Nano to Macro Scale Investigation into Low Salinity Waterflooding in Carbonate Rocks

Abstract

AbstractLow salinity waterflooding (LSWF) in carbonates has proven to improve oil recovery through both fluid-fluid and rock-fluid interactions. However, most of the experimental and modelling studies related to LSWF has been based on macroscale observations such as coreflooding analysis. In this work, a length scale approach from nano- to macro- scale was developed to investigate the underlying mechanisms associated with LSWF and how they impact improved oil recovery (IOR) at each scale.At the fluid-fluid interface, nanoscale characterization of the oil phase after encountering low salinity brine (∼2,000 ppm) showed the formation of water-in-oil micro-dispersions using Fourier transform infrared (FTIR) spectroscopy and environmental scanning electron microscopy (ESEM). Low interfacial tension (IFT) and high dilatational surface elasticity (DSE) at oil-seawater (∼33,000 ppm) interface resulted in more rigid oil-brine interface as compared to using both formation water (∼160,000 ppm) and low salinity brines. At the microscale, injection of seawater brine through a constricted pore throat suppressed oil snap-off and improved sweep efficiency. Microdispersions were also observed after injecting low salinity brine into oil saturated microfluidic devices during a waterflooding process, resulting in oil remobilization and improving the sweep efficiency.Reduced electrostatic bond attraction and repulsive disjoining pressure at the crude oil-brine-rock (COBR) interface observed at the nanoscale, contributed to wettability alteration from oil wet to intermediate wet as brine salinity reduced and in the presence of Mg2+ions on the limestone surface. Calcite dissolution accompanied with a spike in brine pH contributed to the reduced electrostatic bond attraction and impacted the wettability state of the rock. These factors at the nanoscale influenced the improved oil recovery at the macroscale through limestone wettability alteration.This project demonstrated that using a length scale approach provided a detailed understanding of the underlying mechanism influencing the observed wettability alteration and IOR in limestone rocks during LSWF.