Borehole Caliper while Drilling from a 2-MHz Propagation Tool and Borehole Effects Correction

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Abstract The Compensated Dual Resistivity (CDR*) tool computes two apparent resistivities, Rad and Rp8, from the attenuation and phase shift of a 2-MHz electromagnetic wave measured between two receivers. While these measurements are insensitive to the borehole in most situations, the relative phase between a receiver and a transmitter is quite sensitive to the borehole diameter and the mud conductivity. Since the mud conductivity is known, this sensitivity can be used to estimate the borehole diameter during drilling, tripping or washing down. This phase caliper can be obtained in real time using data transmitted to the surface, or it can be calculated after bit runs using data stored in the tool. The borehole diameter can then be used to apply borehole corrections to Rad and Rp8. The borehole correction algorithm can also use any other caliper or bit size as input. Graphs showing the borehole effects for a range of borehole sizes, mud resistivities and formation resistivities are presented. Log examples of the phase caliper and borehole-corrected resistivities are presented.