Pore-Scale Variations of Remaining Oil in Fractured-Porous Volatile Reservoirs under Associated Gasflooding

Summary Waterflooding is an effective method to enhance oil recovery in carbonate reservoirs. However, due to the complex depositional environments, highly developed fractures, and intricate pore-throat structures of carbonate reservoirs, waterflooding often exhibits inefficient performance. Volatile oil reservoirs, with their abundant associated gas resources, can benefit from associated gasflooding, which has become an economical and effective enhanced oil recovery (EOR) method under favorable conditions. Although the mechanisms of associated gasflooding have been well-documented, there remains a significant gap in understanding of the microscopic remaining oil production characteristics and the occurrence states of remaining oil during associated gasflooding. To address this issue, we used nuclear magnetic resonance (NMR) and cryogenic thin-section fluorescence analysis (CTFA) techniques to investigate the microscopic remaining oil production characteristics and the occurrence states of remaining oil during associated gasflooding. Initially, NMR was used to study the microscopic remaining oil production characteristics of cores under different reservoir types, fracture orientations, and gas injection rates. CTFA was then applied to examine the occurrence states of microscopic remaining oil. Based on the formation mechanisms, the remaining oil is classified into different types. We found that waterflooding primarily produces volatile oil in microscale micropores (1–10 μm), with a maximum recovery of 20.2%, while volatile oil in nanopores (<0.1 μm) remains largely nonproductive. Associated gasflooding not only improves oil recovery in existing microscale micropores but also enhances oil production in submicron pores (0.1–1 μm). Through this study, we identified four types of microscopic remaining oil, which are clustered, throat, droplet, and pore lining adsorbed. Notably, associated gasflooding has a limited effect on altering the morphology of deeply trapped oil in the matrix, with remaining oil predominantly occurring in a clustered form. This phenomenon becomes more pronounced with increasing fracture aperture. This nuanced understanding of remaining oil under associated gasflooding offers valuable insights into optimizing EOR strategies in fractured-porous volatile reservoirs.