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Abstract

Since the 1950’s, the use of carbon dioxide to increase heavy oil recovery has attract more attention from industry and laboratory research. The injection of carbon dixode has shown technical and economical advantages for enhancing heavy oil and bitumen recovery, because it can effectively reduce viscosity under the reservoir conditions. When carbon dioxide is injected into the reservoir, it partially dissolves into the heavy oil and mass transfer is the first mechanism to occur. Consequently, the accurate prediction and evaluation of the diffusion coefficient of carbon dioxide in heavy oil is one of the key parameters to develop technology for extraction of heavy oil in a feasible and cost-effective way. However, few experimental data for diffusivity of carbon dioxide in heavy oil are available in the literature. Therefore, this study conducted in order to add to the existing the laboratory data for evaluation and calculation of diffusion coefficient of carbon dioxide into heavy oil. In the past, experimental methods used to determine the diffusion coefficient of a gas in heavy oil were conducted under a constant gas pressure, which assumed that oil phase can be contacted with infinite gas at a fixed pressure. In this study, by employing X-ray Computed Assisted Tomography (CAT) and a non-iterative finite volume method, the purpose is to evaluate and compare experimental diffusion coefficients of carbon dioxide in heavy oil under the constant pressure and decaying pressue at the same time. Moreover, investigation of impacts of pressure on diffusion coefficients is conducted.

It is found that the diffusvity of carbon dioxide in heavy oil is sensitive to the system pressure. The comparison between carbon dioxide diffusion coefficients under the constant pressure and those measured under the decaying pressrue showed an obvious difference. The results of study are essential for understanding oil recovery through carbon dioxide injection.

Introduction

As early as 1950’s, as conventional oil reserves became depleted, interest began to grow in the improved recovery and utilization of heavy oil and bitumen. Many reservoirs in Canada are lelatively small and thin, so techniques need to be developed in order to reduce oil viscosity without application of heat through steam. Compared to the other non-thermal techniques, injection of gas has become as an attractive way to enhance heavy oil and bitumen production since it is more cost-effective. Among a number of kinds of injection gas, carbon dioxide has considerable potential as a solvent fro EOR applications. It is well-known that, compared to methane, nitrogen, or flue gas, carbon dioxide has a much stronger ability to vaporize and extract hydrocarbons. In addition, the pressure required for miscibility with carbon dioxide is basically considerably lower than that for miscible displacement with either methane, nitrogen or flue gas.

When carbon dioxide is injected into the heavy oil reservoirs, the heavy oil viscosity is significantly reduced under the reservoir conditions. In order for mixing and viscosity reduction to occur, mass transfer is the first mechanism, which is bacically controlled by the diffusion coefficient. Accordingly, the measurement and evaluation of the diffusion coefficient is important to understand the mechanism of carbon dioxide extracting heavy oil and develop feasible technology for enhancement of heavy oil recovery. Currently, experimental methods for measuring the diffusivity of a gas in heavy oil can be generally classified into two categories: conventional and nonconventional ^[1]. Several experiments have been published using conventional composition analysis ^{[2, 3]}, monitor pressure decay ^[4-9], monitor gas-oil interface position ^[10], dynamic pendant drop volume analysis ^{[1, 11]}, and low field nuclear magnetic resonance (NMR) spectra ^{[12, 13]}.