A True Multiple Depth of Investigation Electromagnetic Wave Resistivity Sensor: Theory, Experiment and Prototype Field Test Results.

Abstract

Summary This paper presents theoretical, experimental, and field results for a multiple-depth-of-investigation, measurement-whiledrilling (MWD), electromagnetic wave resistivity (EW) sensor. The sensor has four different transmitter/receiver antenna spacings with a phase difference and amplitude ratio at each spacing. The resultant logs provide information on the resistivity radial profile and an accurate Rt evaluation. Comparisons of field logs with wireline logs and with theoretical and experimental results show that this type of sensor can offer an extra-shallow, shallow, medium, and deep investigation depth under most logging conditions. Introduction Interest in providing propagating EW sensors with multiple depths of investigation has grown in the past several years. The introduction in 1983 of MWD propagating-wave electromagnetic resistivity sensors contributed to this interest. These sensors rapidly gained popularity over MWD short, normal sensors because of their good bed resolution and ability to provide high-quality logs under a wide range of borehole conditions. This, in turn, led to more applications for propagating EW logging. Preinvasion MWD resistivities were preferred in some cases over dual-induction logs run days after bit penetration. The good vertical resolution of EW sensors allowed direct measurement of Rt in thin beds and deviated holes without the uncertainties inherent in departure-chart-corrected wireline induction logs. Propagating EW sensors also were run after drilling in problem wells where it was impossible or impractical to obtain conventional wireline logs. In these cases, invasion often was present, but its effect was unknown. In addition, it was sometimes necessary to place the resistivity sensor far from the bit. In other applications where offset information was available, the formation was expected to be sufficiently permeable so that even MWD logs would be affected. To address the invasion question, several propagating electromagnetic wave sensors have been marketed that measure formation resistivity by use of the phase difference between two receiving antennas and by use of the ratio of the signal amplitudes at the same antennas. This approach was selected because of the common belief that amplitude-based measurements always "see" deeper into the formation than phase-based measurements. This conclusion, reached in 1974 by Soviet investigators, now is known to be too simplistic.