Molecular geochemistry of pre-salt crude oils under WAG injection, Santos Basin, Brazil

Enhanced oil recovery methods are applied to pre-salt carbonate reservoir to increase oil production. Among these, water alternating gas (WAG) injection combines the benefits of water and gas injection, promoting more efficient crude oil displacement. In this context, this study aims to assess molecular variations (C₅–C₁₅) in crude oils influenced by WAG injection in two fields, Field A and Field B, both located in the Santos Basin, Brazil. Crude oils collected from wells prior to WAG injection (time zero, t₀) and during WAG operation (t_n) were analyzed by comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-TOFMS). In Field A, crude oils (t₀-t₈) from a single well were evaluated over an eight-year period (2012–2020). The injected gas is the produced gas (mainly composed of methane with low proportions of CO₂) and it behaves as supercritical fluid at reservoir P-T conditions. This injection phase promoted an increase in light hydrocarbons in the produced oil, whereas water injection resulted in their depletion, reflecting the differing chemical affinities of hydrocarbons for the injected fluids (water and gas). In Field B, the gas injection is composed of CO₂ and the reservoir P-T conditions are above the CO₂ critical point, indicating the presence of supercritical CO₂ (scCO₂). The t₀ and t₁ samples collected from the same well showed an overall increase in total hydrocarbon concentration. Branched-alkane, monocyclic, bicyclic, and aromatic compounds also showed enrichment in t₁ samples, whereas n-alkanes exhibited a decrease in concentration. The reduction of n-alkanes, also observed in Field A during gas injection, is attributed to their preferential extraction by the scCO₂ resulting in a relative enrichment of the other chemical classes. Additionally, increases in gas/oil ratio (GOR) and API gravity in t₁ samples from Field B further indicate enrichment in light hydrocarbons in the produced oil. The asphaltene precipitation also showed a correlation between Field B samples and the Field A gas injection phase. Although Fields A and B employed two different gases (methane and CO₂, respectively), both acted as supercritical fluids, and exhibited similar hydrocarbon mobilization trends, due to their apolar character. These results indicate that high-resolution analytical data interpretation reveal trends consistent with those observed in Field A, indicating that Field B samples are influenced by gas injection, which promotes light hydrocarbons enrichment and contributes to improved oil recovery.