Evaluation of gas content prediction models for deep coal seams: A case study from the Daning–Jixian Block, Eastern Ordos Basin

• By integrating pressure-preserved coring technology with geophysical logging data, a gas content prediction model was developed to reveal the intrinsic and extrinsic factors controlling the gas-bearing properties of deep coal seams. • A prediction model for the free and adsorbed gas contents based on geophysical parameters was established to overcome the limitations of traditional models designed for shallow coal seams. • Free and adsorbed gas content prediction models can be applied to evaluate the gas-bearing properties of deep coal seams. Deep coal rock gas resources are abundant. However, high-temperature and high-pressure geological conditions result in different gas occurrence states in deep coal reservoirs than in shallow coal reservoirs. On the basis of pressure-preserved coring data from the Benxi Formation coal seams in the Daning–Jixian Block along the eastern margin of the Ordos Basin, high-temperature and high-pressure methane isothermal adsorption experiments were conducted to elucidate the intrinsic and extrinsic factors controlling the gas content in deep coal seams. A prediction model for the free and adsorbed gas contents was established with geophysical logging data. The total gas contents in the pressure-preserved core coal samples ranged from 15.66 to 35.44 cm 3 /g, of which the free gas contents ranged from 2.27 to 7.96 cm 3 /g, accounting for 12.38% to 24.30% of the total gas content. Gas content variations were significantly influenced by the ash yield, with high-ash coal seams exhibiting relatively low gas contents. Vertically, the middle coal seams were oversaturated, whereas the upper and lower seams were undersaturated. Through the use of geophysical logging data, methods for determining key parameters such as Langmuir volume, Langmuir pressure, water saturation, and porosity were established, and a prediction model for the free and adsorbed gas contents in deep coal seams was developed. The model predicted total gas contents ranging from 6.88–34.18 cm 3 /g, with adsorbed gas contents ranging from 4.75 to 27.3 cm 3 /g (accounting for 66.42% to 84.61% of the total). The free gas contents ranged from 2.31 to 6.91 cm³/g (accounting for 15.43% to 33.58% of the total), which strongly agreed with the measured values obtained from pressure-preserved cores. These findings validated the reliability of the model for predicting the adsorbed and free gas contents in deep coal seams and demonstrated the effectiveness of geophysical logging methods for evaluating the gas-bearing properties of deep coal rock gas reservoirs.