Dynamic Evolution Laws of Free Gas and Adsorbed Gas in Deep Coal Reservoirs and Its Geological Controls: A Case Study of Daning-Jixian Block, Ordos Basin
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摘要: 煤层气的赋存状态是影响深部煤层气资源评价和开发效益的关键因素之一.为了揭示深部煤储层中游离气‒吸附气的动态演化规律及其地质控制，从而深化对深部煤层气成藏过程的认识，本文以鄂尔多斯盆地大宁‒吉县区块太原组8号煤储层为研究对象，构建了游离气含量、吸附气含量、原位孔隙度以及含水饱和度的预测模型，结合煤层埋藏史、受热史、热演化史及储层压力演化过程的系统分析，探讨了煤层气成藏过程中气体赋存状态的动态演化规律.研究表明，该区煤层气成藏过程中游离气‒吸附气的动态演化可划分为4个阶段：（Ⅰ）晚二叠世至早侏罗世，煤层快速沉降阶段，吸附气含量不断增加，几乎不存在游离气；（Ⅱ）早侏罗世至早白垩世，煤层小幅升降阶段，游离气含量逐渐增加，吸附气含量呈现波动式变化；（Ⅲ）早白垩世至白垩纪中期，煤层二次快速沉降阶段，游离气含量先降低后升高，吸附气含量缓慢上升；（Ⅳ）白垩纪中期至今，煤层快速抬升阶段，游离气含量逐渐降低，吸附气含量持续上升.该研究成果可为深部煤层气资源的有利区优选和高效开发提供一定的理论借鉴.Abstract: The occurrence state of coalbed methane (CBM) is one of the key factors influencing the resource assessment and the development efficiency of deep CBM. In order to elucidate the dynamic evolution laws of free gas and adsorbed gas within deep coal reservoirs, as well as their geological controls, thereby deepening the understanding of deep CBM accumulation processes, this study focuses on the No. 8 coal seam of the Taiyuan Formation in the Daning-Jixian Block, Ordos Basin.Prediction models for free gas content, adsorbed gas content, in-situ porosity, and water saturation were established. These were integrated with a systematic reconstruction of the coal seam burial history, thermal history, thermal maturation, and reservoir pressure evolution, to investigate the dynamic evolution of gas occurrence states during CBM accumulation.The results indicate that the dynamic evolution of free and adsorbed gas during CBM accumulation in the study area can be classified into four stages: (Ⅰ) Late Permian-Early Jurassic: a period of rapid coal seam subsidence, accompanied by a sustained rise in adsorbed gas content and negligible free gas occurrence; (Ⅱ) Early Jurassic-Early Cretaceous: a stage of minor fluctuations in burial depth of coal seam, during which free gas content gradually increased, while adsorbed gas content exhibited oscillatory variations; (Ⅲ) Early Cretaceous-Middle Cretaceous: second rapid subsidence phase of coal seam, in which free gas content first decreases and then increases, with adsorbed gas content rising slowly; (Ⅳ) Middle Cretaceous-present: rapid uplift of coal seams, during which free gas content gradually decreases, while adsorbed gas content continues to increase.The findings of this study can provide a theoretical reference for favorable zone selection and efficient exploitation of deep CBM resources.
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Key words:
- deep coalbed methane /
- adsorbed gas /
- free gas /
- dynamic evolution /
- Daning-Jixian /
- coalbed methane geology
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图 1 鄂尔多斯盆地大宁‒吉县区块位置、含煤地层及构造特征（图d引自张雷等，2023）
Fig. 1. Location, coal-bearing strata, and structural characteristics of the Daning-Jixian Block in the Ordos Basin
图 2 煤样覆压孔隙度实验装置示意图
Fig. 2. Schematic diagram of the experimental apparatus for measuring overburden porosity of coal samples
图 3 研究区煤样归一化孔隙度与有效应力（a）以及储层压力（b）的关系
Fig. 3. Relationships between normalized porosity and effective stress (a) and reservoir pressure (b) for coal samples from the study area
图 4 研究区煤样归一化孔隙度实测值与预测值关系
Fig. 4. Relationship between measured and predicted normalized porosity for coal samples from the study area
图 5 鄂尔多斯盆地煤岩总孔体积（无应力加载条件下）与Ro, max关系
Fig. 5. Relationship between total pore volume (under no-stress loading) and Ro, max of coals in the Ordos Basin
图 6 Langmuir体积与Ro, max（a）以及温度（b）的关系
Fig. 6. Plots of Langmuir volume versus Ro, max (a) and temperature (b)
图 8 鄂尔多斯盆地煤样实测含水饱和度与Ro, max（a）以及氦测总孔隙度（b）的关系
Fig. 8. Plots of measured water saturation versus Ro, max (a) and helium-measured total porosity (b) of coal samples in the Ordos Basin
图 9 鄂尔多斯盆地煤样实测含水饱和度与预测含水饱和度的关系
Fig. 9. Relationship between measured and predicted water saturation for coal samples from the Ordos Basin
图 10 大宁‒吉县深部煤层气成藏过程以及吸附气‒游离气动态变化规律
a. 太原组8号煤层埋藏史、受热史和有机质成熟史；b. 8号煤层孔隙度、储层压力和静水压力演化史；c. 吸附气和游离气含量演化史及阶段划分
Fig. 10. Accumulation processes of deep CBM in the Daning-Jixian Block and dynamic variation laws of free gas and adsorbed gas
表 1 研究区煤样覆压孔隙度测试实验方案
Table 1. Experimental scheme for overburden porosity tests on coal samples in the study area
方案序号 有效应力、围压和储层压力设计方案 ① 保持有效应力5 MPa不变 围压（MPa） 6 9 12 15 20 储层压力（MPa） 1 4 7 10 15 ② 保持有效应力10 MPa不变 围压（MPa） 11 14 17 20 25 储层压力（MPa） 1 4 7 10 15 ③ 保持有效应力15 MPa不变 围压（MPa） 16 19 23 25 30 储层压力（MPa） 1 4 7 10 15 
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