Asphaltene gradients and tar mat formation in reservoirs under active gas charging

Based on downhole fluid analysis (DFA) measurements and advanced asphaltene science, a new modified Flory–Huggins regular solution model that has been referred to as the Flory–Huggins–Zuo equation of state (FHZ EOS) has successfully been developed and used to delineate reservoir connectivity recently. It provides the industryʹs first simple explicit EOS for predicting asphaltene grading and has proven reliable to predict connectivity in equilibrated oil columns. In this paper, the methodology for equilibrated reservoirs is extended to a nonequilibrium oil column, especially for a reservoir under active charging of biogenic gas. Based on the properties computed by the Peng–Robinson EOS with methane influx, the FHZ EOS for asphaltenes – originally formulated for equilibrium columns – is used to model the asphaltene (optical density) gradient in the nonequilibrium oil column. In addition, the FHZ EOS with the same parameters is employed to check whether the predicted 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 downward) 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 an asphaltene-rich phase and a tar mat could form if asphaltene flocculation is severe over geological time.