Correction of Acoustic Anisotropy of Bedded Shale and Its Application in Fracturability Evaluation

Shale oil and gas, as typical unconventional resources, have become crucial for stabilizing oil and gas production in China. The calculation of shale reservoir fracturability index is a core step in evaluating engineering sweet spots. However, acoustic logging data of shale formations are susceptible to geological interfaces, borehole deviation, and particularly the bedded structure-induced anisotropy. Such anisotropy causes the measured acoustic interval transit time to deviate from the true formation value, leading to inaccurate in-situ stress calculation and thus compromising the reliability of fracturability evaluation. Accurate acquisition of formation AIT is essential for optimizing fracturing schemes and analyzing borehole stability.

To address this issue, a correction method for acoustic anisotropy of bedded shale in horizontal wells was established. Based on the bedded shale medium model assumption, the formation stiffness coefficients and elastic wave velocities were derived using the equivalent medium theory and elastic wave equation. Shale models with varying bedding angles were constructed to analyze the functional relationships between acoustic velocity, acoustic anisotropy coefficient, and bedding angle.

Numerical simulation results demonstrate that the acoustic interval transit time of bedded shale exhibits significant anisotropic characteristics: both acoustic interval transit time and acoustic anisotropy coefficient increase with the increase of bedding angle, and the acoustic anisotropy coefficient has a good power exponential relationship with the cosine of the bedding angle. A correction model for acoustic anisotropy of bedded shale was thus developed. Field application in horizontal wells of Block L showed that after correction, the peak of the normal distribution curve of acoustic interval transit time in the target reservoir of horizontal wells was highly consistent with that of pilot wells. The corrected acoustic interval transit time was used to calculate Poisson's ratio, Young's modulus, horizontal principal stress difference, and fracture pressure, further deriving the fracturability index. Comparison with the fracturability index calculated from core experimental data indicated that the average relative error of the results was reduced by 9.28%.

This study provides a reliable acoustic anisotropy correction method for bedded shale, which is of great significance for improving the accuracy of shale reservoir fracturability evaluation and guiding the identification of engineering sweet spots.