Quantifying Drillstring-Integrity-Failure Risk: Real-Time Vibration Measurements
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 147747, ’Quantification of Drillstring-Integrity-Failure Risk Using Real- Time Vibration Measurements,’ by Yezid Arevalo, SPE, and Ashley Fernandes, SPE, Schlumberger, prepared for the 2011 SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, 20-22 September. The paper has not been peer reviewed.

Traditional vibration measurements target improving downhole-tool reliability. This effort targeted the effects of vibration on the complete drillstring. Failures associated with drillstring vibration continue despite current measurement sophistication. These failures represent a very significant amount of lost time and are targeted with a proposed solution that is based on real-time measurements of drillstring vibration to estimate an ongoing drillstring-integrity risk for use as a guideline to improve decisions while drilling the well.

Introduction

The understanding of vibrations has improved, recognizing the random nature of downhole vibrations because they often are highly dependent on factors intrinsic to formation and borehole characteristics, which cannot be modeled practically or measured with sufficient detail in real time. These recognized limitations continue to highlight the importance of real-time measurements to manage drilling parameters in an effort to optimize drilling results and manage drilling-integrity risks.

When the risk of drillstring-integrity failure is recognized in a drilling project, it is common to implement advanced vibration-measurement devices to assist the understanding of the drilling-dynamics behavior in real time. Often, the interpretation of these data is complex and requires expert real-time support and follow-up to obtain the desired results of mitigating vibration-related integrity failures. Without appropriate resources, critical data can be ignored easily and not be acted upon, leading to failures and lost time.

Downhole-Vibration Measurement

The measurement device used to quantify vibration severity was a multiaxis vibration chassis (MVC)—a four-axis shock-measurement tool. The first axis refers to the strain gauges and associated electronics used for torsional measurements. The remaining three axes refer to a system comprising the vibration-acquisition board and three offboard accelerometers. The complete system is mounted on a special chassis in the measurement-while-drilling (MWD) tool.

The three accelerometers are mounted in a mutually orthogonal arrangement along the centerline of the MWD tool. The vib-x sensor measures axial shocks, while the vib-y and vib-z sensors measure lateral shocks in orthogonal directions. The mounting arrangement is designed to ensure that the chassis experiences the same vibrations as the MWD tool.

Also, an independent accelerometer is mounted on the MWD tool and normally is used to establish a risk level based on the count of shocks that peak above 50 g. A high risk of tool failure is considered when a level above 10 counts of shocks higher than 50 g is measured. This sensor also registers the highest shock peak within the data-point transmission interval. This information enables identifying situations in which single events of high acceleration can increase the risk of drillstring or downhole-tool failure, because these measurements may not be recorded by a root-mean-square (RMS) measurement that provides an average measurement.