Investigation on acoustic reflection logging of LWD using a flexible finite difference technique

Abstract

ABSTRACTAcoustic reflection logging while drilling (LWD) measurement has overwhelming advantages in detecting reflectors outside the borehole, making it a prominent technology in geosteering during LWD operations. This paper carries out a comprehensive numerical investigation into the acoustic reflection logging responses in LWD environments by combining the analytical real axis integration (RAI) solution and an exquisitely improved finite difference method (FDM). Mutual validation of these two methods using typical and well-known cases ensures the reliability of the modelling methodology. Numerical studies on LWD and wireline acoustic reflection logging reveal that a ring source rather than four azimuthally orthogonal point sources can excite acoustic LWD monopole waves without interference from higher order wave modes. Lower order spatial differential operators are more efficient for FDM simulations of LWD cases when variance in the medium is obvious. Compared with the wireline monopole case, LWD monopole reflection acoustic logging can obtain higher quality reflected waves and has a superior ability to identify the azimuth of a geological reflector away from the borehole.