An oil recovery factor-based method for determination on the lower limit of pore-throat for waterflooding in low-permeability reservoirs

Waterflooding serves as a major secondary tight oil recovery technique due to its merits of low risk, cost-effectiveness, and easy operation. The lower limit of pore-throat is one of the most vital metrics for oil and gas resource potential and productivity evaluation. However, the existing determination methods cannot directly reflect reservoir performance during the production process. In this paper, a new method was proposed by introducing the oil recovery factor (RF) as a criterion to intuitively determine the lower limit of pore-throat for waterflooding in low-permeability reservoirs. Three tight sandstone core samples with representative permeabilities were studied. High-pressure mercury injection and nuclear magnetic resonance measurements were employed to characterize the pore-throat size distribution and the transient variation of oil–water distribution in waterflooding experiments. The lower limits of pore-throat for waterflooding of core #1 (0.128 mD), #2 (0.490 mD), and #3 (0.950 mD) were found to be 43, 46, and 46 nm, respectively. Cores with a higher permeability demonstrated lower overall RFs, which is contrary to expectations. According to the pore-throat size distribution categorized by the lower limit, this observation can be attributed to the increased proportion of unrecoverable region at higher permeabilities, suggesting that permeability, as a macro indicator, might fall short in representing the actual microstructure of flow pathways within a reservoir. Microscopic visualization of the waterflooding experiment was also conducted to intuitively observe the distribution and morphology of residual oil in both unrecoverable and recoverable regions. Our work provides a new method for accurately determining the lower limit of pore-throat from oil productivity perspective and offers data support and a guidance for effective waterflooding exploitation of tight oil reservoirs.