Normal phase high performance liquid chromatography for fractionation of organic acid mixtures extracted from crude oils

摘要

Gas hydrates can form in petroleum production systems of natural gas, water andcrude oil. In some systems the hydrates agglomerate rapidly into large plugs thatcause hazardous blockages of e.g. transport pipelines. The dangers associated withhydrate plugs are severe, and hydrate prevention strategies represents huge economicalcosts for the operators. However, in some systems plugging is never observed.Instead, the hydrates form as tiny crystals that are easily transported within thefluid flow as suspensions. Crude oils that possess low hydrate plugging tendencyare believed to contain natural inhibiting components (NICs). One possible mechanismis that the NICs are anti-agglomerants, i.e. surfactant molecules capableof adsorbing to the hydrate surface. The layer of adsorbed surfactants createsan "oil-wet" hydrate surface that makes the particles less prone to agglomerationand plugging. Knowledge about how to isolate these specific compounds from thecrude oils, and to elucidate their chemical structures, can provide a valuable tool tohydrate plug assessment- and control. These compounds could potentially be developedinto future environmentally friendly low dosage hydrate inhibitors. The originof these crude oil compounds is unknown. The process of biodegradation seemsto be of importance, as most non-plugging oils are biodegraded whereas pluggingoils are typically non-biodegraded. However, there are exceptions to these generalcategories, and this raises the question whether different biodegradation processesinfluence the plugging potential of oils differently.This thesis addresses the challenge of isolation and identification of natural inhibitingcompounds in crude oils, and also includes testing their hydrate wettabilityeffects. One class of crude oil components that has previously been recognised todisplay hydrate anti-agglomerating effects is the polar acidic fraction. In the workof this thesis the attention has been directed to this fraction.A central part of the work has been the development of HPLC- and SPE methodsfor fractionation of petroleum acids. The HPLC separation gives useful information about the distribution of acidic compounds in different oils, and a strongnegative correlation between the relative amount of phenolic compounds in the acidextracts and hydrate wettability has been found. Acid extracts and SPE fractionsisolated from low plugging potential oils impose changes in the wettability of Freonhydrates. One of the SPE fractions displays particularly high effect at low concentration.This distinctly hydrate surface active fraction contains predominantlyweakly polar compounds. FTIR analysis indicates that these molecules containester functionalities that are not found in a corresponding fraction of a high pluggingtendency oil. On the contrary, the profile of the high plugging tendency oilindicates a larger content of phenolic compounds, which is in accordance with theobserved negative correlation between phenols and wettability.Anaerobic- and aerobic laboratory biodegradation experiments produce acidsof different chemical composition. Anaerobically produced acids display highereffectiveness at the oil/hydrate interface than those from aerobic biodegradation.The chemical composition of these acid extracts resembles the trends found forfractions from low- and high plugging potential oils studied in other works; themost hydrate surface active acids hold a larger relative proportion of weakly polarcompounds and lower content of phenolic compounds.The exact chemical structures of the acid fractions isolated in this work remainto be elucidated. The final work of chemical analysis is currently being performedby Dr. Stefanie Pötz at GFZ. Preliminary results from ESI-MS analysis providevery promising data, and reveal clear structural differences in acid compositionsbetween low- and high plugging tendency oils.