A Layman’s Guide to Acoustic Anisotropy

Abstract

Acoustic anisotropy measurements are used in a wide variety of applications, such as fracture characterisation, production optimisation, wellbore stability, and hydraulic fracture monitoring. However, the translation of acoustic waveforms to anisotropic velocities and stress directions is often seen as a “black art”. While the acoustics specialist knows just how factors such as flexural mode dispersion, drilling induced stress, high angle fractures, hole washout, centralisation and tool calibration affect the data, the end user generally doesn’t want to delve deeply into such details, but just wants to know if the results are valid for a particular application.

Thus, while this paper will begin with a review of the common industry methods of processing acoustic anisotropy, the main focus will be on the quality control and understanding how to identify common issues in the delivered results. To that end, a review of quality control plots and their interpretation will be presented, followed by numerous examples of valid and invalid processed results, with the goal of this work being not to instruct the user on how to fix problem datasets, but rather to detail a succinct workflow to identify when to trust anisotropy results and when to consider re-processing the waveform data.

Particular attention is paid to frequently asked questions, such as, “what is the difference between XX/YY processing and shear anisotropy analysis?”, “how do I interpret anisotropy in inclined and horizontal wells?”, “how off-centre can the tool be to get valid shear anisotropy results?”, “what does a dispersion crossover plot really mean?” and “can I get anisotropy from LWD?”

INTRODUCTION

Acoustic anisotropy information is needed for many applications, yet the process for determining anisotropy and associated quality control methods are often poorly understood by the end user. This can cause users to incorporate incorrect results in various applications, leading to a lack of confidence in acoustic data. While there are numerous ways of computing and delivering anisotropy data and accompanying quality control plots, there are relatively simple and practical methods that the end user can practice to determine whether 1) the processed results are valid representations of formation anisotropy (rather than tool or environmental effects) and 2) what those results mean.

This paper is targeted at the non-specialist in acoustics and aims to provide a guide to understanding processed acoustic anisotropy results and to recognize valid and invalid data. It is not intended as a manual on how to process anisotropy; rather it addresses frequently asked questions for the practical user.