Three different periods of CO 2 dissolution into a light crude oil

In this paper, phase behaviours and mass‐transfer mechanisms of a Bakken light crude oil‐CO 2 system are studied. A series of PVT tests of the light crude oil‐CO 2 system are conducted to measure CO 2 solubilities, oil‐swelling factors, and CO 2 ‐saturated oil densities at different equilibrium pressures and the actual reservoir temperature of T res  = 56.0 °C. Five CO 2 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 °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 CO 2 average diffusivity in the light crude oil. It is found that the so‐called density‐driven natural convection can occur in the CO 2 ‐saturated light crude oil as its measured density is much higher than that of the dead light crude oil. Three distinct periods of CO 2 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 CO 2 effective diffusivities ( D eff ) in Period I and CO 2 molecular diffusivities ( D ) in Period II from the GM are in the ranges of 0.28–1.75 × 10 −6  m 2 /s and 0.87–1.94 × 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 .