Functional Components in Low Salinity Waterflood Forming Micro-Dispersion Phase via Fluid-Fluid Interaction in Carbonate Reservoirs

Abstract

Abstract Low salinity water (LSW) enhanced oil recovery (EOR) has gained more attention in carbonate reservoirs with a variety of mechanism hypotheses. Recent research focused on fluid-fluid interaction (FFI) during LSW injection, especially forming water micro-dispersion (MD) as a potential drivers of oil recovery improving mechanism in LSW EOR. This paper elucidates functional components in positive crude oil which showed high MD ration in FFI test and additional oil recovery in LSW core flood experiments. Four stock tank oil (STO) samples were collected from multiple sub-layers (L1, L2, L3, and U). Synthetic brine was prepared as LSW to mimic the sea water (SW) diluted to 1%. The FFI tests measured MD ratios, which represent water content increment caused by the oil-water interfacial chemical reactions, to screen positive oil for low-salinity effect. During the FFI, 3 types of sub-samples were collected as original oil, MD phase, and post-FFI oil. Each sample was fractionated to 7 compositions: Saturates, 1-/2-/3+-ring Aromatics, Polar Resins, Poly Aromatic Resins, and Asphaltenes. Subsequently, all composition were investigated by Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR MS) to find out functional components. Based on MD ratios, three of four STOs were selected as the candidates for FT-ICR MS analysis. STO-L2 and STO-L3 were categorized as positive oil and partially positive oil, respectively. STO-U was picked out as negative oil because of the lowest MD ratio. Functional components, which are generally considered as surface-active components, are assumed to be predominantly contained in positive oil and MD sub-samples compared with negative oil and post-FFI oil, respectively. Therefore, two series of differential analysis were performed for: (a) a group of original oils (STO-L2 vs. STO-U); and (b) a group of positive oils (STO-L2, MD fluid, and post-FFI oil) using the double-bond-equivalent (DBE) vs. carbon number (CN) plot. The differential analysis of positive/negative oils revealed that asphaltenes in positive oil consisted of higher DBE composition. Noticeable differences were observed for asphaltenes and polar resins in a series of positive oil during FFI test. Higher DBE asphaltenes moved from the original oil to MD phase, while majority of polar resins remained in the post-FFI oil. In general, asphaltenes are stabilized with being surrounded by resins. However, analysis result suggests that surrounding polar resins were detached from asphaltene by the interaction between LSW and asphaltenes’ surface-active components. This may result in decreasing polar resins in MD phase. The study demonstrates the change in chemical composition of crude oil depending on positive oil characteristic or contact by LSW. These compositional differences provide us with important clues about the FFI mechanism of LSW through which further oil recovery may be achieved. Deployment of FT-ICR MS analysis elucidated functional components such as higher DBE asphaltenes which might promote the spontaneous formation of water-in-oil micro-dispersion at the oil/LSW interface.