高压压汞联合分形理论分析致密砂岩孔隙结构:以鄂尔多斯盆地合水地区为例

孔隙结构是致密砂岩储层的关键要素，制约油气在储层中的储集与流动，而储层孔隙结构是目前非常规油气勘探开发研究的重点和难点。选取鄂尔多斯盆地合水地区上三叠统延长组长7段10块致密岩心样品，开展高压压汞和X-衍射等实验，运用分形理论研究储层孔喉分形特征，并分析分形维数与储层物性、孔隙结构参数与矿物含量之间的关系。研究结果表明:根据压汞曲线形态和孔隙结构参数将储层分为Ⅰ、Ⅱ、Ⅲ类，其储集性能和渗流能力依次递减，微观非均质性依次增强。不同类型的储层具有不同的分形特点:根据分形曲线存在的拐点0.05和0.02 μm，将实际情况和IUPAC提出的孔隙大小划分标准结合，Ⅰ类储层孔隙分为大孔(＞50 nm)和中孔(50~6 nm)；Ⅱ、Ⅲ类储层据拐点分为中孔A段(50~20 nm)和中孔B段(20~6 nm)；平均分形维数依次增大，分别为2.619 3，2.745 4，2.852 6，非均质性逐渐增强。较少的大孔贡献了主要的渗透率，分形维数反映的主要是孔隙大小非均质性。分形维数与部分矿物存在较好的相关性，矿物成分及其含量是决定分形维数大小的内在因素，进而影响储层的质量和孔隙结构特征。 

Analysis of the pore structure of tight sandstone by high-pressure mercury injection combined with fractal theory: A case study of the Heshui area in the Ordos Basin

Pore structure is the key element of tight sandstone reservoirs, which restricts the accumulate and flow of oil and gas in reservoirs. The pore structure is one of the key and difficult point of unconventional oil and gas exploration and development. In this paper, ten dense core samples from the Upper Triassic Yanchang Group 7 section in the Heshui area of the Ordos Basin are selected to carry out experiments, including high-pressure mercury pressure and X-diffraction. Fractal theory is used to analyze the characteristics of pore throats, then the relationship between fractal dimension and reservoir physical properties, pore structure and mineral content is analyzed. The results show that according to the mercury pressure curve and pore structure, the reservoir is divided into categories Ⅰ, Ⅱ and Ⅲ, its reservoir performance and seepage capacity decrease in turn, and the microheterogeneity is enhanced. Different types of reservoirs have different fractal characteristics: according to the inflection points of fractal curves 0.05 and 0.02 μm. The pore size of class Ⅰ reservoirs is divided into large pores (> 50 nm) and medium pores (50-6 nm) by combining the actual situation with the standard of pore size division proposed by IUPAC. According to the inflection point, the reservoirs of classes Ⅱ and Ⅲ can be divided into medium pores a (50-20 nm) and mesoporous B (20-6 nm). The average fractal dimension increased, which was 2.619 3, 2.745 4 and 2.852 6, respectively, and the heterogeneity gradually increased. The main permeability is contributed by fewer large pores, and the fractal dimension mainly reflects the heterogeneity of pore size. The fractal dimension is related to some minerals. Mineral composition and content are the internal factors that determine the fractal dimension and then affect the quality and pore structure of reservoirs.