Asphaltene Gradients and Flow Assurance in Reservoirs under Active Gas Discharging

Identification of reservoir compartmentalization, quantification of flow connectivity, and assessment of compositional gradients, flow assurance and tar mat formation are of great importance for optimal reservoir characterization, production, and management, especially in deepwater developments. Downhole fluid analysis (DFA) provides a useful tool to measure composition, gas/oil ratio (GOR), density, and color (linearly associated with asphaltene content). In particular, DFA is the method of choice to measure gradients of reservoir fluids vertically and laterally. Based on DFA measurements and advanced asphaltene science (the Yen-Mullins model), the Flory-Huggins-Zuo (FHZ) EOS has successfully been developed and used to delineate reservoir connectivity. Unlike cubic EOS which are a variant of the van der Waals EOS derived from ideal gas law, the FHZ EOS provides the industry’s first predictive asphaltene grading equation of state (EOS) and has proven reliable to predict connectivity in equilibrated oil columns. The theory shows that asphaltene gradients can be large owing to both the gravity term and GOR gradients.

In addition, the FHZ EOS for equilibrated reservoirs can be extended to nonequilibrium oil columns, especially to reservoirs currently undergoing active charging of biogenic gas. Isotope analysis shows that the biogenic methane is not equilibrated in this column. Nevertheless, the local asphaltene concentration within the column is shown to be equilibrated with the local GOR value and gradient. Based on the properties computed by the Peng-Robinson EOS with methane influx, the FHZ EOS for asphaltenes - originally formulated for equilibrium columns - may also be used to model the asphaltene (color) gradient in this nonequilibrium oil column. The obtained 2-nm asphaltene diameter is also consistent with field and laboratory data and is part of the Yen-Mullins model of asphaltene science (size of asphaltene nanoaggregates). In addition, the predicted asphaltene gradients with depth are tested by the same FHZ EOS to see whether the asphaltenes can be stably dispersed or suspended in the oil. The predicted results show that the asphaltenes at the base of the oil column are unstable because late gas charging (diffusing down) results in a reduction of oil solvation power for asphaltenes and makes asphaltenes concentrated near the base of the oil column. The concentrated asphaltenes are too much for the oil and they tend to be flocculated out to form a tar mat. This also explains a main mechanism of tar mat formation in oil reservoirs. This methodology establishes a powerful new approach for conducting DFA color grading analysis and tar mat formation checking by coupling the Yen-Mullins model, and the FHZ EOS with DFA to address reservoir and fluid complexities in reservoirs under active gas charging.