Unraveling the Acoustic Challenges in Horizontal Wellbore Environments - A Case Study in the Delaware Basin

Abstract

Horizontal wellbores are challenging environments in which to understand borehole acoustics measurements, particularly in shale reservoirs. However, with careful analysis, the data offers a wealth of geomechanical and rock physics information not only about the formation in which the tool is located, but about nearby formations as well. This not only allows for production optimization by understanding how fractures will propagate, but also suggests geosteering opportunities, such as placing the lateral a safer distance from nearby boundaries, or steering the well to stay within the most “fracable” portion of the shale.

A case study is presented, considering azimuthal acoustic data that was acquired in a thinly-bedded formation. The target formation is heterogeneous and thinly-bedded with highly-contrasting rocks acting as both source and reservoir in the hydrocarbon petroleum system. In this particularly challenging example, the borehole straddled the interface of two highly contrastive beds for much of the length of the lateral. Initial sonic interpretation using conventional methods led to confusing results; it was difficult to understand whether the velocities pertained to the upper formation, lower formation, or beds even further away.

A new workflow was applied to try to systematically characterize the sonic properties for each layer. This involves visualizing the environment, determining the sonic velocities for each quadrant (up, down, left, right), and using the visualization to determine which velocities (and thus elastic properties) belong to each layer. Once elastic properties for each layer were understood, it is possible to try to solve the problem of how the complex fracture propagation should be modelled in these challenging environments. When further combined with natural fracture information (from image logs or sonic fracture detection), core measurements, and past production from nearby wells, these analyses can be used as a guide for modifying and optimizing completions.

INTRODUCTION

The stratigraphy of the Delaware Wolfcamp is unique amongst “unconventional” plays. It is thinly bedded, composed of mixed fine grain carbonates (siliciclastic lithotypes) and exhibits highly contrastive “stiff” and “soft” facies (Thiercelin and Plumb, 1994).