Acoustic Method for Gas Content Calculation and Productivity Evaluation in Deep Coalbed Methane: A Case Study from Daning-Jixian, Ordos Basin, China

The deep coalbed methane resources in the Ordos Basin are abundant, and the realization of large-scale development is conducive to increasing oil and gas reserves and production, and helping to implement the dual carbon goals. Focusing on the key issue of predicting the gas content of deep coal seams and evaluating the production capacity of deep coal seams, isothermal adsorption and acoustic combined measurement experiments were carried out in the deep coal reservoirs of the Daning-Jixian coalbed methane field in the Ordos Basin, and the free gas and adsorbed gas volumes were calculated while measuring the acoustic attenuation coefficient of coal. By establishing the relationship between the acoustic attenuation coefficient and the gas content and the free adsorption ratio, the calculation model of free gas and adsorbed gas volume based on acoustic attribute parameters was established. Finally, based on the quantitative calculation model for logging, three parameters—gas content, coal structure index, and coal composition index—were established to evaluate the ‘sweet spot’ and productivity potential of deep coal. The results show that the free adsorption ratio calculated by the free gas and adsorbed gas volume calculation model based on acoustic attribute parameters is in good agreement with the free adsorption ratio measured by the experiment, which proves the accuracy of the gas content calculation method. According to the productivity classification standard, the productivity classification of 20 large-scale gas test wells in the target area is carried out, and the success rate of productivity level discrimination is 85%, which is of great significance for the exploration and development of deep coalbed methane.