低频电磁探测技术在煤层气富集区的应用

Application of low-frequency magnetotelluric detection in coalbed methane enrichment areas

以沁水盆地胡底矿区为例,根据实际情况设计野外电磁探测方案,开展探测试验;基于多元电磁探测数据,从地质解译、构造及煤储层参数定量评估等多角度实现煤层气富集区探测。研究结果表明:在地质解译方面,超低频电磁探测数据优势突出,可根据不同地层的电性特征有效识别不同岩层及煤储层;在断裂等构造解释方面,音频大地电磁探测数据获得的视电阻率二维反演结果可准确提取陷落柱及断层信息,且其位置分布及特征与地震数据基本吻合,对小断层识别效果较好;在煤储层参数定量评估方面,超低频电磁探测曲线异常中心深度、范围及幅值平均值分别与煤储层埋深、厚度及含气量相关性较好,为定量评价提供了数据支持,通过采用支持向量机算法构建煤储层含气量评估模型,预测精度达87.33%,与实测含气量的平均误差仅为0.17m3/t。基于多元低频电磁探测数据可从多角度为煤层气富集区的圈定提供参考。

A field experiment of low-frequency magneto-telluric detection was designed and conducted in Hudi,Qinshui Basin.Based on the processed multisource electromagnetic data of this survey,the geological evaluation and coalbed parameter analysis are carried out in order to detect the coalbed methane(CBM).The following understandings are obtained:A.Different lithologic layers and coalbeds can be effectively identified on super-low frequency(SLF)electromagnetic data;B.Collapse pillars and faults can be extracted from 2 Dapparent resistivity inversion of audio frequency electromagnetic(AMT)data and they are consistent with that of seismic data.And subtle beds are also identified;C.The anomaly center depth,anomaly scope,and its average magnitude of SLF data have good correlation respectively with the buried depth,thickness,and gas content of coal reservoirs.The prediction accuracy of gas content is87.33% with a coalbed methane evaluation model supported by vector machines,and the average error is only 0.17 m3/t.Our research proves that the low-frequency magnetotelluric detection is effective in the CBM exploration.