In situ fluid and injection gas compatibility of huff-n-puff gas EOR in a low-permeability shale-sand-carbonate reservoir

Abstract

A sufficient number of choices are available when it comes to injection gas for enhanced oil recovery (EOR) application in unconventional reservoirs, such as CO 2 and field gas to name a few. Most likely, these gases have various degrees of compatibility with the in situ reservoir fluid from the oil recovery increase standpoint. In some cases, they might not even be compatible at all with each other. Instead of creating a better oil extraction, the introduction of the gases could diminish the oil recovery. It has been observed that some gases have exceptional ability in increasing the oil production by decreasing the oil viscosity while moving towards completely vaporizing the oil phase as the gases’ molar fractions increase in the mixture. The optimum injection gas composition is the one that returns the lowest oil and gas viscosity while continuously vaporizes the oil phase at the same time. For instance, pure C 2 is the optimum injection gas composition for the in-situ reservoir fluid used in this study. The significant increase in oil recovery given by the optimum injection gas justifies such injection fluid composition optimization attempt proposed by this study. Furthermore, the injection fluid composition should be among the first to be optimized, before other operational parameters such as huff-n-puff cycle durations. • Some gas types increases the oil recovery of a particular unconventional reservoir better than other gas types. • A good injection gas choice not only lowers the oil viscosity, swells its volume, and vaporizes its heavy components. • A good injection gas choice should also shift the mixture P-T diagram favorably. • A good injection gas choice should therefore in theory be able to vaporize the reservoir oil completely.