Structural Characterization of Functional Molecules Involved in Microdispersion Formation for Low-Salinity Waterflood in Carbonate Reservoirs

Summary Both academia and industry have paid attention to low-salinity water (LSW) enhanced oil recovery (EOR) in carbonate reservoirs with a variety of proposed mechanisms. Our research has focused on fluid-fluid interaction (FFI) during static contact between oil and LSW, forming a water microdispersion (MD) phase, defined as micelle-structure clusters of water molecules surrounded by surface-active species within oil, at the oil/LSW interface as a potential driver of the mechanism in LSW EOR. In the study, LSW-reactive oil forming more MD is defined as positive oil, while LSW-nonreactive oil is defined as negative oil. We elucidated the functional components in positive crude oil which showed high MD ratio (MDR) in the FFI test and additional oil recovery in corefloods. Four stock-tank oils (STOs) were collected (L1, L2, L3, and U) from multiple zones in a target carbonate oil field. Synthetic brine was prepared by diluting the injection seawater (SW) to 1%. To measure microdispersion ratio (MDR), which represents the water content increment caused by oil/water interfacial chemical reactions, FFI tests were performed by statically mixing oils and LSW. Based on the MDR, STO-L2 and STO-L1 were screened as positive oils, while STO-L3 and STO-U were defined as partially positive and negative oils. During the FFI tests, three types of subsamples were collected as original oil, MD phase, and post-FFI oil. Each sample was fractionated to seven components: saturates, 1-/2-/3+-ring aromatics, polar resins, polyaromatic resins, and asphaltenes by means of the extended saturate, aromatic, resin, and asphaltene (SARA) analysis. All of the components were then investigated by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to determine the functional components. Based on the MDR, three STOs were investigated by FT-ICR MS analysis. STO-L2 (positive) and STO-L3 (partially positive) were selected for comparison from the perspective of positive MDR. STO-U (negative) was selected as a control. The functional components, which are generally considered to be surface-active components, were assumed to be more abundant in the positive oil and MD fluid than in the negative oil and post-FFI oil. Therefore, multipairs of subsamples were compared to highlight noticeable variation on double-bond-equivalent (DBE) vs. carbon number (CN) plots. The differential analysis revealed that (1) asphaltenes in the positive oil contained higher DBE composition (i.e., aromatic-core rich) than those in the negative oil, (2) higher DBE asphaltenes moved from the original oil to the MD phase while most polar resins remained in the post-FFI oil, and (3) resin behavior during the FFI differed with the type of heteroatom. The specific heavy-end components stabilized at the oil/LSW interface were considered to promote the spontaneous formation of water-in-oil MD. Finally, the molecular structures of specific asphaltenes and resins that showed the largest variations in the differential analysis plots were characterized assuming that they contained hydrogen-bond-promoting heteroatoms such as nitrogen (N) and oxygen (O), using more than 25,000,000 petroleum molecule databases.