An MWD focused current resistivity system has been developed for' well conditions under which neither the short normal nor the electromagnetic device operates well; that is, in wells where highly conductive drilling fluids are used, where relatively high formation resistivities are encountered, and where large resistivity contrasts are expected. The focused current system employs a guarded electrode design; the dynamic measurement range is 0.1 to 1000 ohm-m at + 5 percent of the resistivity value. The response characteristics of the MWD focused current device compare well to some published responses for equivalent wireline systems.
The focused current system offers superior performance in logging high-angle holes where excellent thin-bed resolution is required, in high-salinity muds where other systems do not perform well, and in fresh muds where the short normal is typically run. It provides exceptional resistivity measurements in all conductive borehole fluids.
A limited number of MWD focused current logs, the system correction charts and comparisons of focused current log responses to wireline log responses in the same wells are shown.
Introduction
The ubiquitous conventional electrode devices dominated the first 20 years of commercial wireline logging and were reasonably successful in measuring formation resistivity when operating, as Figure 1 suggests, where formation resistivities (Rf) were approximately equal to mud resistivities (RM), borehole diameters were not much greater than tool diameters, invasion was shallow or determinable, and beds were thick relative to the tool's electrode spacings. However, logging in salt muds, highly resistive formations and relatively thin pay zones necessitated the development of systems specifically designed to perform well in such environments.
Figures 1 and 2 illustrate why the conventional electrode devices operate well in the former environment and poorly in the latter. That is, when formation resistivities are relatively low, Rf 21 RM, borehole effects are negligible and bed thickness tool spacings, current flows uniformly from the source electrode, the equipotential surfaces are essentially spheres, and the true formation resistivity (Rt) 9 apparent formation resistivity (RAPP), or at least, is correctly determinable from it. We call these conditions the fresh mud environment.
However, when Rf »RM, mud resistivities are very low, formation resistivities very high, boreholes large or bed thicknesses @ conventional electrode device spacings or smaller, the path of least resistance to current flow is within the borehole, equipotential surfaces become elongated spheroids, or football shaped (see Figure 2), and Rt RAPP (and in many cases, is not accurately determinable from it). We refer to these conditions as the salt mud environment.
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