Assessment of Cement Density via Through-Casing X-Ray Logging Technique

In well logging operations, monitoring the cement density is crucial for assessing wellbore integrity and safety. By accurately measuring the cement density, it is possible to assess the curing condition and the quality of the cement, which ensures borehole tightness, minimizes losses, and enhances productivity in oil and gas wells. Although density logging effectively determines cement density, gamma-ray detection can be impacted by various media, including casing, cement, and surrounding formation, complicating the extraction of accurate cement density. Furthermore, the use of the Cs- 137 radioisotope source in conventional density logging raises environmental and health concerns, underscoring the urgent need for research to identify a controlled source for cement density measurement and develop reliable methods for accurately extracting cement density amid the challenge. The X-ray tube, as a controlled source, is a clean and safe option that has proven effective in density logging. We propose a novel method for obtaining cement density through casing using a four-detector X-ray logging system, aimed at enhancing the comprehensive evaluation of cement quality. The method involves analyzing energy spectrum characteristics, developing forward modeling, and inverting cement density. Initially, the scattering gamma spectra from the four detectors are analyzed, the energy windows are divided based on photoelectric effect and Compton scattering. This analysis investigates the relationship between gamma counts in different energy windows and parameters such as cement density, formation density, and cement thickness. Subsequently, a forward model is established to relate gamma counts in each energy window to the cement and formation parameters, considering multiple variable interactions. To accurately determine the cement density in the target formation, the Newton iteration method is applied for inversion, with a regularization approach incorporated to enhance algorithm stability. This integrated method facilitates precise calculations of cement density parameters, thereby supporting a more accurate assessment of cementing quality. A simulation example is conducted to verify the effectiveness of the proposed method. A model of a casing well is established, incorporating various formation lithologies and cement parameters. The multi- detector energy spectra are simulated to invert the cement density. The results indicated that the method maintains a calculation error for cement density within 0.015 g/cm³, achieving accuracy comparable to formation density measurements in open holes. Furthermore, while X-ray is less sensitive to formation parameters compared to Cs-137 because of its detection range, it still offers a capability for semi-quantitative estimation of formation density. Overall, this method enables accurate calculation of cement density and semi- quantitative assessment of formation density, providing significant technical support for the comprehensive evaluation of cementing quality.