Analytical Investigation of Wettability Alteration Effects on Residual Oil Saturation in Mixed-Wet Niger Delta Carbonate-Clastic Reservoirs Using Core Flooding Experiments

Wettability is a fundamental reservoir property controlling the distribution and recoverability of residual oil saturation after water flooding, and its spatial heterogeneity in mixed-wet Niger Delta reservoirs creates significant uncertainty in waterflood recovery predictions. This study analytically investigates wettability alteration effects on residual oil saturation through a systematic series of core flooding experiments on preserved reservoir core samples from two mixed-wet formations in the eastern Niger Delta. Twenty-four core plugs were characterised for initial wettability using the Amott-Harvey and USBM wettability indices. Each core was subjected to primary drainage, spontaneous imbibition, and forced displacement sequences using synthetic formation water and dead crude oil. Wettability alteration from mixed-wet to water-wet conditions was induced by surfactant treatment with a fluorocarbon surfactant system, and the imbibition-drainage cycle was repeated post-treatment to quantify changes in relative permeability curves and endpoints. Results indicate a mean reduction in residual oil saturation of 9.2 saturation units following wettability modification, with the largest improvements in cores exhibiting initial Amott-Harvey indices between minus 0.1 and minus 0.3 indicative of mild oil-wetness. Relative permeability crossover point shifts of 12 to 18 saturation units confirm the wettability change mechanism. Pore network modelling using extracted micro-CT tomography pore structures validates the experimental observations. Implications for surfactant-assisted waterflood design in mixed-wet Niger Delta reservoirs are discussed. Keywords: wettability alteration, residual oil saturation, core flooding, Niger Delta, surfactant.