Carbonate Rock Wettability Interpreted from Capillary Pressure and Imbibition Resistivity Index Analysis

ABSTRACT

The wettability of rock samples from 5 carbonate reservoirs in Western Canada varies from weakly water wet to mixed wet to mildly oil wet. Wettability was determined by USBM and Amott analyses on 17 aged rock samples. Evidence supporting these interpretations was provided in one case by a contact angle measurement of the live oil, and in other instances by relations between un-restored primary drainage and restored secondary drainage capillary pressure curves. Imbibition resistivity index (IRI) measurements were performed on 9 aged sister-plug samples to determine a restored Archie saturation exponent “n”. The saturation exponent measured at the onset of the IRI test provides the most reliable “n” value as it is determined on a restored sample prior to any hysteresis that occurs during increasing Sw. Values for the restored “n” vary from 1.2 to 3.1. Although the imbibition RI tests provide a reliable measure of the saturation exponent, this number cannot be used alone as a valid indicator of wettability. Wettability and pore geometry principally control the shape of the RI profile during imbibition. Comparisons of imbibition RI profiles and water relative permeability curves were made between an oil wet and water wet reservoir with similar pore geometry. The water wet samples generally exhibit suppressed electrical and hydraulic behaviour during imbibition consistent with a more water wet pore system. In contrast, the oil wet samples exhibit an instantaneous flow of water and electric current with increasing Sw that is consistent with a more oil wet pore system. The wettability interpretations and saturation exponent measurements made for these 5 carbonate reservoirs were used to refine expectations of reservoir performance and enhance petrophysical interpretations.

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