Acoustic Fracture Characterisation - Intelligent Interpretation

Abstract

Acoustic data has long been used to characterise fractures, but the methods are often misunderstood, particularly in inclined wells, unconventional reservoirs, and through casing. For example, azimuthal shear anisotropy can be an excellent indicator of fractures, but not if the fractures are perpendicular to the wellbore, which is often the case in horizontal wells. However, Stoneley fracture detection and amplitude/attenuation methods are suited to detect fractures transecting the wellbore. While, in general, the preferred method to characterize near-wellbore fractures is to use electrical or ultrasonic image tools, sonic data offers not only an alternative for identifying fractures when image logs cannot be acquired, but also complement image logs by reading both near and far from the wellbore as well as identifying stress concentrations which are likely to fracture when the stress field is altered by drilling and production. Zones which are stressed (and prone to fracturing) but have not yet fractured are not readily identified with micro-imagers, but can be identified by acoustic logs. Sonic logging also offers possibilities for fracture characterization behind casing, which is not possible with micro-imagers. Acoustic methods also work in all mud types (except air). Much like electrical or ultrasonic imaging methods, there are environments in which sonic fracture characterization techniques work well and those where they are not ideally suited.

This paper begins with a review of acoustic fracture characterization methods, which environments each method is best suited for, and suggesting optimal environments and limitations of the measurements. Examples of the various techniques will be discussed, comparing with non-acoustics methods to understand how different methods complement one another.

INTRODUCTION

Understanding natural and drilling induced fractures is essential to safely drilling a well and maximizing production. While seismic interpretation methods can give us a high level view of areas of fractures, it is at the borehole level we can really understand how fractures intersecting or near the wellbore may interact. By measuring the discrete location of fractures intersecting the wellbore, induced fractures, and altered near-wellbore stress fields, we can understand the effect of the wellbore on the surrounding environment. This understanding is critical to safely drilling a stable wellbore and to optimising production, particularly in tight or shale reservoirs.