陆相页岩热演化过程中孔隙分形特征

Fractal characteristics of pore structure of continental shale in the process of thermal evolution

分形维数可定量表征储层孔隙结构的复杂性,为页岩储层评价提供思路。以热模拟获得的不同热演化阶段的鄂尔多斯盆地长7段页岩为研究对象,应用场发射扫描电镜观察了各演化阶段孔隙变化特征,并通过低温液氮吸附实验,研究各个演化阶段页岩孔隙分形特征,运用FHH模型计算页岩孔隙分形维数,探讨了分形维数与有机碳、矿物成分、孔隙结构参数的关系。研究结果表明:低成熟阶段页岩中纳米级有机质孔发育有限,随着成熟度的增加,在有机质内部开始逐渐发育孔隙,同时黏土矿物颗粒间的有机质也开始分解,出现纳米级层间孔,主要发育墨水瓶状孔和少部分的平行板状孔;孔径峰值主要在2~4 nm和40~50 nm,随着成熟度增加,上述2个孔径段的孔隙相对数量增加,分形维数依次增大,分形维数为2.592~2.717,孔隙非均质性增强。分形维数随着有机碳含量的减少而增加,而与石英、黏土矿物含量相关性不明显;随着成熟度增加,微孔和中孔比例增加,平均孔径减小,孔隙表面越复杂,比表面积和分形维数增加;分形维数与总孔隙体积、微孔体积、中孔体积具有很好的正相关性,而与大孔体积相关性较差。

Fractal dimension can characterize quantitatively the complexity of pore structure of a reservoir and provides ideas for evaluation of shale reservoir.With the shale Chang 7 of Yanchang Formation in Ordos basin as the object,field emission scanning electronic microscope was used to observe the pore characteristics of thermal evolution stages,through the low temperature nitrogen adsorption experiment,the fractal characters of shale pore structure of different evolutionary stages were studied.FHH model was applied to calculate the pore fractal dimension.The relationships between the fractal dimensions and composition,the pore structure parameters were investigated.The results show it had not reached gas window for remaining a low thermal evolution stage for the primary sample,immature source rocks had not begun to generate a lot of hydrocarbons.With thermal maturity increasing,pores began to form gradually in the organic matter,especially,interlayer nanopores generated for the decomposition of organic matter within the clay.There existed mainly ink bottle-like pores and a few of parallel tabular pores,peak pore diameter was in 2~4 nm and 40~50 nm.With increase of maturity,the relative quantity of pores in the two above ranges of pore diameter increased,fractal dimension increased successively,the fractal dimension was 2.592~2.717.The fractal dimensions were negatively correlated with TOC,but show no obvious relationship with the quartz and clay mineral content,the proportion of micropores and mesopores increased with thermal maturity,the average diameters decreased,pore surface was more complicated,the specific surface and fractal dimensions increased.The relationship between total pore volume,micropore volume,mesopore volume and fractal dimensions were positively correlated,while macropore volume and fractal dimensions were poorly correlated.The fractal dimension could be used to evaluate the complexity of reservoir,it provides support for shale reservoir evaluation and adsorption.