Dynamic evolution of residual oil during long-term waterflooding in Middle East carbonate reservoirs: A pore-scale investigation

Long-term water flooding (LTWF) is an efficient way to improve oil recovery (EOR) in carbonate reservoirs in the Middle East. Due to the complex depositional environment and intricate pore-throat structures of carbonate reservoirs, the development characteristics differ significantly from those of conventional sandstone reservoirs. While the mechanisms of LTWF in carbonate reservoirs are well-documented, there remains a significant gap in understanding the microscopic pore-scale displacement characteristics and the dynamic evolution of residual oil. To address this, nuclear magnetic resonance (NMR) and computed tomography (CT) scanning techniques were employed to investigate the behavior of LTWF across various carbonate rock samples. Initially, NMR technology was utilized to elucidate the pore-throat displacement characteristics at the microscopic level for different core samples under LTWF. Subsequently, CT scanning was applied to explore the dynamic evolution of microscopic residual oil and to categorize the types of residual oil based on their formation mechanisms. We found that LTWF predominantly utilizes oil within microscale pores of 1–10 μm and > 10 μm. As the volumes of injected water increase, there is a noticeable improvement in oil displacement within submicron pores (0.1–1 μm). However, residual oil primarily accumulates in nanopores (< 0.1 μm) and submicron pores. The study identified five distinct types of microscopic residual oil: clustered, throat, droplet, corner adsorbed, and pore lining. Notably, the transformation of residual oil in dolomite cores generally shifts from clustered to throat forms, while in limestone cores, it transitions from clustered to predominantly corner adsorbed and pore lining configurations. This nuanced understanding of oil utilization and residual categories under LTWF offers valuable insights into optimizing EOR strategies in complex carbonate reservoirs.