Accuracy of ultrasonic sensor in caliper log

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. These measurements are transmitted to the surface and used to compensate MWD and LWD measurements. Furthermore, with early and accurate real-time caliper measurements, borehole instability can be detected. For example, an elliptical wellbore can indicate the maximum and minimum stress directions. Borehole enlargement or washout can indicate that the mud is too heavy or too reactive with the formation. An under-gauge borehole might indicate bit wear. Caliper measurements are also required to estimate the amount of cement to be used, which is an important environmental and safety concern. This paper describes experiments and specification of the ultrasonic sensor for measuring wellbore diameter. It explains the setup procedure and a rotating machine which is used to simulate the drill string on which the ultrasonic sensor is mounted. The accuracy and problems related to recording and processing of ultrasonic sensor data was investigated. Finally, measured and recorded data of ultrasonic sensor are compared to the simulated wellbore profile.