This paper presents a feasibility study of a novel nuclear sensor design toward finding volumetric flaws in cement behind two or more tubulars in a wellbore. The sensor concept targets plug and abandonment activities and the presented results include experimental test setups built for the purpose, results of simulations and experiments conducted for determining the range of applicability. The measurement is based on a gamma-gamma measurement concept that incorporates a rotating interrogating beam illuminating the wellbore for an azimuthal scan while the tool is in motion in the borehole. The radiation source is a conventional Cs-137 radioisotope emitting 662 keV gamma rays that can penetrate through two metallic tubulars to reach the cement. The detector is a gamma detector that is collimated to accept gamma rays from the volume illuminated with the gamma rays and centrally positioned in the tool body. The detector collimator rotates in synchronization with the rotating interrogating beam providing an azimuthal scanning capability. The feasibility of the concept was initially tested through simulations that provided encouraging results for detection of volumetric flaws in the cement between the outer casing and the formation. This was followed by experiments that were conducted using synthetic test formations embedded with various volumetric cement flaws. The rock sizes were representative of infinite rocks encountered in typical logging runs. The experimental setup provided not only different flaw sizes but also different tubing positions inside the casing annulus. A generic gamma-gamma tool was run vertically in the synthetic rocks without a tubing and with a tubing in place. The vertical runs were repeated around the borehole and images were formed. Another mode of experiment was conducted by rotating the tool around its axis. The motivation behind this was to study the azimuthal response of the gamma-gamma scanner in an eccentered position. The results from the experiments were in line with the simulation results confirming that it is possible to detect volumetric flaws in the cement between the outer casing and the formation. This paper demonstrates a method for detecting cement flaws that is pivotal in plug and abandonment activities. Different measurement techniques are usually sensitive to different flaw morphologies. While nuclear methods cannot detect micro-channeling, they can be very efficient in detecting volumetric flaws in cement. In contrast, such flaws cannot be efficiently detected by other measurement techniques. The results presented here show how a gamma-gamma measurement can be used to detect and characterize volumetric cement flaws that can be encountered in multi-annular completions.
