Benchmark simulations of resistivity-tool response in various environments and complex geometries can aid enormously in explaining tool behavior, and help resistivity-log interpreters to a successful and correct interpretation. It is important to recognize that not every wiggle on resistivity logs corresponds to a resistivity (or conductivity) change in the earth. Conversely, a lack of wiggles does not necessarily guarantee that resistivity in the formations is homogeneous, if the thicknesses of beds are below the tool’s vertical resolution.
Enthusiasm for the new array-type tools with heavy postprocessing and marketing may have led to widely propagated claims on the vertical resolution and multiple depths of investigation of those new tools. However, the fundamental physics of the induction and laterolog tools has not been changed much over the last 50 years. The benchmark studies carried out in this chapter are on the responses of the longtime industry-standard induction, dual-laterolog, and array-laterolog tools (6FF40/ILD, DLT-E, and HRLA) to illustrate the impact of a given environment on resistivity logs. Conditions studied by numerical simulation include bed thickness, main-bed to shoulder-bed resistivity contrast, and dip angle with 1-D induction modeling. Invasion, anisotropic resistivity, and horizontal-well effects were studied with 2-D or 3-D laterolog modeling.
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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.