Estudo da precipitação de asfaltenos induzidos por n-alcanos e dióxido de carbono em petróleos brasileiros por intermédio de titulação em alta pressão

Abstract

The behavior of three different Brazilian crude oils was investigated by intermediate of a high-pressure titration procedure to evaluate asphaltenes stability. These oil samples were named PET-1 (16.5 ºAPI), PET-2 (26.0 ºAPI), and PET-3 (28.0 ºAPI) provided by Petróleo Brasileiro S.A. (Petrobras). The samples were studied in a PVT cell equipped with a solid detector system with light scattering in the near-infrared spectral region. The oil samples were titrated using different compounds (n-alkanes and CO2) as diluent and it was analyzed qualitatively the evidence of asphaltene precipitation. Morphology of precipitated particles and asphaltene onset point was detected by using a high-pressure microscope coupled with the PVT cell. The tests were performed at pressures and temperature conditions typical of petroleum reservoir. No evidence of asphaltenes was found during n-heptane titrations, except for PET-1, which showed the presence of asphaltenes in a molar concentration greater than 85% of titrant. Since the PET-2 + n-heptane and PET-3 + n-heptane systems did not present precipitated particles in the titration, they were submitted to the pressure depletion stage to verify the effect of pressure on asphaltene precipitation. During this test, PET-2 demonstrated no traces of flocculation, aggregation, or precipitation of particles, even with n-heptane molar composition above 85%, indicating highest asphaltene stability. Nevertheless, PET-3 showed significant aggregation and precipitation of particles during fluid depletion in the highest pressures evaluated, confirmed by SDS signal decrease, indicating the presence of asphaltenes. Solids formation presented no-redissolution during repressurization, demonstrating that asphaltene precipitation could be an irreversible process under experimental condition tested. In the systems containing other titrants, a possible liquid-liquid region was qualitatively observed at one of the studied pressures, in the case of n-propane; non-precipitation of asphaltenes during titration and pressure depletion with n-pentane; as well as greater amount of precipitate and aggregate of asphaltenes when CO2 is titrated with PET-3 oil at high pressures, maintaining constant typical reservoir temperature.