Paper xx

Interpreting the Response of the Electromagnetic Propagation Tool in Complex Borehole Environments

Barbara Anderson, Qing-Huo Liu, .Reza Taherian, Julian Singer, Weng Cho Chew, Bob Freedman and Tarek Habashy

Schlumberger

Abstract

The Electromagnetic Propagation Tool (EPT) measures the attenuation and travel time of a 1.1-GHz wave propagating past two receivers. These measurements are used to determine water-filled porosity. Because of its high vertical resolution, another application of the tool is in thin bed analysis, The tool is operable in both water-base and oil-base muds.

Recently developed modeling codes are applied to analyze EPT response in complex borehole environments. The most complicated code, which is based on an efficient numer­ical mode matching method, takes into account the effect of mudcake or standoff in an arbitrary number of thin beds (with or without invasion), Codes based on analytical so­lutions for simpler geometries are also used. All these codes have been extensively verified by comparison with laboratory experiments.

Modeled results examine the effect of standoff on vertical resolution, Normally, thin  beds on the order of 2 in. thick can be resolved, and resolution gradually deteriorates with increasing standoff. However, in some high contrast cases, reflections of electromagnetic waves from bed boundaries or radial interfaces can actually cause the EPT tool to read anomalously higher or lower than it would read in homogeneous media. In laminated zones where bed  thicknesses are 0.5 in. or less, the tool reads some average of the travel times and attenuations of the individual layers.

The effect of conductivity level on the depth of investigation is also examined over typical ranges of formation conductivities and dielectric constants. Although the usual depth of investigation is about 2 in., it can be as much as 1 ft or more in some lossless hard formations.

The effect of antenna polarization is examined. It is confirmed that the Broadside attenuation measurement has better vertical resolution than the Endfire measurement. However, the Endfire travel time measurement has better vertical resolution. It is also shown that the Endfire measurement is less sensitive to mudcake effect and has a greater depth of investigation.