An identification method for thin shale gas reservoirs based on the high-frequency recovery technology in frequency domain: A case study from deep shale gas in the Luzhou area of the Sichuan Basin

At present, the Sichuan Basin is the main battlefield of shale gas exploration and development in China, and the national shale gas production in 2021 is up to 230×108 m3. The deep shale gas reservoirs in the Sichuan Basin are thin, so horizontal wells often run out of the golden boxes (3–5 m) in the process of drilling. In order to solve this problem and improve the high-quality reservoir drilling ratio of horizontal well, this paper takes the deep shale gas in Luzhou area of the Sichuan Basin as an example to decompose the seismic signal in the effective frequency band into fundamental frequency signal by virtue of wavelet transform, based on the high-frequency recovery technology in frequency domain. Then, the harmonics and sub-harmonics of each fundamental wave are calculated according to the harmonic criterion and added back into the original wavelet coefficients to perform the wavelet inverse transform, so as to achieve the high and low frequency recovery of the missing signal. And the following research results are obtained. First, the high-resolution processing method based on harmonic imaging can double the bandwidth while keeping the low-frequency amplitude information and the phase of the marker layer unchanged, so as to maintain the fidelity and amplitude, which is favorable for the identification of thin shale layers. Second, in the reservoir prediction based on the high-resolution seismic data, the vertical resolution is increased from 30 m to about 10 m, which provides support for horizontal well location deployment and well trajectory design. Third, using high-resolution processing results to track and guide horizontal well drilling while drilling can better predict micro fractures and lay a foundation for preventing drilling accidents and improving the drilling ratio of high-quality reservoirs. In conclusion, the theoretical numerical experiment and the actual data processing verify the reliability and scientificalness of this technology. The coincidence rate of the fracture prediction result by this new method to the actual drilling data is more than 90%, and the box drilling ratio reaches 90%. It is indicated that this method ensures the high-quality reservoir drilling ratio of horizontal wells and has a wider application value. © 2022 Natural Gas Industry Journal Agency. All rights reserved.