Abstract: A variable skeleton porosity calculation method based on coal-rock components is proposed in this study to address the issues such as low accuracy in calculating the porosity of deep coal rock reservoirs and the difficulty in applying critical factors methods to evaluate production capacity in deep coal rocks whereas it is applicable in shallow coal reservoirs.Furthermore,two models,namely matrix quality (Q_m) and structural quality (Q_s),are established based on gas saturation calculation models that are developed on the basis of ratios of measured acoustic travel time to reconstructed acoustic travel time.Based on these models and according to production capacity levels,the coal rock reservoirs are classified into three categories.The results show that compared to shallow coal reservoirs,the matrix porosity,cleat porosity,gas saturation,and coal mass structure of deep coal reservoirs are the primary factors affecting production capacity.The accuracy of porosity calculation with key parameters has improved by employing methods such as variable skeleton porosity,dual-lateral cleat porosity,and acoustic travel time method and gas saturation.Theprecision of porosity calculation has increased from 68.1% to 90.5%.The production capacity classification and evaluation method established on these key parameters has significant predictive effectiveness in deep low-medium rank coal reservoirs,achieving a prediction coincidence rate of 91.0%.This classification and evaluation method can provide technical support for the next coalbed methane exploration.

Key words: deep coal rock, low-medium rank coal, variable skeleton porosity, coal mass structure, production capacity prediction