Radial Response of a 2-MHz MWD Propagation Resistivity Sensor

Abstract

Abstract Electromagnetic propagation resistivity sensors have become common in MWD logging applications. These tools are unique among resistivity sensors in that the depth of investigation of the phase shift and attenuation resistivity measurements varies dramatically with the formation resistivity, measuring deeper into the formation in higher resistivity environments. The fact that the depth of investigation can vary by more than a factor of two across a normal range of formation resistivities requires the log analyst to have an understanding of this phenomenon when performing both qualitative and quantitative interpretations performing both qualitative and quantitative interpretations of these logs. Other measurement characteristics such as regions of negative or sharply-rising radial response can produce log responses which may seem peculiar when produce log responses which may seem peculiar when compared to traditional induction logs or laterologs. However, the radial response in virtually any borehole and formation environment can he quickly determined by computer modeling. This modeling, coupled with comparisons between propagation resistivity and induction logs can provide the log analyst with an understanding of the radial response of these sensors and facilitate both qualitative and quantitative interpretations. Introduction Well logs are interpreted in two basic ways: qualitatively through visual inspection by a trained log analyst, and quantitatively through either hand calculation from paper logs or computer processing of digital data. A fundamental component of both types of interpretation is a knowledge of the radial and vertical response of the various sensors. in visually interpreting printed logs, a log analyst mentally compensates for variations in the depth of investigation and vertical resolution of the measurements represented by the log traces. Various deconvolution, resolution-matching, and invasion-correction routines perform a similar function in many computer-processed Interpretations. Thus, he geometric response of a particular sensor must be an integral part of a log analyst's knowledge, and must also be incorporated into computerized interpretation routines. Measurement-While-Drilling (MWD) resistivity sensors that transmit a radio-frequency (usually 2 MHz) electromagnetic wave and measure the phase difference and attenuation between two receivers are available from several service companies and have become the predominant type of MWD resistivity sensor. The velocity (phase difference) and attenuation of a radio-frequency electromagnetic wave is controlled by the conductivity, dielectric permittivity, and magnetic permeability of the medium through which it propagates. Most sedimentary formations have negligible propagates. Most sedimentary formations have negligible magnetic permeability, and at frequencies near 2 MHz, the dielectric contribution is small, particularly at higher conductivities. Thus, formation conductivity (or resistivity) can be derived with reasonable accuracy from either the attenuation or phase difference measurements. A valuable feature of these sensors is that the attenuation and phase difference resistivity measurements made from the same transmitter and receiver antennas have different radial responses, with the attenuation resistivity (R at) generally measuring deeper than the phase difference resistivity (R pd). P. 481