Depth of Investigation
Learning objectives
- Define a tool's depth of investigation
- Relate shallow, medium, and deep readings to the flushed and virgin zones
- Explain how the separation between the curves reveals invasion
- Recognize when deep invasion compromises even the deepest reading
How Deep Does It See?
If the value we need is the deep Rt, how do we get past the invaded zone? The answer is that resistivity is not measured at a single radius. Every tool has a depth of investigation, the radius into the formation that its signal is weighted toward. Tools are built in families, shallow, medium, and deep, and run together they look at the same rock from three different distances.
Three Looks at the Same Rock
The shallow reading is dominated by the flushed zone, so it sits near Rxo. The deep reading reaches past invasion, so it sits near the true Rt. The medium falls between. When the rock is uninvaded the three agree; when filtrate has pushed in, they separate, and that separation is the direct evidence of a permeable, invaded bed, and a clue to the movable fluid (recall that in oil the shallow reads below the deep).
Resolving Invasion, and Its Limits
With three depths and a model of how each tool sees the radial profile, the analyst can solve back for Rxo, Rt, and the invasion diameter, the idea behind the tornado-chart and modern inversion. But there is a limit: when invasion is very deep, even the deep tool loses sight of the virgin rock and is pulled toward Rxo. That is why a fresh, fast-invading mud in a high-permeability sand is the hardest case, and why mudcake, which throttles invasion, is quietly the deep tool's friend.
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.