Effect of fluid-fluid interactions on the performance of low salinity water injection in crude oil reservoirs

Abstract

One of the most promising enhanced oil recovery (EOR) methods is low salinity water injection (LSWI) that has faced several critical challenges, of which understanding the underlying mechanism is still of utmost importance. The importance of understanding the underlying mechanism is essential when screening for suitable oil reservoirs for application of this EOR method. A satisfactory screening method requires a solid data set in the literature of waterflooding and a reliable shred of information about the underlying compounds of crude oil for the positive effect of LSWI. In this treatise, the mechanism of LSWI is scrutinised over a wide range length scale, ranging from micro to pore and core scale. At the beginning, the thesis starts to elaborate on the screening of an extensive crude oil databank including 116 different crude oil samples sourced from different parts of the world. Formation of microdispersion is confirmed by Karl Fischer Titration method (KFT) as a dominant interaction at the oil/water interface, which is promoted by asphaltene molecules of crude oil samples. Following the confirmation of a reliable screening tool for LSWI, the mechanisms of this EOR method are elucidated over pore (chapter 3) and core scale (chapter 4 and chapter 5) experiments. It is visually shown that when crude oil encounters Low Salinity Water (LSW), microdispersion forms spontaneously leading to wettability alteration and crude oil swelling within the porous media. Furthermore, fluid displacement experiments with coreflooding reveal an undeniable link between the potencies of crude oils to from microdispersion and additional oil recovery during tertiary LSWI in sandstones and carbonates. In this dissertation, a comprehensive compositional analysis is carried out to determine the underlying compounds of crude oil that cause microdispersion and additional oil recovery. Wettability alteration and crude oil swelling are shown as the outcomes of microdispersion formation at the oil/water interface leading to additional oil recovery during LSWI. These outcomes of microdispersion will be called low salinity effect (LSE) and it connects the mechanism and the results of the mechanism in terms of additional oil recovery. Using the ionisation techniques of Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy (FT-ICR MS), it was discovered that asphaltene molecules and carboxylic acids with carbon number of C16-C26 and double bond equivalent (DBE) of 1- 4 are mainly involved in microdispersion formation and LSE. Moreover, formation of microdispersion by carboxylic acids and/or acidic asphaltenes and partitioning of low molecular weight carboxylic acids from crude oil into water phase were shown as the dominant interactions at the oil/water interface. It is indicated that formation of microdispersion results in a slightly higher interfacial tension, which stem from engagements of surface-active compounds in microdispersion structures leading to a lower effective concentration at the interface. For the first time, a systematic experimental study on the fluid-fluid interactions between crude oil and water with different salinities was conducted that led to a clear understanding about the mechanism of LSWI. Microdispersion is confidently proposed as the main mechanism of LSWI triggered by carboxylic acids and/or acidic asphaltene molecules. Wettability alteration and crude oil swelling owing to microdispersion were visually substantiated through microfluidic experiments. After determining the underlying compounds of crude oil for LSE, coreflooding experiments corroborate that additional oil recovery by LSWI is only achievable when the crude oil is potent enough to form microdispersion. The findings of this thesis contribute to a reliable screening tool through which suitable crude oil reservoirs for LSWI can be identified and the performance of waterflooding can be controlled based on microdispersion criterion.