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

Abstract

Summary Wettability alteration resulting from asphaltene precipitation in a reservoir affects rock/fluid interactions that have a potential impact on oil production, recovery, and flow in the production network. The current predictive wettability models are inherently inaccurate and do not consider asphaltene stability. This study investigates the impact of pressure-depletion-induced asphaltene precipitation on interfacial tension (IFT) and contact angle for live-oil and water systems at reservoir conditions (high pressure, high temperature), and it presents a graphical (quantitative) method for determining asphaltene onset pressure (AOP) based on interfacial behavior. Water/oil IFT was measured at reservoir temperature using a pendant-drop-shape method for a system of live oils over a range of pressures above and below the AOP, which was already independently determined by means of particle-size-distribution and solid-detection-system techniques. The same pressure and temperature conditions were used to measure contact angle with quartz in the presence of deionized (DI) water as the surrounding medium. The temperature was controlled with an accuracy of ±0.1°C. Some measurements were performed twice to ensure the reproducibility of the experiments and methodology. This work presents the experimental study to quantify the change in interfacial behavior because of asphaltene precipitation and deposition. IFT/contact-angle measurements above and below AOP show that the interfacial behavior follows the normal trends above AOP as observed in other water/hydrocarbon systems. However, as evident when the pressure was reduced below the AOP, a relatively sharp change in the trend is observed in both the IFT and contact angle, which is caused by asphaltene migration to the interface in a way that acts as a 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 quantitative and alternative method to determine AOP, and presents new experimental data on IFT/contact angle of live-oil and water systems at reservoir conditions. Near the wellbore, asphaltene deposition can lead to pore plugging, where a large number of pore volumes flow through the productive life of the well. In this scenario, the size of aggregates (of asphaltene) is an important factor, especially when it is comparable with the pore size. On the other hand, deep in the reservoir, the effects of asphaltene precipitation and deposition on interfacial properties are more important because this can lead to wettability alteration. Thus, the results of this technique can be used to assess the potential impacts deep in the reservoir.