Multiscale Water Ion Interactions at Interfaces for Enhanced Understanding of SmartWater Flooding in Carbonates
Authors: Ayirala Subhash , Saleh Salah , Enezi Sultan , Yousef Ali 
Published: April 2019

In this study, we summarize and discuss the data reported from a series of multiscale experiments to explore the interactions of salinity and water ions at both fluid-fluid and rock-fluid interfaces to understand the pore scale mechanisms responsible for oil recovery in SmartWater flooding. These experimental data on various crude oil/brine/carbonate and crude oil-brine physicochemical changes/effects at elevated temperatures were obtained using a variety of static and dynamic techniques at different scales ranging from atomic-molecular-macroscopic scales. The techniques include surface force apparatus (SFA), cryo-broad ion beam scanning electron microscope (BIB-SEM), zeta potentials, microscope based oil liberation, interfacial shear rheology, and integrated thin film drainage apparatus (ITFDA). The salinities of brines were varied from zero salinity deionized (DI) water to higher salinity injection water in addition to changing the individual ion compositions.

The integration of results obtained from these different multiscale experiments showed that both salinity and individual water ions play a major role not only to determine the oil release from rock surface due to the interactions at rock-fluids interface, but also to impact released oil ganglion dynamics for efficient oil mobilization through the interactions at fluid-fluid interface. The key findings can be summarized as the following: (1) At zero salinity, unfavorably much higher adhesion as well as stronger rigid films to adversely impact crude oil droplets coalescence were observed at rock-fluids, and fluid-fluid interfaces, respectively; (2) An optimal lower salinity containing sufficient amount of sulfate ions is necessary to cause nano-scale ion exchange at the rock-fluids interface that changes the surface charge/potential to favorably alter adhesion and microscopic contact angles for efficient oil release from rock surface; (3) An adequate salinity containing higher amounts of magnesium and calcium ions is desired to form less rigid films at the fluid-fluid interface that promote the coalescence of released oil ganglia for effective mobilization. Based on these novel findings, SmartWater can be defined as a tailored water containing certain salinity and selective composition of three key ions including sulfates, magnesium, and calcium. It must contain lower amounts of monovalent ions and should have the right balance of the three key ions to enable favorable interactions at both fluid-fluid and rock-fluids interfaces and result in faster as well as higher oil recoveries in carbonates.

The analysis on multiscale water ion interactions at both the interfaces performed in this study also sheds the important learning point that not every low salinity water can become a SmartWater for carbonates. These new learnings and the novel knowledge gained would provide useful practical guidelines on how to design optimal injection water chemistries for SmartWater flooding projects in the field.