A novel filtering technique for extracting formation slowness from poorly bonded cased boreholes

Abstract

Extracting formation properties from sonic logging data in poorly bonded cased boreholes is crucial. The importance of this task is underscored by the increasing number of cased wells worldwide and instances wherein sonic measurements are omitted before casing the well. Analyzing formation signals from poorly bonded cased boreholes is highly challenging, especially in the case of free-pipe situations wherein the casing is detached from the formation. We develop a new signal processing technique designed specifically for the sonic data in cased wellbores. This method uses the inherent stopbands of the casing waves for filtering, effectively suppressing the casing wave signal. We provide a frequency range template for various casing types to guide signal processing. Our method has been applied to sonic data from a field cased wellbore to evaluate its performance. We determine its capability to enhance formation signals, even in the presence of dominant casing signals, particularly in cases wherein the formation wave velocity is fast and close to the casing wave velocity, thus providing new perspectives and guidance for the processing of sonic data in cased wellbores.