inaddition, the use of pseudo-relative permeability method can lead to obtaining irrational trends in somecases. We therefore propose an alternative method by adjusting the alpha factors of the mobile components,which avoids the difficulties of modifying the relative permeability curves.This study contributes to the methodology on honoring the non-equilibrium effects and obtaining realisticresidual oil saturation for gas injection in tight formation. The proposed method of adjusting the non-zero alpha factors can be used as an alternative to using pseudo-relative permeability, which avoids the possible drawbacks involved in this method.