A geochemical study of the controls of crude oil water uptake ability at surface conditions

Abstract

The work contained within this thesis is an assessment of the water uptake ability of various crude oils, and an investigation of the relationship between water uptake and the chemical composition of crude oils. U sing the Karl Fischer titration technique it was possible to achieve rapid analysis of the rate of change of the water content of prepared crude oil/water blends with various added water quantities. A set of procedures, called Crude Oil Water Uptake Analysis (COWUA), were established using this apparatus and applied to a group of crude oils, with varying maturity and extent of biodegradation, from two main oil producing provinces, the U.K. North Sea and Santa Maria Basin, California. Water determination from the top and bottom of crude oil/water blends, combined with visual inspection, was used to characterise the water uptake of crude oils by their water retentive (rate of sedimentation of water and/or emulsion droplets) and emulsion formationl/stabilisation (degree of oil/water separation) abilities. Initial results identified that both of these properties altered with the extent of biodegradation of the crude oil in question. Non-biodegraded crude oils exhibited "poor" water retention (rapid water or emulsion sedimentation) and formed stable water-in-oil emulsions, while degraded crude oils exhibited higher water retentive capability (slower sedimentation) yet possessed varied emulsification ability. Water retention was considered to be possibly due to either the physicochemical (viscosity, density etc.) or geochemical (crude oil compositional) properties of the crude oil. It was anticipated that the composition of crude oils, which exhibit "good" water retention would probably contain "emulsifiers", i.e., asphaltene and wax sols as well as oil-soluble surfactants, such as CO-C3 alkylphenols. These emulsifiers promote stable oil/water interfaces and produce good interaction between the immiscible phases, therefore slowing, or preventing, water and/or emulsion droplet growth. However, bulk chemical analysis showed that the effectiveness of asphaltene and wax emulsifiers decreased with increasing biodegradation and that NSO compounds exhibited no association with increasing water retention. Since no relationship between these crude oil geochemical compositions and increased water retention could be detected it is suggested that physicochemical properties of crude oil/water blends are probably responsible for the rate of sedimentation of the water content. The emulsion formation/stabilisation ability. of crude oils analysed varied. Bulk chemical analysis of the major crude oil chemical groups (aliphatic and aromatic hydrocarbons, resins and asphaltenes) showed that crude oils which formed stable water-in-oil emulsions either possessed a composition conducive to asphaltene and wax precipitation (as for the non-degraded North Sea crude oils) or possessed high NSO contents (as for both non-degraded and biodegraded Santa Maria Basin crude oils). However, where none of the above properties were exhibited, as for biodegraded North Sea crude oils, poor or no emulsion formation occurred. Detailed analysis of the CO-C3 alkylphenols found that their concentration was severely reduced in crude oils which were characterised by poor, or no, emulsification. Therefore, the reduction of oil-soluble surfactants (such as CO-C3 alkylphenols), as well as asphaltene and wax sols, is related to poor emulsion formation! stabilisation. The importance of emulsifiers was further outlined by analysis of the organic matter extracted from the crude oil/water interfacial film present in the emulsions. All the above emulsifiers were found to be preferentially enriched, indicating their involvement in the formation and stabilisation of water-in-oil emulsions. The effect of biodegradation upon water uptake was further investigated under controlled conditions, by the laboratory biodegradation of non-degraded and biodegraded North Sea crude oils. The subsequent emulsification of the non-degraded crude oil during biodegradation was not attributed to the presence of asphaltene and wax sols (biodegradation was considered to reduce the presence of these particles) but the result of significant surfactant generation. This phenomenon is associated with the rapid microbial degradation of the easily metabolisable components in the crude oil. Consequently, the observed lack of emulsification, for the previously biodegraded crude oil, was attributed to both the reduced presence of asphaltene and wax sols, as well as poor surfactant generation associated with slow degradation rates