Step Profile (Invasion Model)

Key Takeaways

• Step profile model parameters include three fundamental quantities — flushed zone resistivity Rxo (typically the highest value among the three resistivity measurements used in tornado chart analysis when invasion is resistive, lowest when invasion is conductive), undisturbed zone resistivity Rt (typically determined as the asymptotic deep resistivity reading after invasion correction), and diameter of invasion (typical values 10-100 inches depending on formation conditions and time since drilling); the three parameters together define the step profile invasion that is the simplest representation of the actual invasion conditions; modern computerized invasion correction software automatically determines these parameters from the input array resistivity data through systematic inversion algorithms.
• Step profile vs more complex invasion models trade simplicity vs accuracy — the step profile model is mathematically simple and supports straightforward analytical interpretation through tornado charts and equivalent methods, but the simplification may introduce systematic errors in formations where the actual invasion has gradual transition zones or annulus features; more complex invasion models including transition zone (smooth gradient between flushed and undisturbed zones) and annulus model (specific oil-bearing-formation geometry with intermediate zone of high water saturation) provide more accurate representation of specific formation conditions but require more sophisticated analysis; the operational selection between models depends on the formation conditions and the analytical requirements.
• Operational use of step profile interpretation continues despite availability of more sophisticated alternatives — for many routine formation evaluation applications, the step profile interpretation provides adequate accuracy at acceptable analytical complexity; the conceptual simplicity of the step profile supports educational and explanatory uses where understanding the basic invasion correction concept is more important than precise quantitative analysis; modern integrated petrophysical software supports step profile interpretation as one option alongside more sophisticated alternatives, with the analyst selecting the appropriate model based on the specific application; the continued relevance of step profile interpretation reflects its operational adequacy for routine applications.
• Limitations of step profile model affect interpretation in specific situations — for heavily invaded oil-bearing formations where annulus zones develop (with high water saturation between flushed and undisturbed zones), the step profile model gives systematically incorrect results that should be replaced with annulus model interpretation; for shaly formations with substantial transition zones, the step profile may underestimate the actual flushed zone effects; for formations with azimuthal invasion variation, the step profile cannot capture the directional invasion character; modern integrated petrophysical interpretation considers these limitations and applies appropriate alternative models when the step profile assumptions are inadequate.
• Modern array resistivity tool processing extends the basic step profile concept — modern array resistivity tools (with 4-6 different depths of investigation rather than the 3 inputs of older tools) provide additional constraint that supports more sophisticated invasion analysis; the extended data supports interpretation through the step profile model with greater confidence (when the step profile is appropriate) or interpretation through more complex models when the step profile is inadequate; the integrated array tool processing represents the modern state of the art in invasion correction, with the resulting interpretation supporting reliable formation evaluation across diverse operational conditions.