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鄂尔多斯盆地东缘深部多相态煤层气含量预测模型

Multi-phase coalbed methane content model in deep strata of eastern margin of Ordos Basin

  • 摘要: 鄂尔多斯盆地深部煤层气资源丰富,准确掌握煤层气含量—相态变化规律和内在机理对实现深部煤层气规模开发具有重大意义。目前针对不同相态深部煤层气含量的预测模型尚不完善,煤层气相态变化机理认识不清,这已成为制约深部煤层气高效开发的关键问题。通过梳理鄂尔多斯盆地东缘(鄂东缘)深部煤层气含量以及相态变化的控制因素,建立了多因素耦合的深部多相态煤层气含量预测模型,分析了煤层气含量—相态随埋深的变化规律,揭示了多因素影响下相态转化的内在机理。研究结果表明:① 深部煤层气中吸附气含量变化受到储层温度(T)、储层压力(P)、最大镜质体反射率(Ro,max)、含水饱和度(Sw)的影响;游离气赋存空间受到吸附气和水分子的共同影响;溶解气含量与温度、压力、矿化度(M)呈良好相关性;② 建立了多参数的吸附气含量模型(R2=0.924 6 ),基于吸附膨胀效应建立了游离气含量模型,考虑溶解度随温压和矿化度的变化建立了溶解气含量模型;整合上述模型建立了鄂东缘深部多相态煤层总含气量预测模型。采用该模型计算得到的总含气量与不同煤层气区块的实际含气量具有较高的吻合度。③ 以临界深度(2 000 m)为界,吸附气含量逐渐下降,游离气和溶解气含量逐渐上升,相态由“吸附气”向“吸附气+游离气”变化,最终转变为“吸附气+游离气+溶解气”。研究结果阐明了深部煤层气相态−含量的演化规律和赋存机理,对开展深部煤层气储量评价及甜点优选、推动鄂尔多斯盆地深部煤层气效益开发提供了重要的理论依据。

     

    Abstract: The deep coalbed methane resources in the Ordos Basin are abundant. Accurately understanding the laws and underlying mechanisms of coalbed methane content phase change is of great significance for achieving large-scale and efficient development of deep coalbed methane. Currently, prediction models for deep multi-phase coalbed methane content remain inadequate, and the mechanism of coalbed methane phase change is not well understood. These have become the key problems restricting the efficient development of deep coalbed methane. A multi-factor coupled prediction model for the variation of multiphase coalbed methane content in the deep (>1 500 m) is established by sorting out the controlling factors of coalbed methane content and phase changes in the eastern margin of the Ordos Basin. The general laws and underlying mechanisms of the variation of coalbed methane content phase state with burial depth are summarized and revealed. The results show that: ① The variation of adsorbed gas content in deep coalbed methane is influenced by reservoir temperature (T), reservoir pressure (P), Ro,max and water saturation (Sw); The free gas storage space is affected by the combined effects of adsorbed gas and water molecules; The dissolved gas volume has a good correlation with temperature, pressure, and mineralization degree; ② An adsorption gas content model (R²=0.924 6) is developed based on multivariate factors. A free gas content model is established considering adsorption-induced expansion, while a dissolved gas content model account for variations in solubility with temperature, pressure, and salinity. By integrating these sub-models, a predictive model is constructed to estimate the variation of deep multi-phase coalbed methane content in the eastern margin of the Ordos Basin. The calculated total gas content shows strong agreement with measured values across different blocks. ③ Taking the critical depth (2 000 m) as the boundary, the content of adsorbed gas gradually decreases, while the content of free gas and dissolved gas gradually increases. The gas occurrence state changes from “adsorbed gas” to “adsorbed gas + free gas”, and finally to “adsorbed gas + free gas + dissolved gas”. The findings clarify the evolutionary patterns and occurrence mechanisms of phase states and content of deep coalbed methane, providing an important theoretical basis for assessing the resource potential of deep coalbed methane and promoting its large-scale and efficient development in the Ordos Basin.

     

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