Effect of ionic composition in water: oil interactions in adjusted brine chemistry waterflooding: preliminary results

Abstract

Low salinity or adjusted brine composition waterflooding (LSW or ABCW) is considered a promising improved/enhanced oil recovery (IOR/EOR) method. Despite the large number of studies documented in the literature, there are contradictory results and a lack of consensus regarding the mechanisms that operate in this recovery process. The proposed fluid:rock and fluid:fluid mechanisms are still under discussion and investigation. However, the impact of oil geochemistry and its importance on the fluid:fluid interactions that can occur with brines during LSW or ABCW have been overlooked and studied in a lesser extent.The scope of the present study is to preliminary evaluate crude oil:brine interactions to validate the influence of its compositions. These interactions were evaluated at static conditions for a week and reservoir temperature (60°C) using two oil samples from different Colombian basins and brine solutions of different composition at a constant ionic strength (I = 0.086). Specifically, this investigation evaluated the effect of the type of cation (Na+ and Ca2+) and anion (Cl- and SO4=) on crude oil:brine interactions. The results of these experiments were compared with tests using distilled water (DW). Although a basic characterization of brines (i.e. pH, alkalinity and ionic composition) and oil (oil viscosity) was performed, the main objective of this study is the analysis of water-soluble organic compounds (WSOC) using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). The results demonstrate that water:oil interactions are dependent on brine and crude oil compositions. The main changes observed in the aqueous phase were the increase in inorganic components (desalting effects) and organic compounds soluble in water. Only the system crude oil A and NaCl (5,000 ppm) showed the formation of a micro dispersion. Negative electrospray ionization (ESI (-)) FT-ICR MS data shows that WSOC’s identified in DW and Na2SO4 after the interaction with crude oil A belongs to similar classes but there is marked selectivity of species solubilized with different brines. The relative abundance of classes Ox, OxS and NOx (x > 2) decreases while Ox, OxS and NOx (x ≤ 2) increase their solubility in the presence of Na2SO4 compared to DW. The analysis of O2 and O3S classes using double bond equivalence (DBE) vs. carbon number (CN) contour plots shows that the isoabundance of water-soluble species are within the range of DBE £ 10 and CN £ 20 regardless the brine used in the experiments. Finally, the method of solvent extraction in silica columns used in this investigation for the analysis of WSOC using FT-ICR MS represents a powerful and new approach to study LSW and ABCW.