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

Abstract

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 Γ 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 Γ distribution parameters.

Phase Behavior for Crude Oil and Methane Mixtures: Crude Oil Property Comparison

“…Additionally, there is a variation on the light absorbance of the crude oil sample when the phase evolves, equally changing its phase morphology from a fine dispersed particulate to a droplet-like material. Similar results have been reported by Abutaqiya et al, 30 and according to the authors, these results could be related to the increasing of the asphaltene mass deposition during depletion.…”

Aromatic Ring Index (ARI): A Characterization Factor for Nonpolar Hydrocarbons from Molecular Weight and Refractive Index

“…For the asphaltene pseudo fraction, the aromaticity (γ Asph ) is tuned to reproduce experimental asphaltene onset pressures. Despite the wide success of this characterization procedure and its extensions, ,− it is common for the authors who implement this characterization to report an aromaticity factor for the A + R pseudo fraction that is higher than the aromaticity of asphaltenes. ,− This observation–at first glance–seems inconsistent because asphaltenes are known to be the heaviest and most aromatic fraction of crude oil. − One of the primary goals of this work is to explain the apparent inconsistency of the reported aromaticity of A + R and asphaltenes pseudo fractions by developing a characterization factor that provides a consistent measure of aromaticity.…”

“…One of the most successful characterization methods for modeling PVT properties and asphaltene precipitation using PC-SAFT EOS is the SARA-based method. ,− , In brief, the dead oil is characterized as saturates, aromatics + resins (A + R), and asphaltenes. An aromaticity factor, originally introduced by Ting et al , and later modified by Punnapala and Vargas, is used to obtain the PC-SAFT simulation parameters for the pseudo fractions.…”

Role of Characterization in the Accuracy of PC-SAFT Equation of State Modeling of Asphaltenes Phase Behavior

“…In this work, asphaltene molecular weight is taken as the nanoaggregate size average at ∼1700 g/mol. Other approaches to the variation of the molecular weight is to treat asphaltene as polydisperse component as done by Tavakkoli et al and Abutaqiya et al…”

“…In this work, asphaltene molecular weight is taken as the nanoaggregate size average at ∼1700 g/ mol. Other approaches to the variation of the molecular weight is to treat asphaltene as polydisperse component as done by Tavakkoli et al 18 and Abutaqiya et al 19 The ability to understand asphaltene stability has been a continuous research topic in the oil industry as it offers great promises to limit asphaltene deposition by understanding the pressure−temperature region causing asphaltene precipitation. Asphaltene stabilization is generally explained following two theories.…”