DFA Asphaltene Gradients for Assessing Connectivity in Reservoirs under Active Gas Charging

Abstract

Abstract Identification of reservoir compartmentalization, quantification of flow connectivity, and assessment of compositional gradients are critical 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, a new modified Flory-Huggins regular solution model that has been referred to as the Flory-Huggins-Zuo EOS has successfully been developed and used to delineate reservoir connectivity recently. It 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 this case study, we demonstrate that the methodology for equilibrated reservoirs can be extended to nonequilibrium oil columns. We employ the new asphaltene Flory-Huggins-Zuo (FHZ) EOS for a reservoir 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). This methodology establishes a powerful new approach for conducting DFA color grading analysis by coupling the Yen-Mullins model, and the FHZ EOS with DFA to address reservoir connectivity in reservoirs under active gas charging.