An Evaluation Cement Method Using Gamma-Gamma Density Imaging Logging in a Double Casing Well

Deep wells, ultradeep wells, and some offshore oil wells are characterized by high pressure and high temperature, and their cementing casing is often prone to collapse and rupture. It is difficult to meet the exploration needs by using a single-layer casing cementing method, and nowadays, the double-layer casing cementing method has become more and more common, so the cementing quality evaluation method of double-layer casing has become particularly important. Nowadays, many scholars have analyzed the cement cementing condition of double-layer casing wells using acoustic amplitude and variable density logging, but there are few studies on the evaluation of the cementing quality of double-layer casing wells using nuclear logging methods. Gamma rays have a weaker penetration ability to the medium, and their detection depth is shallow. Gamma rays are emitted from a radioactive source, and the number of particles that can return to the detector area after penetrating the double-layered casing is very small, so it is difficult to obtain information about the cementing quality of the outer casing. The detector counts are affected by the multilayered medium, so it is not easy to distinguish which layer of the inner and outer cement rings are actually missing. This is also the problem of using gamma density logging to study the cementing quality of double-layer casing. In this paper, a new type of multiprobe gamma imaging logging instrument has been designed; the radioactive source is the Cs-137, the proximity detector and the middle detector are composed of single probes, the distal detector is composed of six arrays of probes, and the detector window is composed of metal Be. The 0.662-MeV gamma rays emitted from the Cs-137 radioactive source penetrate through the borehole, inner casing, inner cement ring, outer cement ring, outer cement ring, inner and outer cement ring, and the inner and outer cement ring, and the outer casing, the outer cement ring, and the outer cement ring. casing, inner cement ring, outer cement ring, and ground layer, in which photoelectric and Compton effects occur. By recording the gamma flux distribution of different energies returned to different detectors, the response energy spectrum is obtained, and the total number of scattering counts of each detector in the energy window of 0.1 MeV-0.662 MeV is extracted to derive the logging value under this condition. The near detector has a shallow probing depth and mainly responds to the influence of the inner casing, the middle detector is mainly affected by the inner casing and the inner cement ring, and the far detector is mainly affected by the internal and external casing—the internal and external cement ring. Combining the measurements of the middle and far detectors can reflect the location of the missing layer of the cement ring, and the far detector consisting of six arrays of probes can respond to the missing cement ring in different directions and perform an imaging display, which can visualize the missing cement ring in different directions. In this paper, the diffusion equation of gamma rays under double-layer casing conditions is derived for the process of gamma particles interacting with the medium. Through numerical simulation methods, different inverse models are established to obtain the influence laws of the main control factors, such as inner and outer casing thickness, inner and outer cement ring density, and stratum density, on the count rate of the three detectors, and the response equations are established. The conditions of double-layer casing wells with different cementing conditions are set to verify the inversion effect, and the results show that the cementing quality can be accurately responded to.