Optimizing Design and Performance of an MWD Resistivity Sensor

Abstract

Design specifications of any MWD resistivity sensor are constrained by the sensor being part of the drill string and by the resistivity environment in which the system must operate. Several alternative designs were modeled using Boundary Element Method software, running on a PC. Sensitivity of the device measurement parameters to variations in the system configuration was obtained by modeling and physical testing. The design of the sensor, based on model results, incorporates several unique features including a software control system that may offer significant operational benefits. The resulting guarded current resistivity device (GCR) has a diameter of investigation in the formation slightly less than a LLS under the same conditions of formation to mud resistivity ratios, Rf/Rm, ranging from 0.1 to 1000. For values of Rf/Rm between 0.7 and about 11,000, the borehole correction factor for the 6.75" tool is within 10 percent in boreholes of 8.5" to 9.875" (0.22 to 0.25 m) diameter. Calculation and field tests indicate that formation resistivity can be resolved in beds as thin as three inches (0.08m). Test well data were used to validate the model results and to confirm the operating parameters of the sensor. Actual sensor performance was evaluated downhole using a special test configuration which provided a continuous bi-directional link to the surface permitting pipe movement, circulation and drilling. Formations having a wide range of resistivities and lithologies were logged with the MWD system. Relogging with muds of different resistivity, yielded comparisons with the modeled sensor response over a variety of borehole, formation and operating conditions. Wireline logs and other data from the same well intervals were the basis for a more detailed evaluation of the resistivity sensor response.