Water vapor absorption peaks in infrared spectroscopic gas logging and correlation with water layers

Gas logging is employed in oil and gas exploration to analyze wellhead gases degassed from drilling fluids for hydrocarbon resource evaluation. Aquifer identification plays a critical role in reservoir characterization and drilling optimization. However, conventional resistivity-based methods often exhibit low accuracy, particularly in subtle, low-resistivity, low-porosity, and low-permeability reservoirs. Therefore, Fourier transform infrared (FTIR) spectroscopy was applied to analyze wellhead gases, focusing on characterizing water vapor (i.e., gaseous H2O) absorption peaks in aquifer-associated spectra. First, infrared spectra of water vapor under varying temperatures and pressures were simulated using the high-resolution transmission molecular absorption database (HITRAN). Subsequently, the spectra of wellhead gases were acquired at drilling sites with a developed FTIR gas logger, followed by spectral pre-processing, quantitative analysis, and calibration to determine water vapor volume fractions. Finally, point-biserial correlation and logistic regression were applied to establish the relationship between wellhead water vapor content and aquifer presence. The results demonstrate distinct water vapor absorption features in aquifer-related wellhead gas spectra compared to non-aquifer cases. A statistically significant correlation was confirmed between wellhead water vapor levels and aquifer existence. This study pioneers the use of infrared spectroscopy for analyzing water vapor in wellhead gases. It reveals that the infrared absorption peaks of water vapor can effectively indicate the water content of the formation, offering a novel way for aquifer identification in gas logging.