Asphaltene Precipitation and Alteration of Wetting: Can Wettability Change during Oil Production?

Abstract

Abstract Wett Ability controls the distribution and flow of immiscible fluids in an oil reservoir and thus plays a key role in any oil recovery process. Once thought to be a fixed property of each individual reservoir, it is now recognized that wett ability can vary on both microscopic and macroscopic scales. Polar oil components can adsorb onto the pore-bounding mineral surfaces by several different mechanisms. In this study, we explore a transition in the mechanism of wett ability alteration that relates to the asphaltene fraction and its stability in the oil phase. Asphaltene stability was varied by addition of n-heptane to samples of five crude oils. Conditions ranged from asphaltene stability to mixtures in which aggregates formed and separated from the oil. The onset condition-where the first asphaltene particles become visible-was used as a reference point with respect to asphaltene stability in each crude oil sample. Muscovite mica sheets, treated with brine and with the crude oil-heptane mixtures, were examined using contact angles between decane and water as a measure of altered wett ability on the oil-treated surfaces. A significant increase in oil-wet conditions very near the onset of asphaltene precipitation was observed with four of the five oils, indicating the potential for wetting changes during the course of oil production if conditions of asphaltene instability are approached. Implications of changes in the mechanism of wetting alteration during the course of production from an oil reservoir are considered. Introduction Asphaltenes and Reservoir Wett Ability. The potential for asphaltenes to adsorb onto high energy mineral surfaces and thus to affect reservoir wett ability has long been recognized. A good summary of early work is provided by Anderson.1 More recently, there have been a number of studies of asphaltene adsorption and resulting changes in wetting on smooth surfaces.2–5 What has been missing in previous studies is a simple measure of the stability or incipient instability of the asphaltenes, needed to distinguish between competing interaction mechanisms between these large, aromatic, somewhat polar molecules and surfaces in the presence of brine. COBR Interaction Mechanisms. The polar components in crude oils can adsorb, in the presence of an aqueous phase, by distinctly different mechanisms, depending on factors that include brine composition and the ability of the oil to keep its asphaltenes dispersed.6 Additional interactions that may occur between oil components and dry surfaces are not relevant in the context of an oil reservoir where both aqueous and oleic phases are present.7 Crude oil/brine/rock (COBR) interactions can include:ionic interactions that involve ionization of acids and bases at the oil/water and solid/water interfaces (acid/base, ion-binding, and other specific interactions are included in this category).surface precipitation interactions that depend mainly on crude oil solvent properties with respect to their asphaltenes. While these interaction mechanisms have been demonstrated for specific cases under conditions that should clearly be dominated by one class of mechanisms or the other, little is yet known about intermediate conditions and transition from one set of mechanisms to another. Asphaltenes and Reservoir Wett Ability. The potential for asphaltenes to adsorb onto high energy mineral surfaces and thus to affect reservoir wett ability has long been recognized. A good summary of early work is provided by Anderson.1 More recently, there have been a number of studies of asphaltene adsorption and resulting changes in wetting on smooth surfaces.2–5 What has been missing in previous studies is a simple measure of the stability or incipient instability of the asphaltenes, needed to distinguish between competing interaction mechanisms between these large, aromatic, somewhat polar molecules and surfaces in the presence of brine. COBR Interaction Mechanisms. The polar components in crude oils can adsorb, in the presence of an aqueous phase, by distinctly different mechanisms, depending on factors that include brine composition and the ability of the oil to keep its asphaltenes dispersed.6 Additional interactions that may occur between oil components and dry surfaces are not relevant in the context of an oil reservoir where both aqueous and oleic phases are present.7 Crude oil/brine/rock (COBR) interactions can include:ionic interactions that involve ionization of acids and bases at the oil/water and solid/water interfaces (acid/base, ion-binding, and other specific interactions are included in this category).surface precipitation interactions that depend mainly on crude oil solvent properties with respect to their asphaltenes. While these interaction mechanisms have been demonstrated for specific cases under conditions that should clearly be dominated by one class of mechanisms or the other, little is yet known about intermediate conditions and transition from one set of mechanisms to another.