Experimental Investigation of Effects of Asphaltene Stability on Interfacial Behavior of Live Reservoir Fluid Systems

Abstract

Abstract Wettability alteration due to asphaltene precipitation in a reservoir affects rock-fluid interactions that have potential impacts on oil production, recovery and flow in the production network. The current predictive wettability models do not consider asphaltene stability and are inherently inaccurate. This study investigates the impact of pressure-depletion induced asphaltene precipitation on interfacial tension (IFT) and contact angle for live-oil systems at reservoir conditions (high-pressure high- temperature); and presents a graphical (quantitative) method for determining asphaltene onset pressure based on interfacial behavior. Water/oil IFT was measured at reservoir temperature using pendant drop-shape method for a system of live-oils over a range of pressure above and below the asphaltene onset pressure (that was already independently determined via particle size distribution and solid detection system techniques). Contact angle with quartz was also measured in presence of de-ionized water as surrounding medium at the same pressure and temperature conditions. The temperature was controlled with an accuracy of ±0.1°C. Each set of measurements were performed twice to assure the reproducibility of the experiments and methodology. This work presents the experimental study to quantify the change in interfacial behavior due to asphaltene deposition. The IFT/Contact Angle measurements above and below asphaltene onset pressure show that the interfacial behavior follows the normal trends above asphaltene onset pressure as observed in other water/hydrocarbon systems. However, when pressure was reduced below the asphaltene onset pressure, an evident relatively sharp change in the trend is observed in both IFT and contact angle caused by asphaltene migration to the interface, in a way acting as natural surfactant. As asphaltenes precipitate and deposit in the mineral substrate, the surface turns less water-wet and the contact angle naturally increases to balance the equilibrium forces. This study sets a non-visual quantitative method to determine asphaltene onset pressure and presents new experimental data on IFT/contact angle of live-oil systems at reservoir conditions. The effects of asphaltene precipitation on interfacial properties are more meaningful than the size of aggregates (visual methods), and can be utilized to assess the potential impacts of wettability alteration due to asphaltene precipitation. For example, only when change in contact angle is large, wettability alteration should be considered in reservoir simulation of depletion processes and waterflooding performace, especially in the context of relative permeability changes.