The Resistivity Profile

Part 6, Chapter 6: Resistivity Logs and Invasion Profiles

Learning objectives

Explain why resistivity tools come in shallow, medium, and deep depths
Relate each tool's reading to Rxo, Rt, and the invasion depth
Read the curve separation as an invasion and fluid flag
Identify the deep reading as the closest to the true Rt

Why Three Depths

The resistivity that Archie needs is $R_t$ , the virgin resistivity beyond invasion. But near the borehole, mud filtrate has flushed the pores and set the resistivity to $R_{xo}$ . So a single reading is not enough; the tools come in a suite that looks at different radii, shallow, medium, and deep, to see past the invasion. Each tool reads a blend, weighted by how its depth of investigation compares with the invasion depth $d_i$ :

C_a = G\,C_{xo} + (1-G)\,C_t, \quad G = \frac{d_i^2}{d_i^2 + DOI^2},

with $C = 1/R$ and $G$ the fraction of signal from inside the invasion.

Reading the Separation

The shallow tool, with its small reach, reads almost pure $R_{xo}$ ; the deep tool leans on $R_t$ . When the three curves separate, the bed is permeable and invaded, a useful flag in itself, and the direction of the separation tells the fluid: with fresh mud over saline water the flushed $R_{xo}$ sits above $R_t$ , while in a hydrocarbon zone the virgin $R_t$ is lifted above $R_{xo}$ . The pattern of the spread is a quick fluid and permeability read before any calculation.

The Deep Reads Rt

Of the three, the deep reading is the closest to the true $R_t$ , which is why it is the one carried into Archie. But it is not perfect: as the invasion deepens, even the deep tool is dragged toward $R_{xo}$ , and the reading reads too high or too low. Recovering the true $R_t$ from the three apparent resistivities is the job of the tornado chart, the next section.

References

Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
Ellis, D. and Singer, J. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.