Pore- and Core-Scale Mechanisms Controlling Supercritical Cyclic Gas Utilization for Enhanced Recovery under Immiscible and Miscible Conditions in the Three Forks Formation

Abstract

Tight formations such as the Three Forks Formation in the Williston Basin have very low primary recovery factors (typically 10% or less), which potentially leave billions of barrels stranded in the reservoir. According to the results of gas enhanced oil recovery (EOR) pilot tests that have been conducted in the Bakken Formation, cyclic gas injection appears to be a promising strategy for increasing oil recovery in unconventional reservoirs. Despite the pilot tests and the reported results in Bakken shale, Middle Bakken, and Three Forks Formations, little is known about the mechanisms of cyclic gas injection and the behavior between the injected gases and Three Forks reservoir fluids. In this paper, a series of experiments are applied under different constraints. The parameters that are examined include the effect of soaking time under immiscible and miscible conditions, the effect of fluid state (vapor, supercritical), injection pressure, selected gases (CO2, ethane, and propane), target formations (Upper Three Forks (UTF) and Middle Three Forks (MTF)), and the effect of pore size distribution on the recovery factor. During the Huff-n-Puff process, the nuclear magnetic resonance (NMR) technique was used to analyze the microscopic oil production and the micro residual oil distribution before and after CO2 injection. The results showed that the soaking time enhanced the recovery factor dramatically in both formations under and near the minimum miscibility pressure (MMP) of CO2, while the soaking time was less efficient in cases with injection pressures above the MMP. Propane proved to be the most effective gas at low injection pressure, followed by ethane, with CO2 being the least effective. On increasing the pressure, the performance of ethane and CO2 increased due to the sufficient change in the ethane and CO2 densities. Additionally, according to the NMR tests in UTF, micropores, mesopores, and small pores were predominant in oil extraction during the first cycle. After a few Huff-n-Puff cycles, oil in micropores becomes the primary source of oil extraction. However, in MTF, the dominant pore distributions were mesopores, small pores, and fewer micropores than in UTF. The findings in this study provide new insight to better understand the mechanisms of gas HnP enhanced recovery in the Three Forks Formation, which is of great significance for the efficient hydrocarbon exploitation and greenhouse gas utilization in shale reservoirs.