Wettability Alteration During Co-current and Counter-current Spontaneous Imbibition of Low Salinity Water in Naturally Fractured Reservoir

Abstract

Naturally fractured reservoirs (NFRs) present significant challenges for fluid flow characterization due to their inherent heterogeneity and complex fracture-matrix interactions. A key mechanism governing fluid exchange in such systems is spontaneous imbibition (SI) - the capillary-driven movement of fluids from fractures into the porous matrix. This process manifests in two dominant modes: co-current spontaneous imbibition (COSI) and counter-current spontaneous imbibition (COUSI), determined by the direction of fluid movement relative to boundary conditions. Analytical descriptions of these imbibition processes rely on diffusivity coefficients that combine relative permeability and capillary pressure parameters. Solutions to the resulting equations can be obtained through perturbation techniques. To validate these models, this study compares numerical predictions with experimentally observed in-situ water saturation profiles, acquired through X-ray computed tomography (CT) imaging. The validation has been performed using spontaneous imbibition tests of co-current and counter-current flows in air-brine system for diatomite rock. The validation showed a good agreement between experimental and numerical data, achieving an average relative error of approximately 10%. The study investigates low-salinity water (LSW) injection as a method to alter wettability toward more water-wet conditions and enhance recovery. Oil-saturated carbonate cores were subjected to imbibition experiments in Amott cells using both high salinity water (HSW) and LSW. Six carbonate core samples were analyzed under controlled boundary conditions, with water saturation profiles monitored at multiple time steps using CT scanning. Experimental results reveal a pronounced improvement in water uptake and imbibition depth under LSW conditions. Cores exposed to low salinity brine exhibited higher and more uniformly distributed water saturations, reflecting enhanced wettability alteration and deeper fluid penetration. Conversely, HSW tests showed lower saturation levels with sharper fronts, indicating limited wettability change and reduced displacement efficiency. These trends were corroborated by numerical simulations, which aligned closely with experimental observations, particularly in LSW scenarios. The research introduces an integrated experimental and numerical modeling approach to comprehensively examine wettability alteration mechanisms during SI in oil-wet carbonate reservoirs. Unlike conventional methodologies focusing merely on final oil recoveries or endpoint saturations, this study employs high-resolution X-ray CT scanning to dynamically visualize and quantify evolving water saturation profiles during both COSI and COUSI. A novel comparative analysis between LSW and HSW imbibition processes highlights substantial differences in saturation front propagation and fluid distribution patterns attributable to dynamic wettability alteration that can be modeled by adjusting key parameters such as relative permeability and capillary pressure throughout the imbibition process. This combined experimental-numerical approach provides unprecedented detail into the temporal and spatial dimensions of wettability alteration induced by LSW. Ultimately, this research delivers critical insights and proposes a robust framework for evaluating and optimizing low-salinity EOR strategies, significantly enhancing predictive capabilities and operational outcomes in fractured carbonate reservoirs.