An Experimental Study of CO2 Dissolution into a Heavy Oil Without or With a Porous Medium

In this thesis, phase behaviours of three different solvents (i.e., CO2, CH4, and C3H8) in the Colony and McLaren heavy oils were experimentally studied. The diffusivities of CO2 in the Colony heavy oil without or with an unconsolidated porous medium at different initial pressures and a constant temperature were measured and compared. In addition, the temperature dependence of the CO2 diffusivity in the Colony heavy oil was noted. Three different theoretical diffusion models for determining the diffusivity of a solvent in a liquid phase were applied and analyzed. More specifically, first, two series of PVT tests of the Colony heavy oil– CO2/CH4/C3H8 and the McLaren heavy oil–CO2/CH4/C3H8 systems were conducted to measure the solvent solubilities χ, oil-swelling factors SF, solvent-saturated live oil densities ρo and viscosities μo at different equilibrium pressures and the actual reservoir temperature of Tres1 = 21.0°C. Second, four diffusion tests of CO2 in the Colony heavy oil without a porous medium (Case A) were performed at four different initial pressures (Pi = 1.4, 2.7, 3.9, and 5.0 MPa) and a constant temperature of Tres1 = 21.0°C in a closed constant-volume diffusion cell by using the pressure decay method (PDM). The history matching (HM) method and two graphical methods (GMs), i.e., GM-I and GM-II, were used to analyze the declining pressure as a function of time and thus determine the CO2 molecular diffusivity. Third, four diffusion tests of CO2 in the Colony heavy oil without a porous medium (Case B) and another four diffusion tests of CO2 in the Colony heavy oil with a porous medium (Case C) were performed in a similar initial pressure range to that in Case A but at another constant temperature of Tres2 = 27.0°C. The molecular diffusivity of CO2 in the Colony heavy oil without a porous medium (Case B) and the apparent diffusivity of CO2 in the Colony heavy oil-saturated porous medium (Case C) at Tres2 = 27.0°C were also determined, respectively. Theoretically, the HM method and the two GMs were applied and compared by using the measured pressure vs. time data from Case A. It was found that the assumption of a constant z-factor used in the two GMs can introduce a relatively large error in the determination of CO2 diffusivity. Moreover, representing the infinite terms in the analytical solution of the diffusion model by the first term in GM-I might also introduce a large truncation error, especially at the beginning of each diffusion test. With the same experimental pressure decay data, the determined molecular diffusivities by using the two GMs were two to three times lower or higher than those determined by using the HM method. It was found that for the Colony heavy oil‒CO2 system without a porous medium (Case B) in an initial pressure range of Pi = 1.4–4.7 MPa and at Tres2 = 27.0°C, the determined molecular diffusivities of CO2 were slightly higher than the apparent diffusivities of CO2 in the same heavy oil with a porous medium (Case C) in an initial pressure range of Pi = 1.5–4.7 MPa and at Tres2 = 27.0°C. Specifically, the measured molecular diffusivities ranged from 2.2 to 3.6×10−10 m2/s, whereas the obtained apparent diffusivities were in the range of 1.4 to 2.1×10−10 m2/s. In Case C, the measured diffusive tortuosities were in the range of τ = 1.50‒1.71 for the unconsolidated porous medium used in four diffusion tests. Finally, as the test temperature was increased from Tres1 = 21.0°C (Case A) to Tres2 = 27.0°C (Case B), the measured molecular diffusivity at a similar initial pressure was slightly increased.

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