Asphaltenes constitute a petroleum fraction characterized by low solubility in n-alkanes and high solubility in aromatic compounds. In general, such compounds are molecularly solubilized in the oil or are present in a colloidal dispersion. Changes in the temperature, pressure and composition of the oil may lead to the precipitation of these compounds which can decrease or even interrupt the oil production. The natural gas associated with oils from the Santos basin presents high levels of CO
2. These high levels do not allow the delivery of natural gas for use on land, since the maximum CO
2 content in this stream is 5%. In order to enable the production and maintenance of reservoir pressure, natural gas is fractionated in membranes and the CO
2-rich fraction is reinjected into the reservoir. Thus, precipitation of asphaltenes may occur during the production of the oil caused by the decrease in both the temperature and the pressure of the oil or in the injection well where a variation in the composition of the system occurs. In this work, experimental results are presented for the determination of the asphaltene precipitation onset of the reconstituted oil with natural gas both by the isothermal reduction of the pressure and by the injection of a mixture of CO
2 and methane. For determination of these onsets, a solids detection system composed by a laser at 1550 nm and a transmitted light power detector was assembled on a commercial variable volume cell. To verify the presence of precipitated material, the isobaric filtration of part of the oil was performed using a filter with a cutoff diameter of 0.5 μm. In a preliminary experiment it was observed that there was asphaltene precipitation even with small amounts of CO
2-methane injected.
ACKNOWLEDGEMENTS
The authors acknowledge the support of ANP â Agência Nacional de Petróleo, Gás Natural e BiocombustÃveis and Petrogal Brasil S.A., related to the grant from R&D investment rule.
