Radial Resolution

Key Takeaways

• Radial response functions characterize how each logging measurement integrates response from different radial distances — the differential geometric factor specifies what fraction of the total signal originates from a specific radial distance, with the cumulative geometric factor providing the integrated response from the borehole wall to that distance; for invasion-corrected interpretation, the multi-array resistivity tools provide measurements with characteristic differential geometric factors that peak at different radial distances, supporting separation of invaded zone resistivity from undisturbed zone resistivity through the inversion process; the radial response functions are documented in service company tool specifications and are the fundamental basis for radial resolution analysis.
• Resistivity tool radial resolution variation across the array supports invasion characterization — the array of resistivity measurements at different depths of investigation provides the data needed for invasion-corrected interpretation; modern array resistivity tools (Schlumberger HRLA, Halliburton EM Quad, Baker Hughes Multi-Component Induction) provide 4-6 different radial responses that, when inverted simultaneously, produce the invasion model parameters (Rxo, Rt, invasion diameter, transition characteristics) that support accurate water saturation calculation; without the array of measurements with different radial resolutions, simultaneous interpretation of invaded zones would not be possible.
• Density and neutron porosity radial resolution is generally shallower than resistivity, with peak responses typically at 4-12 inches from the borehole — the shallower investigation makes density and neutron porosity primarily flushed-zone measurements, with the resulting porosity values reflecting the immediately near-wellbore formation properties; for accurate porosity calculation in deeply invaded formations, the porosity measurements may need correction for invasion effects, or the petrophysicist may rely on specific assumptions about flushed zone vs deep formation properties; modern combined density-neutron-resistivity logging provides comprehensive radial characterization that supports invasion-corrected interpretation across multiple measurement types.
• NMR radial resolution is among the shallowest of routine logging measurements, with peak response typically within 1-3 inches of the borehole — the shallow investigation reflects the limited depth that the NMR static and gradient magnetic fields can penetrate the formation; the shallow NMR investigation provides high-quality measurement of near-wellbore properties but limited information about deep formation properties; modern NMR tools (Schlumberger CMR Plus, Halliburton MRIL Prime) include multi-frequency or multi-shell capability that provides slightly different radial responses, supporting some invasion analysis although the depth range is much more limited than resistivity-based invasion analysis.
• Operational implications of radial resolution variation include the need to combine measurements with different radial responses for comprehensive formation characterization — single measurements (e.g., shallow MSFL alone or deep induction alone) provide limited information about radial property variations, while integrated multi-measurement analysis supports the more complete characterization needed for invasion-corrected saturation calculation, formation damage assessment, and other applications; modern integrated formation evaluation software automatically combines measurements with appropriate radial responses to provide the multi-zone characterization that supports reliable interpretation in real-world conditions.