An experimental and numerical modeling program was undertaken to evaluate stability of asphaltenes in a 29 degrees 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 precipitation in live oil were investigated for a range of pressures and temperatures, in presence of CO2 and normal pressures and temperatures, in presence of CO2 and normal alkanes as precipitance. 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 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. properties and in-situ structure. Between 1984 and 1988, Shell Canada conducted a tertiary CO2 Flood Pilot in the Midale Unit in southeastern Saskatchewan. Midale reservoir, an extensively fractured carbonate formation, was discovered in 1953 and has been under waterflood since 1962. Reservoir temperature is 65 degrees C [149 degrees F] and CO2 miscibility pressure is about 15.5 MPa [2250psi].
Shortly after the CO2 breakthrough in production wells, a considerable amount of solid deposit (consisting of asphaltene, wax, and trapped oil) formed in the wellbore equipment and downhole facilities. No such problems occurred during the field primary and secondary production. Similar deposition of solids has been reported by many operators of miscible gas injection projects. Deposit formation adversely affects project economics because of flow impairment, production delays, and costly cleanup operations. Recently, an overview of field experiences with asphaltene deposition and of different ways of dealing with it was provided by Leontaritis and Mansoori.
Crude oil asphaltenes have been studied for the past 50 or so years. Despite the efforts to establish true in-situ asphaltene structure and nature of asphaltene equilibria, a considerable difference in opinion still exists within the scientific community. Asphaltenes are described as molecular entities dissolved in oil, as colloidal particles, or as a combination of both.
In formulating asphaltene thermodynamic model, two distinct approaches have recently emerged. Hirschberg et al. proposed the "molecular solubility" model. Attempts by proposed the "molecular solubility" model. Attempts by others to apply this model met with a mixed degree of success. Mansoori and coworkers formulated the "colloidal suspension" model. Kawanaka et al. described asphaltenes as polydisperse species and introduced continuous polydisperse species and introduced continuous thermodynamics formalism to molecular solubility model. Good overview of past studies, modeling efforts, and of current understanding of asphaltene behaviour is provided in reference 6. It is evident that the physical structure of asphaltenes in crude oils and the mechanism of their precipitation remain to be resolved. precipitation remain to be resolved. P. 599
