Abstract
In formation evaluation, resistivity logs are used to calculate the reservoir water saturation using the well known Archie's law. When high degree of heterogeneity exists, like when vuggy structures are present, the interpretation of electrical logs can be complex. In order to highlight the effect of a vug on a two-phase oil and water flow displacement, a laboratory apparatus was designed. The system employs the 4-contact electrode method to monitor the resistivity response of a rock slab during primary drainage and imbibition cycles. The effect of frequency on the saturation exponent n was also investigated.
Two rock slab models were cut from a strongly water-wet Indiana limestone block: one slab was left untouched while the other one was drilled at its center to simulate the presence of a cylindrical vug (vuggy model). Two transparent windows were placed at both ends of the vug to allow a direct visualization of the fluids distribution with a video camera. The resistivity change was monitored during primary drainage and imbibition cycles on both homogeneous (without hole) and vuggy models. Petrophysical properties of the rock matrix were measured on the homogeneous model (without hole) from the same block.
The visual cell was helpful in understanding the difference of flow behavior between primary drainage and imbibition on the vuggy model. A comparison between the electrical response of the homogeneous and vuggy rock models showed a difference between the two respective saturation exponents at constant frequency. The experimental setup also helped to study the dependence of Archie's saturation exponent n on frequency for the vuggy sample. Because resistivity tools operate at different frequencies (LWD versus wireline), this observation is crucial for the resistivity logs calibration and interpretation for heterogeneous oil reservoirs.