Effect of Molecular-Diffusion Mechanisim on CO2 Huff-n-Puff Process in Shale-Oil Reservoirs

Abstract

Abstract Unconventional resources have played a significant role in changing oil industry plans recently. Shale formations in North America such as Bakken, Niobrara, and Eagle Ford have huge oil in place, 100-900 Billion barrels of recoverable oil in Bakken only. However, the predicted primary recovery is still low as less than 10%. Therefore, seeking for techniques to increase oil recovery in these complex plays is inevitable. In this paper, three different approaches have been combined to investigate the effect of molecular-diffusion mechanisim on CO2 huff-n-puff process in Bakken formation. Firstly, comprehensive review has been conducted on the reported experimental studies for diffusion mechanism of CO2 injection in shale cores. Then, numerical simulation methods were used to mimic CO2 performance in these poor-quality reservoirs. Different scenarios of these shale reservoirs conditions have been simulated, which in turn, generated different diagnostic plots of CO2 performance. Finally, these diagnostic plots have been fitted with some of the CO2 fields’ pilots which were performed in Bakken formation of North Dakota and Montana. This study found that the integration method of different tools such as experimental, simulation, and pilot tests is the proper technique to accurately diagnose the role of CO2 diffusion mechanism in these poor-quality reservoirs. This work concluded that there is a clear gap between lab-works conclusions and pilot tests performance, and this gap mainly happened due to the misleading prediction for the role of CO2 diffusion mechanism in field conditions by upscaling lab observations. However, pilot tests performance generally denied any significant role for diffusion mechanism on CO2 performance. The poor role for CO2 diffusion mechanism in Bakken formation happened due to that either of kinetics of oil recovery process in productive areas of these reservoirs are too fast or CO2 diffusion rate in field conditions are too slow. Therefore, upscaling the same CO2 diffusion rate, which has been obtained from injecting CO2 in chips of cores in lab conditions, to field scale needs to be reconsidered. This study explains how CO2 diffusion mechanism in unconventional reservoirs is different in macroscale level (field scale) from microscale level (lab conditions), and how that affects CO2 performance in improving oil recovery from unconventional resources.