AAPG Middle East Region Geoscience Technology Workshop

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Understanding and Accounting for the Role of Pore Micro-Geometry in Contributing to Low Resistivity Pay in Heterogenous Carbonates


It has long been observed that resistivity-based saturation in heterogenous carbonate reservoirs immediately above the oil-water contact (OWC) indicates considerably higher water saturation than that evident from pulsed neutron logs or that expected from oil cut in production tests. Given that much higher resistivity values would have been expected in order to match the evidence from other sources, this phenomenon is termed low resistivity pay in carbonates. In this talk we develop further on an effective medium conductivity model of carbonate rocks that has been shown over the past couple of decades to accurately predict Archie’s cementation exponent in water saturated carbonates of the Middle East. The new formulations account for the presence of hydrocarbons in the rock and are shown to correctly model the conductivity response in both drainage and waterflood. Petrophysical principles governing the ingress and extraction of hydrocarbons in the rock are brought to bear on the solution of water saturation from resistivity measurements. In the process we explain the apparent enhanced conductivity of such rocks close to the OWC and also the observed hysteresis in the resistivity index curves over the entire saturation cycle. The model provides considerable insight into the link between pore geometry and conductivity, allowing the user to anticipate when LRP might exist and also improve saturation estimates post waterflood. We demonstrate the applications on several Middle East wells.