Quantitative analysis of coal-bed methane components by downhole laser Raman spectroscopy

Abstract

Laser Raman spectroscopy can be used to acquire the unique fingerprint of a specific molecule, and it is widely used to identify substances and to study the spectral line characteristics of molecular structures. The measurement of coalbed methane (CBM) content is essential in the exploration and development of CBM fields for optimizing the fracture design. For this purpose, laser Raman spectroscopy can be extremely beneficial because it detects the gas content rapidly and accurately. Moreover, conventional gas content testing methods are laborious, time-intensive, and expensive, and they yield inaccurate results. Therefore, we have integrated a laser Raman spectroscopy system with coiled tubing (CT) equipment for downhole deployment in gas wells to accurately determine the CBM content in situ. The developed system can directly determine the CBM content at a specific location in the target layer. The trace test characteristics enable this system to rapidly detect downhole gas components and contents. The real-time detection data are transmitted via a cable to a computer on the surface and are processed using a baseline correction algorithm and a data enhancement algorithm. The Fourier transform and the wavelet transform are used to identify the Raman spectral lines, whereas analysis of Raman spectra is used to determine the CBM content. By using this equipment, we can shorten the cycle of depressurization, drainage, and recovery processes from multiple days to just a few hours. Furthermore, the integrated laser Raman spectroscopy-CT system enables a flexible operation and possesses strong site operability, making it suitable for complex and high-risk wells.