Pore-Scale Fluid Saturation and Oil Trapping in Heterogeneous Carbonates: Insights from Brazilian Pre-Salt Reservoirs

Summary In this study, we investigate fluid saturation and oil trapping mechanisms at the pore scale in a highly complex carbonate sample from the Barra Velha Formation, Santos Basin, Brazil. For this purpose, we performed drainage and imbibition cycles followed by X-ray imaging to evaluate the impact of sedimentological features, such as primary and secondary textures and minerals, on fluid saturation. Under the applied conditions, the sample behaved mixed-wet in macropores and water-wet in micropores. Rock texture was a critical factor influencing fluid distribution during primary drainage and forced imbibition. Both irreducible water and residual oil were predominantly trapped in intracrystalline pores, where narrow pore throats limited connectivity and accessibility. During primary drainage, oil initially behaved as a nonwetting phase, transitioning to oil-wetting at higher pressures in macropores, while its entry into smaller pores remained restricted. Aging and spontaneous imbibition caused fluid redistribution, with water returning to tighter regions, especially in dolomite- and quartz-rich areas. Oil trapping was primarily dictated by rock texture, with minor influence from mineral species on local trapping. This behavior is likely due to the dominance of calcite, which displayed a higher affinity for oil during aging and spontaneous imbibition. Imaging secondary drainage revealed complex wettability patterns: smaller pores retained their water-wet behavior, while macropores exhibited mixed conditions. Mineral precipitation, recrystallization, and dissolution contributed to surface roughness, promoting water-wet conditions in some pores, resulting in highly heterogeneous saturation patterns. These findings provide detailed insights into fluid behavior in complex carbonate systems, emphasizing the interplay between rock texture, wettability, and fluid distribution, with implications for carbonate reservoir characterization and enhanced oil recovery.