Joint Inversion of Formation Shear-Wave Transverse Isotropy from LWD Multipole Dispersion Data

Summary Seismic wave anisotropy has long been an important topic in the exploration and development of unconventional reservoirs, especially in shales, which are commonly characterized as transversely isotropic ([TI] or vertical TI [VTI]) media. At present, the shear-wave TI properties have mainly been determined from wireline acoustic logging measurements, but the feasibility of those obtained from logging-while-drilling (LWD) acoustic data needs to be established. We have developed a joint inversion method for simultaneously determining formation shear-wave transverse isotropy and vertical velocity from LWD multipole acoustic data. Our theoretical analysis shows that the presence of anisotropy strongly influences on LWD Stoneley- and quadrupole-wave dispersion characteristics, while the monopole Stoneley and quadrupole waves are both sensitive to the formation shear-wave TI parameters, they suffer from the typical non-uniqueness problem when using the individual wave data to invert parameters alone. Thus, the respective dispersion data can be jointly utilized to estimate the formation shear-wave TI properties. By the joint inversion, the non-uniqueness problem in the inversion can also be effectively alleviated. The feasibility of the method has been verified by the processing results of field LWD acoustic-wave data. The result, therefore, offers an effective method for evaluating formation anisotropy from LWD acoustic data.