基于热模拟实验的页岩孔隙连通性与形状系数演化研究及其地质意义

Study on evolution of shale pore connectivity and shape factor based on thermal simulation experiments and its geological significance

富有机质页岩的孔隙演化与页岩气赋存富集关系密切,孔隙连通性与形状演化特征对储集空间精细表征和页岩气赋存机理的揭示具有重要意义。本文对山东半岛北部古近系黄县组低成熟富有机质页岩不同热演化阶段的模拟产物开展低温液氮吸附和扫描电镜实验,并辅以数字图像处理技术分析页岩孔隙连通性和形状系数演化过程。结果表明:(1)低成熟阶段,有机孔少量发育,无机孔主要为矿物基质孔;随热演化程度增加,大量近圆形有机孔逐渐形成,少量溶蚀孔和黏土矿物层间孔生成;到过成熟阶段,压实作用导致部分孔隙减小或消失;(2)随热演化程度增加,有机孔和无机孔孔径均呈先增大后减小的趋势,孔隙形状系数均呈现近“V”型变化趋势;(3)高温高压导致样品孔隙系统第二个优势孔径峰值变大,表明孔隙系统增加了狭长-裂缝型孔隙,有助于沟通其他类型孔隙,提高孔隙系统的连通性;(4)热演化过程中生成的大量近圆形有机孔能为甲烷提供更多吸附位,生成的黏土矿物层间孔等狭缝型无机孔能够提高孔隙连通性,从而有助于提高页岩气的赋存和运移能力。本研究可为页岩气储层表征与页岩气赋存富集研究提供基础。

The pore evolution of organic-rich shale is closely related to the occurrence and enrichment of shale gas.It is of great significance to study the characteristics of pore connectivity and shape evolution for fine characterization of reservoir space and revealing the occurrence mechanism of shale gas.The low mature shale rich in organic matter in the Huangxian Formation of Paleogene was selected in this paper,and thermal simulation physical experiments were carried out to obtain simulation products with different thermal evolution degrees.Low temperature liquid nitrogen adsorption and scanning electron microscope experiments were undertaken,and digital image processing was carried out to extract pores,and the evolution characteristics of pore connectivity and shape coefficient were studied.The results show that:①During the low mature stage,a small amount of organic pores developed,while inorganic pores were mainly mineral matrix pores.Along with the increase of thermal evolution,a large number of nearly circular organic pores are gradually formed,and a small number of dissolution pores and clay mineral interlayer pores are formed.At the over-mature stage,some pores are reduced or disappeared due to compaction.②With the increase of thermal evolution degree,the pore diameters of both organic and inorganic pores increase at first and then decrease,and the pore shape coefficients all show a nearly V-shaped change trend.③High temperature and high pressure causes the peak value of the second dominant pore diameter of the sample pore system to become larger,which indicates that the narrow-slit pores are added in the pore system,which is helpful to communicate with other types of pores and improve the connectivity of the pore system.④A large number of nearly circular organic pores generated during thermal evolution can provide more adsorption sites for methane,and slit inorganic pores such as interlayer pores of clay minerals can improve pore connectivity,thus helping to improve the occurrence and migration capacity of shale gas.The research can provide a basis for shale gas reservoir characterization and shale gas occurrence and enrichment research.