Bulk composition control on insoluble reservoir bitumen formation.
Résumé
Pyrobitumen is solid, insoluble bitumen originating from the thermal degradation of liquid oil (Jacob 1993). 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 (Lorenz 1998). Due to its relatively low density, it also leads to incorrect wireline log estimates of the oil in place (Elkins 2001). 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 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. References Elkins, L. F. (2001). "Uncertainty of oil-in-place in unconsolidated sandstone reservoirs, - a case history." Journal of Petroleum Technology 24: 1315-1319. Jacob, H., in, eds., p. (1993). Nomenclature, classification, characterization, and genesis of natural solid bitumen (migrabitumen). Bitumens in ore deposits. H. K. J. Parnell, and P. Landais. New York, Springer- Verlag: 11- 27. Lorenz, J. C. B., R.L.; Evans, L.W. (1998). "Permeability reduction by pyrobitumen, mineralization, and stress along large natural fractures in sandstones at 18300 ft depth: destruction of a reservoir." SPE Reservoir Evaluation and Engineering 1(1): 52-56.