Quantitative study on the blockage degree of pores due to asphaltene precipitation in low-permeability reservoirs with NMR technique

With the nuclear magnetic resonance (NMR) technology, a new experimental method was proposed to study the blockage degree of pores due to asphaltene precipitation during CO2 flooding in Low-Permeability Reservoirs. By comparing the T2 spectrum measured before and after CO2 flooding, the blockage degree by asphaltene precipitation was quantitatively determined for the pores with different sizes. It was found that, at the immiscible flooding stage, oil was mainly recovered from the larger pores (1.0-100.0 μm) and the asphaltene tended to precipitate in such pores. Compared with the larger pores, nearly no asphaltene precipitation was formed in the smaller pores (0.1-1.0 μm) at the immiscible stage. At CO2 injection pressures above the minimum miscibility pressure (MMP), asphaltene precipitation formed in the lager pores; precipitation also took place in the smaller pores, filling up a significant portion of space in the smaller pores. At the miscible flooding stage, the total amount of asphaltene precipitation was reduced. The observed permeability reductions correlate reasonably with the amount of asphaltene precipitation for all flooding stages. By analyzing the T2 spectrum in different pores before and after CO2 flooding, it was observed that the smaller pores were more apt to be blocked by the asphaltene precipitation than the larger pores. Since the asphaltene-blocking phenomena significantly affect the smaller pores, the core permeability may be substantially compromised after CO2 flooding.