Applicability of simple asphaltene thermodynamics for asphaltene gradients in oilfield reservoirs: The Flory-Huggins-Zuo Equation of State with the Yen-Mullins model

Abstract

In recent years, the Flory-Huggins-Zuo Equation of State (FHZ EoS) has been utilized to treat asphaltene gradient data in many oilfield reservoirs. The FHZ EoS employs asphaltene species from the Yen-Mullins model consisting of molecules, nanoaggregates and clusters of nanoaggregates; the specific species used depends primarily on the concentration of asphaltenes but also on the asphaltene stability. Reservoirs with equilibrated asphaltenes are repeatedly found to produce oil as a single flow unit addressing a key reservoir property. For crude oils with moderate concentrations of asphaltenes (a few%), asphaltenes are typically dispersed as nanoaggregates. In crude oils with high concentrations of asphaltenes (∼10%), asphaltenes are typically dispersed as clusters of nanoaggregates. This paper examines a reservoir with a crude oil of moderate concentration of asphaltenes where a secondary charge of methane created some limited instability of asphaltenes. Many pressure and fluid measurements are made to determine that indeed the fluid column is equilibrated. The FHZ EoS and the Yen-Mullins model in this equilibrated oil column are then compared with the asphaltene gradient; excellent agreement is obtained extending the use of this thermodynamic modeling to crude oils with limited asphaltene instability. Downhole fluid analysis (DFA) is used to delineate the asphaltene gradient, and both DFA and lab data are used for determination of solution gas content of oil samples. Conventional gas chromatography, two-dimensional gas chromatography (GC × GC) and gas isotope analysis are performed to analyze the oils, all with a focus to test and ensure thermodynamic equilibrium of the oil column. The mixed charge of an oil and a secondary gas is confirmed identifying the source of asphaltene instability. The result of the asphaltene instability is to create a fraction of crude oil at the base of the oil column with asphaltenes dispersed as clusters of asphaltenes, thus exhibiting a larger asphaltene gradient there. Fluid inclusion measurements are useful to indicate the sequence of events in gas and oil charging into the reservoirs in geologic time. The Flory-Huggins-Zuo EoS and the Yen-Mullins model continue to be confirmed in oilfield studies as well as in the laboratory, even in cases of limited asphaltene instability observed here.