Downhole fluid analysis (DFA) has been successfully used to delineate reservoir attributes such as vertical and lateral connectivity and properties of the produced fluids. The new-generation DFA tools not only measure bulk fluid properties such as gas/oil ratio (GOR), density, and light-end compositions of CO2, C1, C2, C3–C5, and C6+ more accurately but also color (optical density) that is related to the heavy ends (asphaltenes and resins) in real time at downhole conditions. In addition, the color measurement is one of the most robust measurements in DFA. Therefore, color gradient analysis in oil columns becomes vital to discern reservoir complexities by means of integrating advanced asphaltene science with DFA Fluid Profiling.
In this paper, a thermodynamic asphaltene grading model was developed to describe equilibrium distributions of heavy ends in oil columns using the multicomponent Flory-Huggins regular solution model combined with a gravitational contribution. The variations of oil properties such as molar volume, molar mass, solubility parameter, and density with depth were calculated by the equation of state (EOS). A three-parameter Gamma distribution function was employed to characterize asphaltene components. The primary factors governing asphaltene distribution in reservoirs are the gravitational term, which is determined in part by the size of the asphaltene molecular or colloidal particle, and the solubility term, which is determined in large part by the GOR. Consequently, it is critical to accurately measure both the fluid coloration and the GOR to understand the asphaltene distribution. The two field case studies showed that colored resins (asphaltene-like heavy resins) were molecularly dissolved in condensate oil columns whereas asphaltenes were dispersed as nanoaggregates in crude oils. The heavy ends (resins or asphaltenes) have a preference of going to the bottom of the oil column both because of gravity and the variation of the liquid-phase (live oil mixture) solubility parameter. The results obtained in this work were in accord with the observations in recent advances in asphaltene science. The asphaltene distributions were consistent with an equilibrium distribution implying reservoir connectivity. In both cases, the subsequent production data proved the reservoir connectivity and the methods developed herein were validated. This methodology establishes a new powerful approach for conducting DFA color and GOR gradient analyses by coupling advanced asphaltene science with DFA Fluid Profiling to address reservoir connectivity.