Compositional dependence of pyrobitumen formation in petroleum reservoirs
Résumé
Pyrobitumen is solid, insoluble bitumen originating from the thermal degradation of liquid oil. Prospecting for petroleum in deep, hot reservoirs increases the risk that the reservoir will contain pyrobitumen which acts as a porosity and permeability-reducing cement and is thus considered highly detrimental to reservoir quality. Due to its relatively low density, it also leads to incorrect wireline log estimates of the oil in place. The aim of this work was to determine the pyrobitumen formation potential of immature, heavy type II oil and its constituents as a result of thermal cracking. Closed system gold tube pyrolysis was used to artificially mature asphaltic oil (Boscan, Venezuela) at temperatures from 300-550°C for 4 hours. Samples of the same oil were fractionated into saturate, aromatic, resin, and asphaltene (SARA) components which were pyrolysed isothermally under the same conditions as for the whole oil. At 500°C all fractions produced pyrobitumen although asphaltenes yield more insoluble residue (62%wt) compared to the saturates (19%wt). In addition the asphaltene and resin fractions produced pyrobitumen at lower temperature than the aromatics and saturates. Petrographic analysis of the insoluble residues indicates that reflectance evolution and graphitizing potential vary according to the composition of the oil precursor. This work shows that whilst pyrobitumen is a natural consequence of thermal cracking disproportionation, the amount of pyrobitumen produced and its physico-chemical and optical properties are strongly dependent on precursor oil composition.