3942 r2010 American Chemical Society pubs.acs.org/EF Energy Fuels 2010, 24, 3942–3949 : DOI:10.1021/ef1001056 Published on Web 06/03/2010 Theoretical Treatment of Asphaltene Gradients in the Presence of GOR Gradients Denise E. Freed,* ,† Oliver C. Mullins, † and Julian Y. Zuo ‡ † Schlumberger-Doll Research, 1 Hampshire Street, Cambridge, Massachusetts 02139, and ‡ DBR Technology Center, Schlumberger, 9450 17th Avenue, Edmonton, Canada T6N 1M9 Received January 28, 2010. Revised Manuscript Received May 10, 2010 The modeling of hydrocarbon fluids in oil-field reservoirs is essential for optimizing production. In particular, the often large compositional variations of reservoir crude oils must be understood and modeled. The two most important chemical constituents that govern many chemical and physical properties of subsurface reservoir crude oils are the dissolved gas content, described by the gas-oil ratio (GOR), and the asphaltene content. The modeling of GOR variations of crude oils in reservoirs has been practiced routinely for many decades. However, proper modeling of the asphaltenes and/or heavy ends of reservoir crude oils has been precluded because of the lack of understanding of the chemical and physical properties of asphaltenes in crude oils. Recently, the modified Yen model has codified advances in asphaltene science by providing a framework for understanding the molecular and colloidal structure of asphaltenes in crude oils. Here, a thermodynamic model of asphaltenes in reservoir crude oils is developed that can incorporate the modified Yen model and thus can be used to treat reservoir crude oils. Our objective is to analyze the distribution of reservoir fluids, in particular the asphaltenes. This deviates from most previous studies of asphaltene thermodynamics, which were focused on the phase behavior of asphaltenes. Here, compositional gradients of asphaltenes, as well as the GOR of reservoir crude oils, are analyzed. Asphaltene gradients are shown to be strongly affected by both gravity and solubility. The latter effect is heavily dependent on the dissolved gas content of the reservoir crude oil. Case studies are provided that exhibit the power of this modeling. Introduction The two most important chemical constituents that govern many chemical and physical properties of subsurface reservoir crude oils are the dissolved gas content, described as the gas-oil ratio (GOR), and the asphaltene content. For exam- ple, surface or sea floor facilities are built according to gas and liquid volumetric handling. Flow rates are critically dependent on the fluid viscosity, which is a function of both the light-end (e.g., gases) and heavy-end (e.g., asphaltenes) contents of the crude oil. Moreover, reservoir fluids can exhibit variation of important chemical properties, especially the light- and heavy- end contents, from a variety of mechanisms. 1 These variations must be understood for the building of optimal production strategies. In addition to reservoir fluid complexities, reservoirs gen- erally possess complex architecture. In particular, reservoirs can have (infrequent) large compartments or can consist of numerous small compartments. (A compartment must be penetrated by a well to be drained.) That is, in extreme descriptions, reservoirs can be similar to a kitchen sponge that has a connected porosity or similar to a spool of bubble wrap that has many small individual compartments. Compartmen- talization or its inverse, reservoir connectivity, is the biggest problem in almost all deepwater projects around the world. 1 There are many mechanisms that can produce reservoir fluid variability. Often this variation in the fluid can address com- partmentalization because different compartments are likely to be filled with different fluids. If a stair-step discontinuous fluid property is found (in a single phase) in the reservoir, then compartmentalization is often indicated. 1 In contrast, contin- uous and monotonic trends of reservoir fluid properties often imply connectivity because they suggest that there is massive fluid flow across the reservoir. If the reservoir fluids are equilibrated, especially in the heavy ends, then reservoir con- nectivity is more strongly implied. 1 This follows because the mobilities of the heavy ends are very low, so equilibrated heavy ends are not compatible with substantially restricted flow in the reservoir. Consequently, it becomes more important than ever to model all components of reservoir fluids. Modeling GOR in reservoir fluids has been performed for decades using variants of the van der Waals equation of state [or cubic equations of state (EOS)]. These equations have been very useful for describing a variety of fluid parameters. For example, heuristics have been developed to indicate when GOR gradients are expected. 2 In large measure, in compres- sible fluids, the greater expansion of the lightest components as they go higher in the column will create the thermodynamic drive to give GOR gradients. In contrast, incompressible fluids do not yield density variations and thus do not yield GOR gradients. 1,2 The success of cubic EOS to predict GOR gradients in reservoir crude oils has been confirmed in live crude oil centrifugation experiments. 3 (Live crude oils are those oils that, under reservoir pressure and temperature conditions, contain dissolved gases.) However, treating solids, such as asphaltenes, as a pseudocomponent in a gas-liquid *To whom correspondence should be addressed. E-mail: freed1@ slb.com. (1) Mullins, O. C. The Physics of Reservoir Fluids; Discovery through Downhole Fluid Analysis; Schlumberger Press: Houston, TX, 2008. (2) Hoier, L.; Whitson, C. SPEREE 2001, 4, 525–535. (3) Ratulowski, J.; Fuex, A. N.; Westrich, J. T.; Sieler, J. J. Theoretical and experimental investigation of isothermal compositional grading; SPE 8477; SPE: Dallas, TX, 2003.