Identification and characterization of dominant seepage channels in oil reservoirs after polymer flooding based on streamline numerical simulation

Abstract

Reservoir heterogeneity, water-oil mobility ratio, cementation degree, and strong injection and production significantly influence long-term waterflooding and polymer flooding processes in oil reservoir development, leading to the formation of low-resistance dominant seepage channels in the lower part of thick oil layers without internal laminations. Understanding the spatial distribution and evolution of dominant seepage channels in oil reservoirs during waterflooding and polymer flooding is crucial for improving reservoir development and recovery rates. It uses a streamlined numerical simulation method to quantitatively show the spatial and temporal distribution of dominant seepage channels in the oil reservoir. A model and chart were created to demonstrate the relationship between permeability and throat radius, capturing changes in reservoir permeabilities over time. The study developed a comprehensive mathematical model to identify dominant seepage channels, considering parameters such as inter-well flow ratio, injection efficiency, permeability change value, and water saturation. Research findings indicate a 14.8 % increase in reservoir permeability after polymer flooding, with slight changes in porosity and a 20.3 % decrease in clay content. The established mathematical model and identification criteria were used to comprehensively quantitatively evaluate flow channels, describing the distribution characteristics and evolution process of dominant seepage channels in the experimental area over four periods: pre-polymer flooding, low water-saturation period during polymer flooding, post-polymer injection, and subsequent 10 years of water flooding. Both polymer flooding and subsequent water flooding have significantly altered the distribution pattern of predominant fluid pathways: 254 well layers now demonstrate robust predominant fluid pathways (19 %); another 272 well layers feature moderately predominant fluid pathways (22 %); while an additional 440 well layers display weaker predominant fluid pathways (approximately 35 %). There has been a noticeable increase in both weak and moderate predominant fluid pathways, which are now more widespread in the area. However, localized occurrences of strong predominant fluid pathways persist with no significant change in their distribution characteristics. Analysis at four different time points shows a clear trend of increasing intensity as we move from polymer to subsequent water flooding in this decade. Additionally, examination along an axis from upper left to lower right reveals a linear progression that indicates clear connections among different categories of dominant seepage channel. The verification results show a strong correlation between the calculated dominant seepage channel distribution and actual water-injection profile test results, indicating that the method effectively identifies dominant seepage channels in oil reservoirs after polymer flooding and has the potential to enhance oil recovery further.