Effect of Variable Saturation Exponent on the Evaluation of Hydrocarbon Saturation

Summary Petrophysical measurements on effectively clean, desaturated core plugs from different fields have shown that the assumption of a constant saturation exponent for a given sample frequently is violated, although the conventional bilogarithmic distribution of resistivity-index/water-saturation data is often approximately linear. These variations in saturation exponent are attributed to the effects of pore geometry, in particular the nonuniform distribution of electrolyte within a heterogeneous pore system as desaturation progresses. The data indicate that, if unrecognized, a variable saturation exponent can induce errors of more than 10 saturation units (s.u.) in the petrophysical evaluation of water saturation. Procedures are outlined for the identification of those physicochemical reservoir conditions that can give rise to a variable saturation exponent and for accommodating these variations in reservoir evaluation.

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