Information on the phase behaviour of crude oils under pressure is a subject of great interest in view of the problem of production and transport of hydrocarbons from offshore zones in the Arctic, which will be widely expanded in the near future. The main goals of experimental investigation presented in this article were to answer:
- which physico-chemical properties of crude oils have the most influence on glass transition; and
- does glass transition depend on the crystallisation process in crude oils?
In this article, the results of the investigation of transition in crude oils with quite different properties are presented. The specimens were from oilfields in Russia and Kazakhstan. Properties of the samples are shown in Table 1. In addition to these samples, one model oil was also investigated. To prepare the model oil, we took Komsomolsk oil (sample 2 from Table 1) and added 10 mass% of petroleum wax extracted from Kumkolsk oil (sample 3 from Table 1). All samples were degassed and dried before the investigations.
The following considerations were suggested before experiments. A difference in viscosity in crude oils is mainly due to the different content of light components; a higher content of light components lowers the viscosity. This should result in more difficult access to the glass transition, i.e. this transition should take place at higher pressures and lower temperatures.
Another consideration deals with the possible specific role of different components in the formation of the glass transition. For example, petroleum wax can easily form crystalline clusters and precipitate from solution. The crystalline clusters are formed during a many-step first-order phase transition, which includes the crystallisation of hydrocarbons with different molar masses. If the crystalline network occupied the whole volume of a sample forming, for example, as some sort of molecular crystal,1 this would drastically diminish the degree of configurational mobility of molecules in the non-crystallised part of the oil matrix. In this case, one should expect the strong influence of crystallisation on the position and properties of the glass transition (including the possibility of its total suppression). However, this overall crystallisation occurs usually only at high concentrations of crystallising components. The fact that in low-viscosity crude oils at atmospheric pressure the crystallisation of petroleum wax is accompanied by the precipitation process indicates that molecules of petroleum wax form many separate crystalline clusters rather than one overall network.2
The last assumption is in agreement with earlier experimental evidence that the glass transition in oil systems at atmospheric pressure is nearly independent of wax content.3–5 If this is also true at high pressures, crystallisation would not have a great influence on the glass transition.
Experimental Details
We measured the thermal conductivity (l) and heat capacity per unit volume (rcp) using the transient hot-wire method. The experimental arrangements of this method are described in detail elsewhere.6,7 The uncertainty of a determination of l and rcp is estimated to be 2.5 and 4%, respectively.
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