The high energy intensity of steam-based heavy oil extraction processes is leading the industry to investigate solvent-aided processes for heavy oil/bitumen recovery. The diffusion coefficient of solvent in heavy oil is a key parameter to determine how effective these processes are. Measuring the diffusion coefficient is challenging, especially at in-situ conditions, i.e., at elevated temperature and pressure. This paper presents a microfluidics-based method for measuring the diffusion coefficient of propane and butane, the most commonly used light hydrocarbons in solvent process, into heavy oil at high temperature and pressure conditions.

A silicon-glass microfluidic chip was designed and fabricated using the Deep Reactive Ion Etching (DRIE) and anodic bonding. The diffusion tests were performed at temperatures ranging from 20°C to 120°C and pressures up to 100 bar. Upon blue light excitation, heavy oil naturally fluoresces at visible wavelengths, and the intensity varies with solvent concentration. Based on this mechanism, the light intensity change of the heavy oil in a 100-micron channel was recorded with a camera connected to the microscope during the diffusion process. An image processing method was developed accordingly to create a map of fluorescence light intensity versus diffusion time and distance, which was further processed numerically to calculate the diffusion coefficient with measured correlations between light intensity and solvent concentration.

At all testing conditions both solvents were liquid which inevitably would cause asphaltene precipitation at oil-solvent interface as demonstrated by the observations during the diffusion tests. It was also found that the asphaltene precipitation phenomena were more pronounced with butane than propane. To measure the diffusion coefficient, a method using solvent diluted oil was developed to avoid the asphaltene precipitation issue.

The measured diffusion coefficient of propane was around 0.6 × 10-10 m2/s using pure solvent method, and that of butane was in the range of 1—9 × 10-9 m2/s, at their corresponding testing conditions. For both solvents the diffusion coefficients increased with temperature while no satisfactory trend was observed with pressure within 100 bar.

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