Wellbore Instability Monitoring by Using 4D Imaging of Ultrasonic Caliper Logs in Real Time Drilling
Authors: Elahifar Behzad , Esmaeili Abdolali , Fruhwirth Rudolf K. , Thonhauser Gerhard 
Published: May 2016

Abstract
Drilling programs continue to push into new and more complicated environments. As a result, accurate measurement and analysis of drilling data in real time are becoming more critical by means of minimizing failure costs.
The measurement of actual wellbore shape in real-time can be considered as one of the key components to detect problems such as borehole instability. Abnormal wellbore shape will allow drawing conclusions on the stress field. Accurate wellbore caliper measurements in real-time will also allow determining cement volume requirements with less measurement time. Measuring flow rate accurately behind the bit with ultrasonic sensor will also help the driller to detect washouts along drill string in real time.
This paper describes experiments related to the accuracy of ultrasonic sensor measurements for estimation of wellbore diameter in drilling fluids with different mud weights and additives in real time. A fully automated test robot has been designed and tests have been performed in different drilling conditions. The test robot allows moving vertical as well as lateral movement of a sensor head in an artificial wellbore which can be run with different fluids. In addition, bubbles were added to the mud while measuring borehole diameter by the sensor. Tests have been performed in relation to the radial position of the caliper tool inside the wellbore, including different objects inside the borehole simulating latches or dog legs. The wellbore profiles have been measured with different axial and rotational surveying speeds.
Numerical simulation of ultrasonic measurements and comparison of the results to the recorded data gives an estimate on the measurement accuracy at different RPMs of drill string. It is also shown that gas bubbles can be detected and compensated with an appropriate accuracy if circle fitting methods like the Kasa method in combination with robust error models are applied.

Introduction
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 help 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.
Real time application of ultrasonic caliper tools also provides a method for calculating borehole volumes. On a final bit run for instance, the sensor may collect the caliper data during trip-out for the estimation of cement volumes. Other applications of ultrasonic caliper tools might include real time casing wear and borehole stability detection, evaluation of borehole cleaning and determination of tight spots or formation ledges (Maranuk et al., 1997).
This paper reviews tests and experiments which have been performed by the ultrasonic sensor in drilling fluids with different mud weight. Effects of air bubbles on the recorded data by ultrasonic caliper were determined. However, even with the modern technology improvements, ultrasonic caliper cannot provide absolute reliable information for all drilling environments and conditions. Several operational factors such as tool centralization, mud density, borehole wall roughness, tool position, gas bubbles and drillstring vibration 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.