Pore-Size-Dependent Wettability and Multiscale Remaining-Oil Distribution in an Ultra-Thick Carbonate Reservoir: A Case Study of the MB1 Reservoir in the Middle East

Understanding the multiscale distribution of remaining oil in ultra-thick porous carbonate reservoirs is essential for improving recovery during the middle-to-high water-cut stage. This study investigates the MB1 reservoir of H Oilfield in the Middle East by integrating core-scale physical experiments with reservoir-scale saturation-field analysis. Waterflooding experiments on six selected cores, supplemented by NMR measurements on three cores and qualitative CLSM observations on two cores, characterized displacement and oil retention within the tested samples. The three NMR-tested cores exhibited a consistent pore-size-dependent sequence comprising water-wet micropores, predominantly water-wet small pores, predominantly oil-wet medium pores, and oil-wet macropores. The wettability sequence was associated with distinct displacement behavior among pore classes. More than 70% of the pore-classified remaining oil occurred in micro- and small pores. The oil persisted mainly as isolated, blind-end, film-like, and corner-associated forms. Reservoir-scale saturation maps and cross-sections were then extracted from previously history-matched models for three architecture zones at a common development time. The model-derived saturation fields were classified into three reservoir-scale patterns: interlayer contrast, band-margin enrichment, and reservoir-interior enrichment. The independently obtained core-scale observations and reservoir-scale model outputs were then compared interpretively. Within the MB1 case study, depositional architecture and connectivity were associated with the modeled distribution of reservoir-scale sweep, whereas the core experiments indicated pore-size-dependent displacement and oil retention within the tested samples. This comparison provides a case-specific distinction between unswept oil and microscopically retained oil.