The influence of crude oil acids on natural inhibition of hydrate plugs

Abstract

Gas hydrates can form in petroleum production systems of natural gas, water and crude oil. In some systems the hydrates agglomerate rapidly into large plugs that cause hazardous blockages of e.g. transport pipelines. The dangers associated with hydrate plugs are severe, and hydrate prevention strategies represents huge economical costs for the operators. However, in some systems plugging is never observed. Instead, the hydrates form as tiny crystals that are easily transported within the fluid flow as suspensions. Crude oils that possess low hydrate plugging tendency are believed to contain natural inhibiting components (NICs). One possible mechanism is that the NICs are anti-agglomerants, i.e. surfactant molecules capable of adsorbing to the hydrate surface. The layer of adsorbed surfactants creates an "oil-wet" hydrate surface that makes the particles less prone to agglomeration and plugging. Knowledge about how to isolate these specific compounds from the crude oils, and to elucidate their chemical structures, can provide a valuable tool to hydrate plug assessment- and control. These compounds could potentially be developed into future environmentally friendly low dosage hydrate inhibitors. The origin of these crude oil compounds is unknown. The process of biodegradation seems to be of importance, as most non-plugging oils are biodegraded whereas plugging oils are typically non-biodegraded. However, there are exceptions to these general categories, and this raises the question whether different biodegradation processes influence the plugging potential of oils differently. This thesis addresses the challenge of isolation and identification of natural inhibiting compounds in crude oils, and also includes testing their hydrate wettability effects. One class of crude oil components that has previously been recognised to display hydrate anti-agglomerating effects is the polar acidic fraction. In the work of this thesis the attention has been directed to this fraction. A central part of the work has been the development of HPLC- and SPE methods for fractionation of petroleum acids. The HPLC separation gives useful information about the distribution of acidic compounds in different oils, and a strong negative correlation between the relative amount of phenolic compounds in the acid extracts and hydrate wettability has been found. Acid extracts and SPE fractions isolated from low plugging potential oils impose changes in the wettability of Freon hydrates. One of the SPE fractions displays particularly high effect at low concentration. This distinctly hydrate surface active fraction contains predominantly weakly polar compounds. FTIR analysis indicates that these molecules contain ester functionalities that are not found in a corresponding fraction of a high plugging tendency oil. On the contrary, the profile of the high plugging tendency oil indicates a larger content of phenolic compounds, which is in accordance with the observed negative correlation between phenols and wettability. Anaerobic- and aerobic laboratory biodegradation experiments produce acids of different chemical composition. Anaerobically produced acids display higher effectiveness at the oil/hydrate interface than those from aerobic biodegradation. The chemical composition of these acid extracts resembles the trends found for fractions from low- and high plugging potential oils studied in other works; the most hydrate surface active acids hold a larger relative proportion of weakly polar compounds and lower content of phenolic compounds. The exact chemical structures of the acid fractions isolated in this work remain to be elucidated. The final work of chemical analysis is currently being performed by Dr. Stefanie Pötz at GFZ. Preliminary results from ESI-MS analysis provide very promising data, and reveal clear structural differences in acid compositions between low- and high plugging tendency oils.