DFA Profiling of Oil Columns With Asphaltene Gradients

Abstract

Abstract Downhole fluid analysis (DFA) has successfully been used to delineate reservoir attributes such as vertical and lateral connectivity and properties of the produced fluids. The new-generation DFA tools measure GOR, density, light-end compositions of CO2, C1, C2, C3–C5, and C6+, and color that is linearly related to the heavy ends in real time at formation conditions. Color grading analysis in oil columns becomes vital to discern reservoir complexities by means of integrating advanced asphaltene science with DFA Fluid Profiling. Modeling of asphaltenes and heavy ends of reservoir crude oils is only now becoming possible because of recent advances in the understanding of the molecular and colloidal structure of asphaltenes in crude oils. The Flory-Huggins polymer model has been widely used in modeling of asphaltene precipitation in the oil and gas industry. However, it has been excluded to describe asphaltene gradients in oil columns because the gravitational term requires knowledge of the sizes of asphaltenes which have been unclear until now. On the other hand, typical cubic equations of state (EOS) cannot represent asphaltenes well because of inaccurate asphaltene characterization and the lack of critical points for asphaltenes. Here, we treat the gas-liquid components of crude oil with the Peng-Robinson EOS. Heavy end gradients are treated with the Flory-Huggins formalism explicitly incorporating the size of asphaltene molecules and aggregates. All parameters in the model were related to DFA measurements such as compositions, GOR, density, etc. The variations of gas and oil properties with depth were calculated by the Peng-Robinson EOS based on DFA compositions using specifically developed delumping, characterizing, and OBM correcting techniques. In particular, oil solubility parameters are linearly related to DFA density. Filed case studies prove the value and simplicity of our heavy end treatment. A volatile oil case study shows that colored resins (heavy asphaltene-like resins) are molecularly dissolved in crude oils. Several case studies show asphaltenes are dispersed as nanoaggregates in stable crude. These case studies are in accord with the observations in the recent advances in asphaltene science. Heuristics can be developed form results herein corresponding to prediction of asphaltene gradients. For example, heavy ends (resins, asphaltenes) preferentially reside at the bottom of the oil column both because of gravity and the possible GOR gradients, thus variable solvation strengths. This methodology establishes a powerful new approach for conducting DFA color grading analysis by coupling advanced asphaltene science, the Flory-Huggins grading model, the new workflow and other information with DFA Fluid Profiling to address reservoir connectivity.