A novel method for cement placement diagnosis behind multiple strings based on sonic dipole cutoff mode processing

Abstract

Acoustic measurements are widely used to diagnose the condition and placement of cement in cased oil and gas wells, and its bond to interfaces in contact with it. However current methods, encompassing high frequency sonic CBL-VDL and ultrasonic measurements, are designed for single steel casings and insufficient to probe behind more than one casing, and so cannot address the diagnosis of the placement and bond of cement behind a second casing that have become critical in the oil and gas industry. We therefore look at the use of lower frequency sonic measurements with deeper radial probing capability, to identify features appropriate for such a diagnosis. In particular, in this work we look at the modeling of sonic monopole and dipole modes as generated by existing tools and identify a set of cutoff modes that are indicative of annular fill behind a second casing when the annulus behind the first is known. Therefore the acquisition and processing of such modes in conjunction with the use of traditional cement evaluation techniques could become a viable avenue for the acoustic diagnosis of cement placement in multiple casing string geometries. Modeling results for such scenarios will be presented along with potential application to experimental and field data to indicate the feasibility of such a diagnosis using feature extraction.