Schlumberger

Technical Paper: Effective BHA Design and Vibration Modeling for Improving the Quality of Sonic Measurements

Society: IADC
Paper Number: 140326
Presentation Date: 2011
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Abstract

The first logging while drilling (LWD) sonic tool was developed in 1996. For the oil industry, sonic tools have now become a critical part of the existing suite of LWD tools. They allow the determination of formation velocities while drilling, enabling real-time decision making. Compressional and shear velocities obtained can be used for calculating rock mechanical properties, which can then be used for drilling optimization, ensuring hole stability, or as inputs into mechanical earth models. Compressional velocities are also sensitive to pore pressure and can be used to determine pore pressure trends while drilling. This allows the calculation of a safe mud weight window and the ability to update it in real time. LWD sonic tools also allow the calculation of interval transit time, which can be used to update seismic data while drilling. Acoustic tools, therefore, offer a variety of measurements critical for drilling successful wells, so it is imperative to ensure that the best quality data are delivered at all times.

The detrimental physical effects of shock and vibration on drillstring and bottomhole assembly (BHA) components are well known. However, vibrations have also been known to have adverse effects on downhole LWD tool measurements. Measurements of acoustic signals are sensitive to excessive sources of downhole noise, such as shock, vibrations, and poor drilling practices. It is therefore critical to the quality of the sonic log to minimize vibrations while drilling, to design BHAs that minimize tool movement, and to optimize drilling parameters. The effects of shocks and vibration can be clearly seen by comparing acoustic logs over zones with high downhole shocks to those zones in which shocks were minimized. The primary way to reduce the likelihood of these problems is to design a BHA that is less prone to vibrations and is correctly configured to optimize the acoustic measurement. Today, many companies have a range of software available, from simple vibration to finite element analysis software that can create models required for this type of design.

Background information on acoustic measurement and its uses is presented, along with an overview of some current BHA vibration modeling techniques. We review data from projects with poor quality sonic data due to lack of vibration modeling and from projects in which vibration modeling was done so that the results can be compared and analyzed. We look at what is required at the design phase for planning BHAs to minimize vibrations and to optimize the acoustic measurement. Some examples of what is done today are discussed, and a list of general guidelines for BHA design that should be considered when running acoustic tools is included.

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