Formation Anisotropic Constants From Sonic Data Acquired While Drilling

Abstract

Formation anisotropic constants play an important role in the determination of formation stresses, fractures, lithology, and mechanical properties. These formation characteristics provide critical inputs to wellbore stability during drilling and optimal completion designs of hydrocarbon bearing reservoirs. However, there are no available techniques for the estimation of a sub-set of formation anisotropic constants using Logging-While-Drilling (LWD) sonic data. Inversion of LWD-sonic data for anisotropic constants is rather challenging because of a strong coupling between the collar-flexural and formation-flexural modes propagating along the borehole. The influence of a strong coupling between the collar and formation flexural modes on LWD-sonic data are significantly different in fast and slow formations. Consequently, frequency dependent sensitivities of the measured collar and formation flexural dispersions to changes in the formation anisotropic constants are significantly different as well. New inversion algorithms provide estimates of a sub-set of Transversely-Isotropic (TI)-constants from measured collar and formation flexural dispersions in both fast and slow formations. Computational results confirm validity of the proposed algorithms using synthetic dispersion data obtained in a fast Bakken shale and slow Austin chalk TI-formations in the presence of a drill-collar to account for the LWD sonic tool in a liquid-filled borehole.