Acoustic anisotropy and heterogeneity of unconventional formations with ultrasonic slowness and reflectivity images acquired with a newly developed logging-while-drilling (LWD) tool is studied. Conventionally, acoustic anisotropy has been evaluated with dipole or unipole sonic tools. However, if there is formation heterogeneity around the wellbore, what the dipole measurement can probe is the equivalent homogeneous mechanical properties. Unipole sonic tools can evaluate azimuthal heterogeneity and anisotropy. However, their ability to reveal fine formation features is largely limited due to spatial resolution limits of a few feet. The current study of using ultrasonic technologies is to overcome those limitations. The new LWD acoustic tool uniquely defines rock mechanical properties in inch scale as a function of azimuth in unconventional formations.
The newly developed logging-while-drilling (LWD) ultrasonic tool is equipped with pitch-catch and pulseecho modules. It has been field tested in both vertical and horizontal wells. The pitch-catch measurement operates with a wide band ultrasonic frequency centered at 250 kHz, which enables the evaluation of formation mechanical properties at inch scale as a function of azimuth. The ultrasonic slowness log acquired with the new LWD tool is compared with the slowness log acquired with the conventional wireline dipole sonic tool run in the same well. Numerical modeling is also conducted to verify the novel ultrasonic pitchcatch measurements in the corresponding anisotropic formations.
The field data reveals that the inch-scale ultrasonic slownesses image together with the ultrasonic reflectivity image logs can show characteristic features that clearly correlate to the formation lithology and structure. Those images also clearly show presence of heterogenous rock properties which cannot be evaluated with the standard sonic logging. In high gamma ray intervals, where the intrinsic transverse isotropy is expected, the vertical and the horizontal shear slowness with the ultrasonic pitch-catch measurements are different. Thus, the newly developed acoustic tool is effective to probe both the heterogeneity and anisotropy of unconventional formations while drilling.
This work highlights the importance of evaluating acoustic anisotropy and heterogeneity of unconventional formations with high resolution slowness and reflectivity images. The formation mechanical properties around the wellbore in horizontal wells are not always homogenous, which is the common assumption in conventional sonic slowness analysis. Therefore, the target application of the current study with this new and unique LWD acoustic tool is completion optimization and geology and structure analysis with the detailed rock mechanical properties both in depth and azimuth.