An ultrasonic sensor has been developed for MWD (measurement while drilling) and LWD (logging while drilling) applications that perform real time borehole caliper measurements and also wellbore stability monitoring. The caliper determines the oval shape of the borehole with a defined accuracy and resolution, and will allow drawing conclusions on the stress field.
Recognition of variations in borehole shape in real time drilling allow the drilling engineer to actuate appropriate counteractions to avoid costly failures, or to implement alterations in the drilling practices to optimize the shape of the borehole and thus improve the drilling efficiency. The received data helps the driller to make proper decisions such as reaming a critical zone, changing the flow rate to reduce erosion or modifying the string rotation speed to reduce vibrations. This paper reviews tests and experiments, which have been performed using two different ultrasonic sensors in different drilling fluids and also different wellbore conditions. The comparison of the results of high frequency and low frequency sensor will help to design and manufacture an ultrasonic caliper which can detect instabilities in the wellbore with high resolution recorded data. Effects of drilled cuttings and gas bubbles on both sensors are analyzed and the attenuation and scattering of the signals were reduces by the new design of the caliper. In the new design of the ultrasonic caliper, sound velocity could be determined in real time at the pressure and temperature of the downhole condition. This will increases the accuracy of the measured diameter and the shape of the borehole in real time application.
Finally, even with the modern technology improvements, ultrasonic caliper cannot provide absolute accurate information for all drilling environments and conditions. Several operational factors such as mud density, borehole wall roughness and tool position in the borehole among others need to be considered in order to optimize the accuracy of the borehole caliper data. For this reason analysis of circle fitting algorithms based on the Kasa method have been implemented. Using this method and the new design of the caliper tool, accuracy of the recorded data increases.