Abstract
The advanced waterflooding technologies through salinity and ionic content adjustment can make favorable impacts on rock wettability and oil recovery. In carbonate reservoirs, SmartWater at low ionic strength showed strong chemical interactions with carbonate minerals and oil components. As a result, several hypotheses are proposed in literature as ionic exchange, rock dissolution, surface charges and others. The applied macroscopic and microscopic technologies have certain limitations in identifying the structures at interfaces especially at monolayers. In this paper, advanced Sum Frequency Generation (SFG) spectroscopy is utilized for the first time to characterize the chemical structures of molecules at the brine/oil interfaces. Different brines recipes and model oil are tested to determine the effects of individual and combined ions on the monolayer structures. Stearic acid is also mixed with hydrocarbons to mimic the acidity condition of fluids in the reservoir. The change in the chemical structure is mo nitored with time at a broad wavenumber range from 1,000 to 3,800 cm-1. Distinct spectral signatures of oil components and water ions are detected at different pH conditions. The SFG data is compared with the previous macroscopic wettability results to predict the components that are highly affected during waterflooding and enhanced oil recovery (EOR) processes. This study brings new insights on understanding the chemical structures at the thin monolayers of flat and curved geometric 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 is three folds higher when compared to high salinity water. This indicates that the interactions at oil/water interfaces are enhanced at lower ionic strengths. In addition, these findings are also confirmed with similar behaviors at a higher salinity brine as connate formation brine. 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.
Highlights
Over the last decade, laboratory studies, single well chemical tracer tests and several field pilots, in clastic and carbonate reservoirs, have demonstrated that additional oil can be recovered after modifying the injection water chemistry [1,2]
The results indicated that the adsorption film of stearic acid was highly aligned and subsequently caused drastic changes in the interfacial structure of n-dodecane
Stearic acid was combined with hydrocarbons to mimic the acidity condition of fluids in the reservoir
Summary
Laboratory studies, single well chemical tracer tests and several field pilots, in clastic and carbonate reservoirs, have demonstrated that additional oil can be recovered after modifying the injection water chemistry [1,2]. There is a high contrast between the concentration of the monovalent and divalent ions as well as the dissolved ions ratios. Such variations can complicate the chemical interactions between fluids and reservoir minerals especially that certain ions comes in different forms as free and/or complexes. Diverse hypothesis causes the observed changes in wettability and oil recovery as the following: 1) adsorption of sulphate ions associated with co-adsorption of divalent cations, 2) change in the surface charges at interfaces, and 3) microscopic dissolution of anhydrites [4]
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