Abstract

Effective geomechanical analysis requires a thorough understanding of the complexities of the formations drilled, such as stress-induced anisotropy, intrinsic anisotropy, azimuthal heterogeneity, drilling induced and natural fractures, and/or borehole damage. The identification and classification of these features can be complex, particularly when multiple different types are present together. The conventional sonic tools can struggle to differentiate these effects when more than one is present and often are lacking in the capability needed to clearly resolve them mainly because of the limited spatial resolution associated with the probing sonic frequencies. A new logging-while-drilling (LWD) dual ultrasonic tool has been developed, that provides high-resolution, full-azimuth, ultrasonic compressional and shear (P&S) slowness images together with ultrasonic borehole images and caliper that can be used to help address this formation complexity on multiple scales and so to improve geomechanical models and analysis. In this paper, we study feasibility of classifying formation geomechanical features, such as intrinsic anisotropy, stress-induced anisotropy, and azimuthal heterogeneity, using the ultrasonic P&S slowness from the LWD dual ultrasonic tool, and discuss conditions when such classification is expected to be successful and when more integrated approach is needed to address complexity of formations.

Introduction

Understanding the complexity of formation mechanical properties is very important for effective geomechanical analysis, such as the optimal design of hydraulic fracturing or wellbore stability analysis for safe and cost-effective drilling. Conventionally, standard sonic logging has been used to provide key acoustics measurements to characterize the formation mechanical properties axially, azimuthally, and radially (Pistre et al., 2005). These measurements are made at sonic frequencies, which results in an intrinsic limitation for resolving layers smaller than 1-ft thickness, because of the probing wavelength at these operating frequencies. A recently developed LWD dual ultrasonic tool demonstrated the ability to deliver P&S slowness measurements and images that resolve inch-scale bedding (Blyth et al., 2021). The P&S slownesses measured at ultrasonic frequencies deliver measurements with 2-in vertical resolution and in 16 azimuthal sectors around the wellbore. The tool also provides borehole images and calipers with ultrasonic pulse-echo measurements. P&S images from the field test data showed evidence that both intrinsic and stress-induced anisotropy can be revealed using LWD dual ultrasonic data at a similar fine resolution scale.

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