Canadian Journal of Chemical Engineering, Vol.97, No.1, 330-343, 2019
Three different periods of CO2 dissolution into a light crude oil
In this paper, phase behaviours and mass-transfer mechanisms of a Bakken light crude oil-CO2 system are studied. A series of PVT tests of the light crude oil-CO2 system are conducted to measure CO2 solubilities, oil-swelling factors, and CO2-saturated oil densities at different equilibrium pressures and the actual reservoir temperature of T-res = 56.0 degrees C. Five CO2 diffusion tests in the light crude oil are performed at five different initial test pressures (P-i = 4.1, 5.2, 6.3, 7.7, and 9.0 MPa) and T-res = 56.0 degrees C by applying the pressure decay method (PDM). The graphical method (GM) is used to analyze the measured test pressure versus time data of each diffusion test and determine CO2 average diffusivity in the light crude oil. It is found that the so-called density-driven natural convection can occur in the CO2-saturated light crude oil as its measured density is much higher than that of the dead light crude oil. Three distinct periods of CO2 dissolution into the light crude oil are identified: the natural convection-dominated period (I), the transition period, and the molecular diffusion-dominated period (II). The determined CO2 effective diffusivities (D-eff) in Period I and CO2 molecular diffusivities (D) in Period II from the GM are in the ranges of 0.28-1.75 x 10(-6) m(2)/s and 0.87-1.94 x 10(-9) m(2)/s at the initial test pressures of P-i = 4.1-9.0 MPa, respectively. Finally, it is also found that the constant Z-factor approximation used in the GM may cause a relatively large error in the determination of D-eff or D.