Understanding Acoustic Data in Unconventional Reservoirs

Abstract

Acoustic velocities are one of the key measurements for production enhancement and increased drilling efficiency in unconventional reservoirs, but the environment can present considerable complications to accurate data interpretation. Because these reservoirs are often produced by way of horizontal boreholes through shales, the sonic logs respond to intrinsic shale anisotropy, natural fractures, drilling induced stress, and nearby beds above or below the target formation. These intertwined effects frequently lead to misinterpretations such as picking a compressional slowness from an approaching bed but the shear slowness from the current formation, leading to odd (and confusing) geomechanical results. Other times, shale anisotropy is inadvertently assumed to be linked to natural fracture networks. Interpretations from different vendors, and even different analysts within the same team might vary greatly due to different processing methods. Thus, we need to consider the various aspects of sonic response in horizontal wellbore through unconventional reservoirs and proscribe a workflow and quality control regime to ensure consistent and valid results.

We begin with a discussion of the unconventional reservoir applications which require sonic data as key inputs, followed by a brief review of 3D anisotropy. The remainder of the work focuses on understanding the complex sonic log responses in horizontal shale reservoirs. A workflow is described, with the goal of correctly determining the compressional and shear velocities for the bed in which the tool resides as well as the nearby beds. Several field examples will be examined in detail, describing the correct methods for interpretation and the common pitfalls. Examples of how incorrectly interpreted results impact formation evaluation calculations follow. Finally, operational recommendations to optimise sonic data collection in shale reservoirs will round out the discussion.