Estimation of Diffusion Coefficients in Bitumen Solvent Mixtures as Derived from Low Field NMR Spectra

ABSTRACT

Use of solvents for the extraction of heavy oil and bitumen appears to be an increasingly feasible technology. Both vapour extraction and direct solvent injection are considered for conventional exploration and production schemes, while solvent dilution of bitumen is a standard technique in oil sands mining. Mass transfer between solvent and bitumen is a poorly understood process. In some cases, it is totally ignored compared to viscous force effects. In other cases, phenomenological estimations of diffusion and dispersion coefficients are used.

Low field NMR has been used successfully in determining both solvent content and viscosity reduction in heavy oil and bitumen mixtures with various solvents. As a solvent comes into contact with a heavy oil or bitumen sample, the mobility of hydrogen bearing molecules of both solvent and oil changes. These changes are detectable through changes in the NMR relaxation characteristics of both solvent and oil. Relaxation changes can then be correlated to mass flux and concentration changes. Based on Fick’s Second Law, a diffusion coefficient, which is independent of concentration, was calculated against three oils and six solvents.

Introduction

There are extensive deposits of heavy oil and bitumen in Alberta. These fields are primary candidates for thermal and solvent based processes for recovery of oil and bitumen. Both vapour extraction and direct solvent injection are considered for in situ exploration and production schemes, while solvent dilution of bitumen is a standard technique in oil sands extraction as part of the secondary froth flotation. Mass transfer between solvent and bitumen is a poorly understood process. Only a few experimental values of the diffusion coefficient of various organic substances into bitumen are available in the open literature (1-4). To better understand oil/solvent mass transfer phenomena, more experimental data are necessary, especially in liquid-liquid systems. In this paper, such experimental data are obtained for three oils and six solvents.

Most methods for studying mass transfer between two liquids employ some type of an optical system to record time dependent patterns that can be photographed and then analyzed to yield either the binary or the ternary diffusion coefficients for the system of interest ( 5). The work presented in this paper is a deviation from such methods. In the presented work, low field Nuclear Magnetic Resonance (NMR) was used as the tool for mixing pattern recording.

Low field NMR has great potential as a tool for measuring properties of reservoir fluids and produced liquid streams (6). From a single NMR measurement of a fluid stream containing oil and water, the relative fractions of both liquids can be determined (7). Low field NMR has been used successfully in determining both solvent content and viscosity reduction in heavy oil and bitumen mixtures with various solvents (8, 9). As a solvent comes into contact with a heavy oil or bitumen sample, the mobility of hydrogen bearing molecules of both solvent and oil change. These changes can be correlated to mass flux and concentration changes. If Fick’s Second Law of diffusion is used to model such mass flux and consequent concentration changes, then an apparent diffusion coefficient can be calculated.

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