Systematic Investigation of Asphaltene Deposition in the Wellbore and Near-Wellbore Region of a Deepwater Oil Reservoir Under Gas Injection. Part 1: Thermodynamic Modeling of the Phase Behavior of Polydisperse Asphaltenes

Thermodynamic modeling is conducted for a high-asphaltene, high-resin crude oil produced from a deepwater reservoir using the perturbed-chain statistical associating fluid theory (PC-SAFT). The asphaltenes are characterized as a polydisperse fraction following a three-parameter Gamma distribution function with resins included as the lightest cut of the asphaltene distribution. Modeling results with 55 mol % injection indicate that the driving force of precipitation is sufficiently large that a significant amount of non-asphaltene components co-precipitate with asphaltenes. As pressure decreases from the upper asphaltene onset pressure (UAOP) to the bubble pressure (BP), the amount (by weight) of the asphaltene-rich phase surpasses the amount of the asphaltene-lean phase. Interestingly, as pressure decreases below the BP, the asphaltene-lean phase dissolves into the asphaltene-rich phase until the pressure reaches the lower onset pressure, where the lean phase is completely dissolved. As a result of the high driving force of precipitation, all asphaltenes precipitate out of solution before the BP and significant amounts of the other pseudo-fractions also co-precipitate. This causes the composition of asphaltenes in the asphaltene-rich phase to decrease as the pressure decreases, and the asphaltene composition in the asphaltene-rich phase at the upper onset is 23.6 wt %, which is uncommonly low for a precipitating phase. Experimental images from high-pressure microscopy show that the shape of the formed asphaltene-rich phase changes from a rigid solid-like structure near the UAOP to a soft liquid-like structure as the pressure decreases. This indicates a decrease in asphaltene composition during depressurization, similar to the simulation results produced by PC-SAFT. A sensitivity analysis is performed to evaluate the assumption of poly- and monodisperse asphaltenes from a modeling perspective and the effect of the Gamma distribution parameters.