Crude oil is a complex mixture of hydrocarbons whose physical properties vary significantly with its composition, temperature and pressure. Viscosity is a particularly important property influencing the flow of oil in hydrocarbon reservoirs and its displacement by water and other fluids during production processes. The modelling and optimisation of such processes would be greatly aided by models which predict the viscosity of crude oils at (high) reservoir temperatures and pressures (HTHP), ideally from a knowledge of the oil composition. This research has involved making accurate HTHP viscosity measurements on a range of hydrocarbon systems and using these to evaluate the ability of an effective hard-sphere model to predict the data with minimal calibration.
In the first phase, the viscosity and density of a range of pure hydrocarbons, representative of those found in crude oils, and their mixtures, were measured at temperatures and pressures covering typical reservoir conditions (up to 448.15 K and 135 MPa). The vibrating wire technique was used for viscosity in conjunction with a vibrating U-tube densimeter. The ability of the Dymond-Assael (DA) effective hard-sphere model to correlate and predict the viscosity of both the pure components and the complex mixtures was investigated. Agreement for pure components was within ± 5 % whereas for the mixtures this ranged from ± 5% to ± 25 % depending on the complexity. The same thermophysical properties were determined for two North Sea crude oil samples at temperatures ranging from (298.15 to 448.15) K and pressures up to 135 MPa.
The effect of adding an alkane mixture diluent was also investigated. It was found that by treating the crude oils as effective single hydrocarbon components, the Dymond-Assael model could correlate their viscosity to within the experimental uncertainty and that of the diluted crudes to within ±10%. The overall study gives encouragement that a limited number of calibration viscosity/density measurements on a crude oil should enable prediction of its viscosity over a wide range of temperatures and pressures and enable viscosity changes to be predicted when crude oils are mixed with components whose DA parameters are known.