Abstract: Deep coal reservoirs are generally characterized by high gas content, a high proportion of free gas, and high gas saturation. However, the production behavior of deep coalbed methane (CBM) remains poorly understood, and in particular, there is a lack of systematic investigation into the differences in gas production associated with different coal macrolithotypes. In this study, the pore-fracture morphology, gas content distribution, and reservoir sensitivity of different macrolithotypes were systematically compared. Based on the mechanisms of free gas flow and adsorbed gas desorption and diffusion in deep CBM reservoirs, a horizontal well productivity prediction model was developed to clarify the dynamic changes in gas production during pressure depletion. The reliability of the model was further verified using actual gas-well production data. Results show that bright coal (bright and semi-bright coal) exhibit a higher degree of pore-fracture development and better connectivity, resulting in stronger flow capacity, whereas dull coal (semi-dull and dull coal) has relatively poor pore-fracture development, with gas production dominated by desorbed gas. A horizontal well productivity model for deep CBM that accounts for adsorbed and free gas was proposed. Based on the Langmuir equation and real-time pressure-field data, the variation in adsorbed gas content during production was calculated using the time-difference method. Meanwhile, the dynamic changes in porosity, real-time pressure, and reservoir parameters were incorporated to quantify the variation in free gas production. The total gas production calculated by the productivity model agrees with the actual production data over 700 days, with a coincidence rate of more than 90%. Furthermore, the model quantitatively characterizes the spatiotemporal variations in reservoir pressure depletion, clarifies the contributions of free and adsorbed gas production in different reservoir zones, and predicts the estimated ultimate recovery (EUR) of gas wells. These results reveal the differences in gas production behavior among different coal macrolithotypes and establish a productivity evaluation model applicable to deep CBM wells.

Key words: gas production mechanism, flow model, productivity evaluation, free gas, Ordos Basin

摘要： 深煤层总体具有高含气量、高游离气含量和高含气饱和度特征，深层煤层气产气规律尚不清晰，尤其是不同煤岩类型影响的产气差异缺少系统解剖。通过系统对比不同宏观煤岩类型煤样孔裂隙形态、含气量分布及储层敏感性差异，基于深层煤层气游离气流动与吸附气解吸扩散过程认识，建立了水平井产能预测模型，明确游离气、吸附气在压降过程中的产出动态变化，并结合实际气井产能验证了模型的可靠性。研究结果表明，亮煤（光亮煤和半亮煤）储层孔裂隙发育程度较高，连通性较好，具有较强渗流能力，而暗煤（半暗煤和暗淡煤）储层孔裂隙发育程度相对较低，其产气贡献以解吸气为主；提出了考虑吸附气和游离气产出比例的深层煤层气水平井产能模型；基于兰氏方程，结合实时压力场数据，通过时间差分法计算了生产过程中吸附气含量的变化；结合孔隙度、实时压力与储层参数动态变化明确了游离气的产出量变化；产能模型计算的总产气量与气井生产700 d的实际数据吻合率大于90%；进一步定量刻画不同时间、不同距离储层压降变化，明确了不同区域内储层游离气、吸附气产出贡献，实现了气井最终采收 率（EUR）预测。上述结果揭示了不同煤岩类型储层产气过程差异，提出了可以应用于深层煤层气井产能评价的计算模型。

关键词: 产气机理, 渗流模型, 产能评价, 游离气, 鄂尔多斯盆地