Linking pore scale mechanisms with macroscopic to core scale effects in controlled ionic composition low salinity waterflooding processes

Abstract The controlled ionic composition low salinity waterflooding processes through tuning of salinity and ionic composition are lately becoming popular for oil recovery in carbonates. Most of the experimental work reported in this area has been limited to macroscopic and core scale measurements and these results consensually pointed out wettability alteration towards water-wet state to be the main recovery mechanism. It is widely believed that wettability is the macroscopic to core scale effect and the link between such effects with molecular scale interactions and phenomena occurring at both carbonate-oil/brine and oil-brine interfaces is not yet well understood. The current study aims to address this knowledge gap by acquiring data on various crude oil/brine/carbonate and crude oil-brine physicochemical interactions using several macroscopic, pore, and core scale experimental techniques. These techniques include contact angle, zeta potential, cryo-broad ion beam scanning electron microscope (BIB-SEM), sum frequency generation (SFG) spectroscopy, and core flooding relative permeabilities. Both high salinity water (HSW) and a controlled ionic composition low salinity water (CICLSW) containing sufficient amount of three key ions (sulfates, magnesium, and calcium) are used to delineate water chemistry effects on resulting physicochemical interactions and changes occurring at carbonate-oil/brine and oil-brine interfaces. The macroscopic contact angle data showed strongly oil-wet behavior with HSW and favorable wettability alterations towards water-wet conditions with CICLSW. The opposing polarity of zeta potentials is observed at calcite/brine and oil-brine interfaces with HSW, which can collapse the thin water film and cause the direct contact of crude oil on to the rock surface to result in oil-wetness. In contrast, CICLSW showed increasingly negative zeta potentials at oil/brine interface and the polarity reversal to negative zeta potentials across calcite/brine interface to stabilize the water-film on the rock surface and subsequently render water-wet conditions. The cryo-BIB-SEM imaging showed trapping of oil ganglia in larger pores and relatively less contact of crude oil with carbonate surface for CICLSW flooded sample. The phase distributions in the sample further confirmed less amount of oil trapped in carbonate pores, which provides a clear indication of CICLSW effect on wettability alteration and better oil mobilization in carbonates. It is also understood from sum frequency generation spectroscopy CH3/CH2 stretching results that CICLSW increased the conformation order of hydrocarbon chains to enhance the interactions at thin interfaces and ultimately alter the wettability. Significant increase in the intensities of free OH stretches and hydrogen bonding are observed at oil/water interface with CICLSW. Such result provides a direct indication that dissolved salt ions displace some water molecules at the interface, resulting in reduced free OH bond densities to correlate well with zeta potential drop observed in HSW. The shifts in relative permeability curves obtained by history matching the coreflood experimental data also confirmed the favorable effect of CICLSW in altering the wettability towards water-wet conditions. This study thereby establishes a good agreement between macroscopic to core scale effects and pore scale interactions occurring at carbonate-oil/brine and oil-brine interfaces to clearly demonstrate the beneficial effects of CICLSW on wettability alteration and oil recovery in carbonates.