(308c) Comparative Analysis of Dynamic Crude Oil-Water and Crude Oil-Brine Interfacial Properties

In this paper, crude oil-water and crude oil-brine interfacial properties are investigated qualitatively and quantitatively over wide ranges of temperatures and pressures. Dynamic interfacial tensions (IFTs) of crude oil-water and crude oil-brine systems were measured and compared, and relevant influence factors were evaluated.

One crude oil from the Middle East, de-ionised water and two synthetic brines were investigated. Dynamic IFTs of the crude oil against deionised water and the two synthetic brines were measured at temperature and pressure up to 120 °C and 300 bar, respectively. Additionally, a microscope fitted with a heated stage was used to search for precipitation of (possibly surface-active) components in the oils within the target temperature range. Several mathematical models were employed to aid the interpretation of the interfacial phenomena through the quantifications of molecular diffusion and adsorption/desorption.

The dynamic behaviour of the crude oil surface tension (ST) and the oil-water/brine IFT was found to be qualitatively different. Upon forming a pendant oil drop in air, the surface tension was found to increase monotonically with time, approaching an asymptotic limit. Conversely, with a sessile oil drop formed in deionised water or brines, the interfacial tensions were observed to decrease in most cases, so that the ‘equilibrium’ IFT was usually lower than that at first contact. Furthemore the oil-water and oil-brine IFTs behaved differently. The oil-water IFTs were found to be higher than the oil-brine cases, and the latter exhibited somewhat greater variation with temperature and pressure changes. Another important observation is for the target crude oil-water/brine systems, the IFT rapidly decreased over an initial period of up to about 1000 s and then either (a) reached a plateaux or (b) passed through a minimum and increased again towards an asymptotic limit in between the initial and minimum values. The initial rapid decrease in IFT is attributed to surface-active components diffusing towards the interface, where they are more- or less-strongly adsorbed. A subsequent increase in the IFT might be attributed to surface reconfiguration through dissolution of surface-active components into the surrounding water and/or formation of asphaltene flocs at the interface. The abnormal IFT increase after the initial rapid reduction is strongly influenced by the temperature, pressure and brine composition. Specifically, the abnormal phenomenon appears when the temperature is raised to or above 70 °C for all the three systems and it was found to be weaker with increasing pressure. Moreover, the degree of this abnormal increase depended upon the composition of the aqueous phase. Last but not least, temperature and pressure effects are much different for the cases which respectively involve water and brine or even for the cases with exactly same conditions but at different time.

Crude oil-water and crude oil-brine interfaces are of great importance in oil/gas exploration and production and also in carbon geological storage. Most previous scientific studies focused on synthetic hydrocarbons while actual crude oil samples have not been widely studied in the literature. This study addresses key knowledge gaps in respect of crude oil-water and crude oil-brine interfacial phenomena by combining experimental measurements with theoretical interpretation.