New insights into the interfacial phenomena occurring between hydrocarbon solvent and heavy oil

Abstract

Solvent injection is an alternative method for heavy and extra heavy-oil recovery in situations where thermal methods cannot be applied, such as thin reservoirs, wormholed reservoir after-CHOPS (cold heavy-oil production with sands), or fractured reservoirs. Solvents normally exist in their liquid or supercritical phase under reservoir conditions and may not be miscible with heavy-oil at first contact. Coupling with the fact that diffusion into highly viscous fluids tends to be very slow and an interface exists in the first contact of liquid solvent and oil, displacement by capillary imbibition may take place. This displacement eventually improves the contact area between oil and solvent resulting in an enhanced mixing process by diffusion.To understand this phenomenon and fully capture the interaction of solvent and heavy-oil in different rock systems, experimental investigations were conducted using sandstone and limestone core samples. The samples saturated with different types of oils (viscosities ranging between 14 and 170,000 cP) were immersed into a tube filled with different types of solvent (propane, heptane, decane, and naphtha). Two characteristics of the experiments, namely the areal coverage as a measure of capillary imbibition and the rate of color change as a measure of diffusion, were used to analyze the imbibition-diffusion process. Both the color change of the solvent surrounding the rock and the amount of oil expelled from said rock were used to analyze the interaction process to identify the existence of a transient interfacial tension period and the effect of coupling imbibition and diffusion processes on the interaction qualitatively (visually) and quantitatively. The color change of the medium surrounding the core (becoming darker) points to the prominence of diffusion over capillary forces.We observed that in the solvent/heavy-oil system, in which molecular diffusion is a slow process, a dynamic (or transient) interfacial tension (TIFT) exists before fully mixing. During this period, both imbibition and diffusion become effective––acting symbiotically, and depending on the oil/solvent viscosity ratio, the period may dominate a great portion of the whole process. This critical viscosity ratio was found to be 20,000–50,000––corresponding to a viscosity range of 30,000–50,000 cP. For this range of oil viscosities, a medium carbon number (typically heptane) was observed to be the most effective solvent to yield the most efficient TIFT type interaction.