LWD Sonic Logging in Cased Holes

摘要

While LWD sonic logs have been routinely used in open holes for nearly a decade, LWD sonic logging in cased holes can also provide useful information, including cement evaluation logs and formation velocities. Though it is common practice to acquire wireline sonic data through casing, it is a relatively unexplored facet of LWD sonic logging. While much of the theory is the same for both environments, there are critical differences related to the presence of the large LWD tool and the resulting small fluid annulus which modify the properties of the guided waves and multiply reflected waves. Some of the differences are theoretically illustrated through full wave synthetic seismograms and field data examples. Well-bonded and poorly bonded situations are discussed. In the latter case, examples of a poor casing-cement bond or cement-formation bond are both investigated. Examples of compressional velocity data acquired through casing are presented along with comparisons to wireline data. Examples of qualitative cement bond logs based on the attenuation characteristics of the compressional wave train are also presented. Finally, operational recommendations and processing techniques for casing bond logs and compressional velocities through casing are discussed and quality control displays illustrated.