Characterization of high molecular weight hydrocarbons (>C 40 ) in oils and reservoir rocks

Abstract

Abstract The presence of high molecular weight hydrocarbons in crude oils has been known for some time, particularly in view of their ability to precipitate and form barriers and blockages in pipelines. Recent advances in analytical technology have led to the development of capillary columns for gas chromatography capable of withstanding high temperatures (350 to 450°C), which allow the characterization of hydrocarbons with carbon numbers as great as C100 and above. Consequently, the individual high molecular weight hydrocarbon compounds in oils and waxes can now be identified. In the present paper, results are presented from high temperature gas chromatographic analyses of naturally occurring waxes. These clearly show the presence of significant concentrations of hydrocarbons with carbon numbers in excess of C 40 in many of these samples. In order to assist in the quantification of these high molecular weight hydrocarbons, we have also assessed the potential of thermal microdistillation coupled with a field ionization detector, to obtain more accurate molecular weight distributions in wax samples. This is based on the concept that high temperature gas chromatography has an upper carbon number limit in the range C 100 to C 120 . Thermal distillation may provide the ability to exceed that limit and determine the presence of compounds above C 120 . More detailed characterization of the wax fractions on a molecular level has the potential to provide clues as to the composition and origin of the residual components left in a petroleum reservoir at the end of its production history. A better understanding of the nature of this material will lead to improved secondary and tertiary recovery mechanisms, and other enhanced oil recovery procedures.