Modified Starch Flooding in Carbonates: Impact on Oil Recovery and Crude Oil Composition.

Enhanced oil recovery (EOR) in carbonate reservoirs has been a persistent challenge due to their complex wettability and heterogeneity. In a previous study, the introduction of quaternary ammonium groups was shown to increase the amphiphilic character and solubility of the starch derivatives, promoting wettability alteration from oil-wet to mixed-wet conditions at concentrations of 1 mg/mL on the limestone outcrop surface. Motivated by these findings, three 2-hydroxy-3-(trimethylammonium)-propyl starch derivatives (HTPS), previously synthesized from corn starch with different cationic degrees of substitution (DS) and molar masses (MM), were evaluated as potential enhanced oil recovery (EOR) agents. Core-flood experiments on carbonate plugs revealed incremental oil recoveries of 6-11% over conventional waterflooding, with higher DS and MM enhancing oil displacement efficiency through improved carbonate wettability alteration and sweep efficiency. To provide molecular-level evidence for the oil recovery mechanisms, the original crude oil and recovered oils from core-flood tests were analyzed by ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI(±) FT-ICR MS). The compositional analysis demonstrated significant changes in acidic and basic polar compounds, particularly within nitrogen- and oxygen-containing classes, suggesting selective adsorption and fractionation processes at the rock-fluid interface. These findings highlight ESI(±) FT-ICR MS as a key analytical tool linking macroscopic recovery performance with molecular-level fractionation in the recovered oil, thus validating the mechanisms behind the enhanced oil recovery observed in carbonate cores.