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乌鲁木齐河东矿区煤储层渗流孔孔隙分形特征研究

Study on fractal characteristics of seepage pores of coal reservoirs in Hedong Mining Area of Urumqi

  • 摘要: 为准确描述乌鲁木齐河东矿区煤储层渗流孔孔隙特性,结合压汞试验及煤质分析试验,采取分段分形方法定量探讨了煤储层渗流孔孔隙结构特征及其影响因素,将渗流孔孔隙度与孔隙结构参数进行耦合,并进行了煤岩渗透率预测。研究结果表明:乌鲁木齐河东矿区43号和45号主力煤层孔隙结构较为复杂,但连通性较好,43号煤层样品压汞曲线表现进汞饱和度相似,退汞效率较高,煤样微小孔及大孔较为发育。45号煤层样品进汞饱和度为45%~85%,且退汞效率较低,煤样微小孔隙占有绝对优势,故43号煤层孔渗性优于45号煤层;煤岩渗流孔隙分形维数分布在2~3,中、大型孔隙均存在明显的分形特征,煤样的中孔分形维数D2高于大孔分形维数D1,故研究区煤储层中孔较大孔复杂;大孔分形维数随着镜质组含量的增加而减小,随着惰质组含量的增加而增加,随着煤中水分的增加而呈现倒“U”型的相关关系,随着灰分的增加呈现下降的趋势等;煤岩渗透率高低是渗流孔孔隙度和孔隙结构耦合而决定的,在95%置信带内大孔孔隙度P1与渗透率呈现强相关关系,中孔孔隙度P2与渗透率呈现中等程度相关,分形维数D1、D2则呈现弱相关关系。基于支持向量机方法将渗流孔孔隙度和分形维数作为煤岩渗透率的自变量进行训练,经检验得出的渗透率与实际测试渗透率拟合程度很好,渗流孔孔隙度与孔隙结构耦合可有效反映煤岩渗透率。

     

    Abstract: In order to accurately describe the coal seam seepage pore characteristics from Hedong mining area in Urumqi,combined with the mercury injection test and coal quality analysis test,a section fractal method was used to quantitatively discussthe pore structure characteristics and influencing factors of seepage pores in coal reservoirs, and then coupledwith the porosity and pore structure to evaluate the permeability of coal reservoir.The results show that the pore structure of No.43 and No.45 main coal seams in Hedong mining area are more complex but the connectivity is better. The mercury intrusion curves of samples from No.43 coal seam show similar mercury saturation and high mercury removal efficiency, indicating that micropores and macropores are relatively developed. The maximum mercury intrusion saturation in samples from No.45 coal seam is between 45% and 85% and the mercury removal efficiencyis low, revealing that the micropores are dominantly developed. It is concluded that the permeability of No.43 coal seam is better than that of No.45 coal seam.The fractal dimension of coal seepage pores ranges from 2 to 3, showing that there are obvious fractal features in medium and large pores, and the average fractal dimensions of the mesoporous (D2) are commonly higher than those of the macroporous (D1). It is concluded that the mesopores are more complex than the macropores in the studiedcoal reservoirs.The macropore fractal dimension decreases with the increase of the the vitrinitecontent and ash yield, but it increases with the increase of the inertinite content, and it presents an inverted "U" correlation with the increase of moisture content in the coal. The permeability of coal is determined by the coupling of seepageof both porosity and pore structure. In the 95% confidence interval, the macropore porosity (P1), mesopore porosity (P2) and fractal dimensions are strongly, moderately and weakly correlated with permeability respectively, and fractal dimensions D1 and D2 have a weak correlation.Based on support vector machine method, the porosity and fractal dimension of the seepage pores are trained as independent variables to calculate coal permeability. The calculated permeability fits well with the experimental results with a goodness-of-fit. It is concluded that the coupling results of seepage pore porosity and pore structure can be effectively used to evaluate the coal reservoir permeability in the study area.

     

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