Since well logging began more than 80 years ago, the petroleum industry has relentlessly sought the true formation resistivity, Rt, the crucial property needed to estimate formation hydrocarbon saturation and reserves. Determining Rt, one of the earliest recognized challenges in well logging, is now going through a qualitative leap with the application of rigorous tool response modeling. This chapter looks at costs, benefits, and tool-specific issues involved in determining true formation resistivity.
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Understanding resistivity-tool response and resistivity-log interpretation for formation evaluation is vital for the matching of the reconstructed deep-reading resistivity logs with the field log curves. AAPG Archie 2 introduces the fundamental concepts required. Resistivity-logging-tool physics and measurement accuracy are reviewed, and forward- and inverse-modeling resistivity-tool responses are introduced. In the case studies presented, well-deviation, shoulder-bed, bed-thickness, borehole, mud-resistivity, and invasion effects on restivity-log responses are discussed. This volume has been written for geoscientists and engineers working with and interpreting resistivity logs, petrophysicists and reservoir engineers integrating resistivity-based and capillary-pressure-based quantitative calculation of formation water saturation, and formation evaluation specialists.