关键词: 产气机理, 渗流模型, 产能评价, 游离气, 鄂尔多斯盆地

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