Heuristics for Equilibrium Distributions of Asphaltenes in the Presence of GOR Gradients

Abstract

Production of crude oil from subsurface reservoirs is greatly impacted by many complexities such as reservoir fluid flow, connectivity, viscosity gradients, and tar mat formation. In situ fluid analysis in oil wells has enabled facile measurement of fluid gradients of dissolved gases and dissolved solids in reservoir crude oils; these gradients have proven very useful for analysis of reservoir complexities. The analysis of solution gas generally uses the cubic equation of state. However, until recently, there had been no predictive equation of state to model asphaltene gradients. Recently, the different nanostructures of asphaltenes in crude oil have largely been resolved and codified in the Yen–Mullins model. In turn, this has enabled equation of state development for asphaltene gradients in crude oils. For example, the Flory–Huggins–Zuo EoS is now ubiquitously utilized in modeling asphaltene gradients. Here, the magnitude and dependencies of the three terms of this EoS, gravity, solubility, and entropy, are considered in detail. Simple expressions for ratios of these terms are obtained as a function of the gas/oil ratio of the crude oils. In particular, the transition from gravity dominance to solubility dominance is examined. A variety of heuristics are developed to guide interpretation of asphaltene gradients that are so routinely measured. Expressions are given that could be used for real-time interpretation upon measurement of these gradients. The utility of EoS modeling of asphaltene gradients is significantly enhanced when incorporating these heuristics.