Numerical study on the characteristics of multipole acoustic logging while drilling in cracked porous medium

Cracks and pores universally exist in earth formation rocks, significantly affecting LWD acoustic waves. Using a cracked porous medium elastic wave theory, we derive LWD acoustic wave field for a fluid-filled borehole containing a drill collar and study the velocity dispersion, attenuation, and sensitivity of multipole wave modes (e.g. Stoneley, flexural and quadrupole waves) as a function of crack density and gas saturation. Increasing crack concentration and gas saturation can produce significant velocity change and attenuation. Relatively speaking, the velocity is more sensitive to crack density, while the attenuation is more sensitive to gas saturation. The dispersive LWD dipole and quadrupole waves characterized by a "Airy Phase" are strongly attenuated by gas saturation. This phenomenon can be used to detect the presence of gas in tight formations. The modeling and analysis results of this study are in agreement with results from field LWD data analyses.