鄂尔多斯盆地吴起—志丹地区长8段致密砂岩储层孔喉结构分形特征

Fractal characteristics of pore throat structure of Chang 8 Member tight sandstone reservoir in Wuqi-Zhidan area,Ordos Basin

为精细表征鄂尔多斯盆地吴起—志丹地区长8段致密储层孔喉结构,在铸体薄片、扫描电镜、X射线衍射、高压压汞、恒速压汞以及油水相渗等相关实验的基础上,利用分形理论研究了长8段致密储层的孔径分形特征和喉道分形特征,系统分析了分形特征与储层孔隙度、渗透率、微观孔喉特征之间的关系。结果表明:研究区岩性以岩屑长石砂岩为主,平均孔隙度为8.2%,平均渗透率为0.16×10^(-3)μm^(2),为典型的致密储层。储层孔隙类型主要为粒间孔和溶蚀孔,根据毛管压力曲线形态和高压压汞参数将储层孔喉结构分为I类和II类2种类型。恒速压汞实验表明研究区样品总进汞饱和度中,喉道进汞占比较多,恒速压汞计算的喉道分形维数具有两段特征,即细喉道段(D_(T1))和粗喉道段(D_(T2))。全孔径分形维数和喉道分形维数均与物性、进汞饱和度和退汞效率等参数具有良好的负相关性,喉道分形维数与喉道进汞饱和度、主流喉道半径呈良好负相关性,孔喉的非均质性增强,致使油相流体在含水储层中更加难以通过,油相相对渗透率变小。

In order to accurately characterize the pore throat structure of tight reservoir in the Chang 8 Member of the Wuqi-Zhidan area of the Ordos Basin,based on the analyses of cast thin section,scanning electron microscopy,X-ray diffraction,high-pressure mercury injection,constant velocity mercury injection,and oil-water phase infiltration,the pore throat fractal characteristics and throat fractal characteristics of tight reservoirs in Chang 8 Member were studied by using fractal theory.The relationship between fractal characteristics and reservoir porosity,permeability,and microscopic pore throat characteristics was systematically analyzed.The results show that the lithology of the study area is dominated by lithic feldspar sandstone,with an average porosity of 8.2%and an average permeability of 0.16×10^(-3)μm^(2).The reservoirs are generally tight.The pore types of the reservoir are mainly intergranular pores and dissolution pores.According to the morphology of capillary pressure curve and high-pressure mercury injection parameters,the pore throat structure of the reservoir is divided into two types:type I and type II.The constant rate mercury intrusion experiment showed that the mercury saturation of the samples in the study area was mainly contributed by the throat.The fractal dimension of throat calculated by constant velocity mercury injection has two characteristics:Fine throat section D_(T1) and coarse throat section D_(T2).The full aperture fractal dimension and throat fractal dimension have good negative correlation with physical properties,mercury injection saturation and mercury removal efficiency.The throat fractal dimension has a good negative correlation with the mercury injection saturation of the throat and the radius of the mainstream throat.The more homogeneous the pore throat is,the more difficult it is for the oil phase fluid to pass through the water-bearing reservoir and the lower the relative permeability of the oil phase.