Evaluation Method for Cementing Quality of Double-Casing Wells Based on Continuous Wavelet Transform

Accurate evaluation of cementing quality plays a crucial role in ensuring the safety and efficiency of oil and gas field development. To address the limitations of traditional methods in dual-casing wells, this study proposes a cementing quality evaluation method for dual-casing wells based on continuous wavelet transform (CWT). This paper systematically elaborates on the basic theory of CWT, uses the Morlet wavelet as the basis function to perform time-frequency transformation and in-depth analysis on actual well logging data, and extracts multi-dimensional characteristic parameters such as first arrival time and amplitude. It clarifies the thresholds and judgment logic for characteristic parameters, establishes a multi-scale time-frequency analysis framework and quantitative identification criteria for the cementation state at the multi-interfaces (casing-cement sheath-formation), and conducts comparative verification by combining numerical simulations and logging data from single-casing and dual-casing wells. The research results show that: ① It successfully distinguishes different states, including good cementation at all interfaces and poor cementation at 1 to 4 interfaces, and identifies the differences in time-frequency characteristics under different cementation states. For example, when cementation is good, obvious formation waves exist in the frequency band of 8～12 kHz; when cementation at the first interface is poor, the energy in the time window of 0.30～0.45 ms and frequency band of 15～30 kHz exceeds the threshold of 500. ② In the application of the 35～220 m interval of Well A, it is found that the proportion of poor cementation at the second and third interfaces is significantly higher than that of other interfaces. Poor cementation at the third interface occurs concentratedly in the 110～150 m interval; continuous poor cementation at the second interface appears in the 65～70 m and 72～82 m intervals; and continuous poor cementation at the fourth interface is observed in the 195～220 m interval. ③ Compared with traditional methods, the adaptability of the proposed method to complex formations is significantly improved. It can accurately distinguish the cementation quality of the second and third interfaces, which are difficult to judge by traditional methods, and solves the problems of reduced resolution and insufficient collaborative evaluation of multi-interfaces in traditional methods. ④ Through the comparison between simulated data and actual data, the reliability of the quantitative relationship model is verified, and the evaluation results of poor cementation at the four interfaces of dual-casing wells are in good agreement with those of single-casing wells. It is concluded that this quantitative evaluation method based on CWT provides an effective new approach for cementing quality evaluation of dual-casing wells, significantly improves the accuracy and reliability of evaluation, and promotes the development of related evaluation technologies in oil and gas exploration and development.