Basics of Resistivity Tools
Resistivity logs use electrode arrays or coil arrays for focusing. The accuracy of the measurement is enhanced by designing the array to focus into the zone of interest.
The most significant difference between laterolog and induction devices is that laterologs put the borehole fluid, the invaded zone, and the undisturbed zone in series in the radial direction of the borehole as regards current flow, whereas the induction devices put these regions in parallel. If the injected currents are constant, the measured potentials for laterolog tools increase proportionally with resistivity. The eddy current induced in the formation and the electric potential measured by induction devices will increase as the conductivity increases (i.e., as the resistivity decreases). This chapter briefly describes laterolog and induction tool response theories.
Figures & Tables
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.