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基于多参数融合的煤层气富集区预测方法

Prediction method of coalbed methane enrichment area based on multi parameter fusion

  • 摘要: 煤层气是一种以吸附态为主赋存于煤储层中的气藏。利用地震勘探手段进行煤层气富集区预测是降低勘探风险,提高煤层气井采收率的重要方法。由于煤层气的赋存机理和常规油气完全不同,而且煤层气赋存的地震岩石物理机制尚不明确,常规油气预测方法应用于煤层气预测往往失效。提出一种以煤层气赋存地质要素预测为研究对象,融合多种地震属性参数的煤层气富集单元评价方法。选择沁水盆地武乡南区块作为研究区域,通过岩石物理分析确定围岩和煤岩的弹性参数特征,利用反演波阻抗属性对煤层厚度和顶底板岩性的展布进行了预测,采用波形聚类分析方法研究了聚煤前后的沉积微相,通过对不同煤体结构的弹性参数交会发现密度对原生煤和构造煤具有良好的区分特性,利用密度属性预测了构造煤的平面分布,地震各向异性反演得到的裂隙密度与钻井揭示煤层渗透率相关性良好,利用裂隙密度预测了煤层渗透率的分布,基于前人研究成果,认为弹性模量×密度(λρ)和剪切模量×密度(μρ)属性对于煤层吸附气含量较为敏感,利用λρ×μρ属性预测了吸附气含量。在此基础上,采用BP神经网络算法,以已知井点的测试含气量和五类地质参数为样本,进行神经网络模型训练,建立煤层顶底板岩性、聚煤前后沉积微相、煤体结构、煤层渗透率和煤层含气性与煤层气富集区之间的非线性映射关系。结果表明,利用多地质参数融合方法进行煤层气富集性预测吻合率高,能够降低单因素预测多解性的影响,降低勘探风险。

     

    Abstract: Coalbed methane (CBM) is a kind of gas reservoir which mainly exists in coal reservoirs in adsorption state. The prediction of coalbed methane enrichment area by seismic exploration is an important method to reduce the exploration risk and improve the recovery of coal-bed methane wells. For the reason that the occurrence mechanism of coalbed methane is completely different from that of conventional oil and gas, and the seismic petrophysical mechanism of coalbed methane occurrence is still unclear, conventional oil and gas prediction methods are often ineffective when applied to coalbed methane prediction. In this study, an evaluation method of coalbed methane enrichment unit is proposed, which takes the prediction of geological elements of coalbed methane occurrence as the research object and integrates multiple seismic attribute parameters. Setting the Wuxiang South Block in Qinshui Basin as the study area, the thickness of coal seams and the lithology of roof and floor were predicted basing on the physical analysis of coal seams and surrounding rock and wave impedance inversion,. The sedimentary microfacies were analyzed using waveform clustering analysis method before and after coal accumulation, which led to the significant difference of the density of coal rocks with different coal body structures, thus the den-sity attribute was used to predict the plane distribution of tectonic coal. Since the fracture density has a good correlation with the coal seam permeability, it is used to predict the coal seam permeability. The forward modeling results show that the attribute of λρ and μρ is sensitive to the content of adsorbed gas in coal seams, therefore it is selected to predict the adsorption gas content. Relying on these abovementioned five geological parameters: the lithology of the coal seam roof and floor at the well point, the sedimentary microfacies before and after coal accumulation, the coal body structure, the permeability and the gas bearing property as samples , the BP neural network model of the coalbed methane enrichment area is constructed for the known coal seam gas content at the well point,, which presents as the building of the nonlinear mapping relationship between the five types of geological elements and the enrichment area through sample training, which results in the prediction of coalbed methane enrichment area . The results show that the coalbed methane enrichment predicted by multi geological parameter fusion method has a high coincidence rate with current drilling, which reduces the multi solution of single factor prediction.

     

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