Free Volume Estimates of Thermodynamic and Transport Properties of Heavy Oils with CO2

Carbon dioxide is to be used to displace crude petroleum oil in the underground reservoirs.  Usually some dissolution of CO₂ into the oil and some evaporation of oil into the gas phase, lead to the condition of miscibility which greatly improves the displacement process.   Even where no miscibility is reached, as in heavy crudes, there are still some advantages.  When CO₂ dissolves in oil, the volume of oil increases squeezing it out of narrow capillaries.  Further, the viscosity of oil also decreases, at times by an order of magnitude.  This aids the displacement process.  Consequently to quantify the process it is necessary to know the CO₂ solubility, the swelling produced, and the changes in viscosity and diffusivity with the CO₂ content.

There is not much in commonly available that unifies both thermodynamic and transport data in a single model with the exception of free volume theory.  The free volume is the fraction that is not taken up by the bodies of the molecules.  When the free volume changes, the occupied volume remains the same.  As a result, free volume changes can be related to the coefficient of volumetric expansion and isothermal compressibility. What is more important, is that when the free volume is low, it can also be used to quantify the viscosity and solubility.  Many parameters are needed and we start to the density-pressure-temperature without CO₂ data available for heavy crudes in the literature to establish some and then with the viscosity-pressure-temperature to find the rest which allows us to predict the remaining viscosity data limited only by the nature of the data. 

Then we interpret the data on swelling by CO₂ which leads us to the volume fraction of CO₂.  At 3000 psia, it is quite large ~ 0.3.  The viscosity data leads us to the free volumes of CO₂ in oil and allows as to calculate the diffusivities of CO₂.  Some attempt to compare the results to experimental observations is made the latter are represented as averages and the conclusions are not very sharp.