The dual-resistivity MWD tool uses sections of the drill collar as electrodes and provides two independent resistivity measurements: a principal well-focused lateral measurement of the current flowing radially from a defined segment of the drill collar and a secondary axial or bit measurement of the current flowing from a defined portion of the lower bottomhole assembly (BHA). In oil-based muds, a single (bit) measurement is made of the current flowing to the formation through the drill bit.

A new "Toroidal" Measurement While Drilling (MWD) resistivity tool was introduced to the oil and gas industry in 1986. The tool utilities the drill collar as an electrode to provide two independent resistivity measurements: a focused lateral resistivity measurement and a trend resistivity at the drill bit. In oil based muds the tool provides a single high-resolution measurement at the bit-to-formation contact. Since its introduction the tool has been operated in a variety of diverse drilling environments, both in the North See and in North America, and in a wide range of drilling fluids including oil based muds. This paper reviews the performance of the tool with specific log examples highlighting the qualitative and quantitative responses of both resistivity measurements in comparison to conventional wireline logs. Particular attention is also given to the advantages and limitations of logging in the MWD environment. As a result of these operating experiences several changes were necessary in order to improve the reliability of the tool and to ensure good log quality, especially in fast drilling formations. These problems together with the effected solutions will also be presented. presented. Finally a brief review will be made of future enhancements to MWD system which will provide additional sensors for formation evaluation and reservoir analysis. Introduction The "toroidal" resistivity tool, based on a proposal by J. Arps in 1967, was developed in 1983 and combined with the MWD Gamma Ray measurement, which was already available at the time. This MWD Gamma Ray/Resistivity tool, the RG tool, was run for the first time in October 1984, to prove the validity of the resistivity measurements. The basic tool structure and electronics package of this RG tool were then re-designed to incorporate directional and deviation measurements, resulting in the RGD tool. This new resistivity tool provided a well-focused Lateral measurement, with a 2 feet receiver spacing, and an axial Bit measurement. The tool was introduced in 1986 and included a downhole processor, which manages the data acquisition system and controls, via commands from surface, the parameters to be measured, the transmission sequence and the transmission rate (Bits per second). Provision was made for the inclusion of sensors for downhole Provision was made for the inclusion of sensors for downhole annulus and drillpipe pressure Measurements shortly afterwards. The first commercial run with the now MWD tool was made in October 1986. Since then, structural improvements have been made to the tool to increase the protection of the toroids. p. 73