An experimental and numerical modeling program was undertaken to evaluate stability of asphaltenes in a 29°API gravity reservoir undergoing tertiary CO2 injection. The study aimed to develop a better understanding of asphaltic solids formation and possible ways of controlling it.

Asphaltene phase behaviour and the extent of asphaltene precipitation in live oil were investigated for a range of pressures and temperatures, in presence of CO2 and normal alkanes as precipitants. The nature of the solids formed was also studied using a variety of techniques. A molecular thermodynamic model with asphaltene-asphaltene and asphaltene-resin association was used to correlate the asphaltene precipitation data. The model not only matched the trends in asphaltene behaviour, but did so to a degree quantitatively.

The thermodynamic model calculations provided valuable insight into the behaviour of this complex system Destabilization of asphaltenes by CO2, the degree of which varied with system pressure and temperature, can be shown to be related to specific changes in the properties of the liquid (oil) phase, brought about by changes in system composition, pressure, and temperature. In agreement with observations in a recently completed CO2 flood pilot project, the model indicated asphaltene destabilization along the entire wellbore under the conditions prevailing in the pilot producing wells. The tuned asphaltene model allowed examination of asphaltene stability under a variety of wellbore conditions and gave indication of operating scenarios that should be avoided in a field project to minimize the solids formation.

The predictive capabilities of the model are critically evaluated in view of scarcity of reliable data on asphaltene properties and in-situ structure.

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