鄂尔多斯盆地镇北地区延长组超低渗透储层孔隙结构及其分形特征

选取鄂尔多斯盆地镇北地区延长组7块超低渗透岩心样品,分别开展铸体薄片、扫描电镜、X衍射、高压压汞和核磁共振等实验,明确了超低渗透储层孔喉分布特征、孔隙类型、矿物组成及其含量;运用分形理论研究了储层孔喉分形特征,系统分析了分形维数与储层物性、孔隙结构参数和矿物成分及其含量之间的关系。结果表明:研究区储层孔隙类型主要为残余粒间孔、溶蚀孔和晶间孔。储层矿物成分以石英和长石为主,黏土矿物中绿泥石含量最高。根据毛管压力曲线形态和排驱压力可将储层孔隙结构分为Ⅰ类、Ⅱ类和Ⅲ类3种类型,其储集性能和渗流能力依次变差,孔隙结构非均质性逐渐增强。高压压汞所得的孔喉大小分布具有多重分形特征,依据分形特征曲线存在的明显转折点,将孔隙空间分为大孔、中孔和微孔,微孔相对于大孔和中孔而言孔隙分布较为均匀和规则,对应分形维数最小。核磁共振技术可以更全面地表征储层的孔隙空间,T2T2cutoff段可动流体孔隙空间和有效孔隙符合分形结构,对应的分形维数均反映相互连通孔隙的复杂程度。储层孔喉分形维数与孔隙度和渗透率之间具有较好的负相关性,与孔隙结构参数之间也存在较好的相关关系,储层的矿物组成及其含量是决定分形维数大小的内在因素,进而影响储层质量和孔隙结构特征。

Pore structure and fractal characteristics of ultra-low permeability reservoirs in Yanchang Formation in Zhenbei area,Ordos Basin

Seven ultra-low permeability core samples of Yanchang Formation in Zhenbei area,Ordos Basin are selected to carry out casting thin section,scanning electron microscope(SEM),X-ray diffraction,high pressure mercury injection and nuclear magnetic resonance(NMR)experiments,respectively.The characteristics of pore throat distribution,pore type,mineral composition and its content of ultra-low permeability reservoirs are clarified.The fractal theory is used to study the fractal characteristics of reservoir pore throat,and the relationships between the fractal dimension and the reservoir physical properties,pore structure parameters as well as mineral composition and its content are systematically analyzed.The results show that the reservoir pore types are mainly residual intergranular pores,dissolution pores and intercrystalline pores.The mineral composition of the reservoir is dominated by quartz and feldspar,and the content of chlorite in clay mineral is the highest.According to the morphology of capillary pressure curve and displacement pressure,the reservoir pore structure can be divided into three types:Type Ⅰ,Type Ⅱ and Type Ⅲ.The corresponding reservoir performance and percolation capacity become worse in turn,and the heterogeneity of pore structure gradually increases.The pore throat size distribution obtained by high pressure mercury injection has multi-fractal features.There are two obvious turning points in the fractal feature curve,which divides the pore space into macropores,mesopores and micropores.Compared with macropores and mesopores,the pore size distribution of micropores is relatively uniform and regular,and the corresponding fractal dimension is the smallest.NMR can provide a better characterization of the reservoir pore space.The pore throat distribution of the T2T2cutoff section conform to the fractal structure,and the corresponding fractal dimension reflects the complexity of interconnected pores.The fractal dimension of reservoir pore throat has a good negative correlation with porosity and permeability,and also has a good correlation with pore structure parameters.The mineral composition and its content of reservoir are the internal factors that determine the size of fractal dimension,and then affect the reservoir quality and pore structure characteristics.