煤层顶板非均质性识别及对煤层气富集的控制作用

Coalbed methane(CBM) which self-generation and self-bearing is unconventional gas reservoir. The CBM enrichment regularity is still the research difficulty, because of the unique occurrence and the heterogeneity of reservoir. Previous summarized many influencing factors of the CBM enrichment. The roof stratum of coal seam(RSCS), as one of parts of reservoir closed system, is very important. However, the RSCS itself is characterized by the heterogeneity, and its compactness affects CBM preservation. To study the heterogeneity of the RSCS and its control to CBM enrichment, the low-frequency rock physics experiment will be studied. Under seismic frequency condition, the experiment will test the velocity of the p-wave and s-wave and calculate the wave impedance reference value. The four kinds of stones will be collected from the seam roadway roof stratum or the field section. The stones include fine sandstone, siltstone, mudstone and coal. The wave impedance reference value of different stones will matches the seismic attributes to accurately identify the heterogeneity of the RSCS. The CBM content characteristic will be studied based on single well CBM content and AVO attribute. Then the heterogeneity contrasts with the CBM content characteristic to explore its control to coalbed methane enrichment. This study will reach two results. On the one hand, the study will establish one identifying method of the heterogeneity of the RSCS by 3-d seismic. On the other hand, this study will establish one assistant forecasting method of the CBM enrichment by contrasting analysis.

煤层气属于残余型非常规气藏，由于独特的赋存状态与储层的非均质性，煤层气富集规律一直是研究的难点。前人总结了煤层气富集的影响因素，其中顶板盖层作为封闭系统十分重要。顶板盖层本身存在着非均质性，其致密程度影响煤层气保存。为了研究顶板盖层非均质性及对煤层气富集的控制，本项研究以低频岩石物理实验为基础，采用地震频率的声波测试不同岩性的纵、横波速度等, 计算岩石波阻抗参考值。岩石样品选择巷道或剖面煤层顶板的细砂岩、粉砂岩、泥岩及煤岩四种，将不同岩性的波阻抗参考值与三维地震属性进行匹配，对顶板盖层非均质性进行高精度识别。同时利用单井含气量、AVO反演属性等，开展煤层含气规律研究。将顶板非均质性与含气规律进行对比研究，探索顶板非均质性对煤层气富集的控制作用。一方面，建立利用三维地震识别煤层顶板非均质性的研究方法；另一方面，丰富煤层气预测理论，探索一套通过分析顶板盖层性质辅助预测煤层气富集区的研究方法。

为了研究沁水盆地南部煤层气富集的主控因素与规律，本文以寺河矿一个较成熟的煤层气开发区块为研究对象，采用地质研究、地球物理与低频岩石物理实验相结合的方法，利用研究区内单井数据、三维地震数据及含气量数据，对3# 煤层含气量的影响因素及煤层气的富集规律进行了分析。结果表明，研究区煤层气富集的影响因素有煤层构造发育、地层压力、煤层结构、顶板盖层性质及水文地质条件等。其中，地层压力、煤层结构和顶板盖层性质是煤层气富集的主要控制因素。地层压力会影响吸附态的煤层气含量饱和度，煤层结构决定了储集空间的大小，煤层顶板的致密程度对煤层气成藏与保存具有重要的影响作用。利用三维地震属性识别了煤层与顶板盖层性质，分析了煤层结构与盖层致密性对煤层气富集的控制作用。低频岩石物理实验丰富了低频条件下的实验数据，实验数据与地震属性具有较好的匹配关系，进一步验证了顶板盖层致密性是煤层气富集的主控因素。研究区煤层气富集规律分析表明，煤层压力与储集空间越大，对煤层气富集越有利。埋深较大的向斜部位满足地层压力与煤岩结构需求，致密的泥岩顶板使得煤层气更优先在该部位富集与成藏。