Interactions of Ions at Fluids Interfaces in Carbonate Reservoirs: Molecular Studies

Summary The advanced waterflooding technologies through salinity and ionic content adjustment can make favorable impacts on rock wettability and oil recovery. Reducing the injection water salinity enhances the physiochemical interactions with carbonate rock sites and oil components. Therefore, multiple mechanisms are proposed in literature including rock dissolution, adsorption of ions, zeta potential alteration, diffusion from macro to micro pores and others. Most of the applied macroscopic and microscopic technologies have certain limitations in characterizing the interfacial interactions in-situ at molecular scales. In this paper, Zeta Potential and advanced Sum Frequency Generation (SFG) spectroscopy are utilized to study the electrokientics behaviors and capture any subsequent changes in the chemical structure at interfaces. Different brines recipes and model oil are tested to determine the effects of ionic strength on the monolayer structures. Stearic acid is also mixed with hydrocarbons to mimic the acidity condition of fluids in the reservoir. The samples are pre-equilibrated with connate formation brines and different wetting condition, water-wet and oil-wet, are applied. The change in the chemical structure is monitored with time at a broad wavenumber range from 1,400 to 3,800 cm-1. Distinct spectral signatures of oil components and water ions are detected to define the correlation between surface charge alternation and chemical structure. Zeta potential of calcite particles and oil droplets are monitored at moderate temperature condition. The SFG data is compared with the electrokinetic results to predict the components that are highly affected during waterflooding processes. This study brings new insights on understanding the chemical structures at the thin monolayers at different aqueous interfaces. The measured spectra, coupled with a wide range of laser polarization settings, and signal intensity trends are discussed in terms of composition, and structure of organic and inorganic components. For example, the intensity for SmartWater at certain wavenumber significantly affected by the monovalent concentration. Similarly, the zeta potential magnitude decreased as decreasing the sodium chloride concentration. This indicates that the interactions at oil/water interfaces are enhanced at lower ionic strengths. These findings are also confirmed with similar behaviors for oil wet particles only. The novelty of this interfacial study can provide better understanding of the reaction mechanisms altering the ionic strength and salinity of injection water and its impact due to the changes in geometric interfaces. Such understanding is also crucial to optimize the chemistry of injection water and its interaction with oil components and carbonate rock, to ultimately alter wettability toward water-wet.