A New Method for Measuring Solvent Diffusivity in Heavy Oil by Dynamic Pendant Drop Shape Analysis (DPDSA)

Abstract

Abstract This paper presents a new experimental method and its computational scheme for measuring the diffusion coefficient of solvent in heavy oil under the practical reservoir conditions by dynamic pendant drop shape analysis (DPDSA). In the experiment, a see-through windowed high-pressure cell is filled with the test solvent at desired pressure and temperature. Then a heavy oil sample is introduced slowly through a syringe delivery system to form a pendant drop inside the pressure cell. The subsequent diffusion process of solvent into the pendant oil drop causes its shape and volume to change until an equilibrium state is reached. The sequential digital images of the dynamic pendant drop are acquired and digitized by applying computer-aided digital image processing techniques. Physically, variations of the shape and volume of the pendant drop are attributed to the interfacial tension reduction and the well-known oil swelling effect as solvent gradually dissolves into heavy oil. Theoretically, the dynamic pendant drop profile is governed by the Laplace equation of capillarity and the molecular diffusion process of solvent into the pendant oil drop is described by the mass diffusion equation. An objective function is constructed to express the discrepancy between the experimentally observed and the numerically predicted profiles of the dynamic pendant drop. The solvent diffusivity in heavy oil and the mass-transfer Biot number are used as adjustable parameters and thus determined once the minimum objective function is achieved. This novel experimental technique is tested to measure diffusivities of carbon dioxide in a brine sample and carbon dioxide in a heavy oil sample, respectively. It should be noted that, with the present technique, a single diffusivity measurement can be completed within an hour and only a small amount of oil sample is required. The interface mass-transfer coefficient at the solvent-heavy oil interface can also be determined. In particular, this new technique allows the measurement of solvent diffusivity in an oil sample at constant pre-specified high pressure and temperature. Therefore, it is especially suitable for studying the mass transfer process of injected solvent into heavy oil during solvent-based post-cold heavy oil production (CHOP).