Advanced chemical analysis of heteroatomic molecules in Danish North Sea crude oil

Crude oil is a highly complex material composed of thousands of distinct molecular species. The material is classified as a non-uniform mixture of high hydrocarbon content, up to 97% by weight. Besides carbon and hydrogen as primary elements, this complex mixture contains minor amounts of organic heteroatoms, including nitrogen, oxygen, and sulfur. Oxygen and sulfur-containing compounds significantly influence crude oil's characteristics and play a key role in interfacial behaviors between crude oil, rock, and brines in natural reservoirs. In oil production, enhanced oil recovery (EOR) processes using advanced water flooding aim to explore novel and cost-effective ways to improve the extraction and reduce the residues of oil saturation in reservoirs. Despite mounting studies, the dependence between flooding water composition and oil recovery enhancement must be better understood. Therefore, a deeper insight into crude oil heteroatoms is desirable and required to optimize EOR methods and investigate underlying physicochemical mechanisms.
This thesis focuses on analytical method developments and surface interactions of carboxylic acids and sulfur-containing species in crude oil. A high-resolution fractionation method based on liquid chromatography (LC) followed by gas chromatography-mass spectrometry (GC-MS) analysis was developed to isolate aromatic molecules, including oxygen and nitrogen-containing compounds (Paper I). This method serves as an integral technique to enhance the detection of traditional petroleum analysis with chromatographic methods. The method increases the number of identified compounds by approximately 60 to 150% compared to solid phase extraction pre-fractionation combined with GC-MS and direct two-dimensional GC-MS. A new analytical method was developed using a labeling technique and high-resolution Orbitrap MS to characterize carboxylic acids in crude oil (Paper II). The method successfully identified the carboxylic acid profile of crude oils from different Danish fields of the North Sea. The proposed approach can selectively separate and pre-concentrate carboxylic acid for high-resolution mass spectrometry (HRMS) analysis accompanied by different pre-fractionation techniques. Based on knowledge about carboxylic acid in crude oil, we examined the equilibrium of selected represent acid tracers in solid-liquid and liquid-liquid systems (Paper III). The dependence between carboxylic acid structures and their surface activity was observed in the adsorption on calcite surfaces and the oil-water partitioning. Also, the ionic composition is perceived as an essential factor in influencing the mobility of crude oil molecules and altering the wetting condition of the reservoir rock surface. These findings are premises for the following study about the fate of organic acid in core-flooding experiments to interpret the EOR mechanism with low salinity flooding fluid (Paper IV). The acid tracers in crude oil were monitored during the core-flooding experiment using Danish North Sea reservoir chalk and modified brine flooding fluids. We established a low salinity effect in mobilizing crude oil in a carbonate chalk core sample and subsequently improving oil recovery observed through simple two-phase systems and complex two-phase flow experiments. Moreover, a micro-extraction method using silver-modified HKUST-1 metal-organic framework material was studied to separate sulfur-containing compounds for HRMS. Through analytical development and surface interaction investigations in multi-component systems, this thesis provides advanced tools to characterize heteroatomic molecules and support EOR and other petroleum chemistry studies.