致密气储层孔喉分形特征及其与渗流的关系——以鄂尔多斯盆地下石盒子组盒8段为例

Fractal Characteristics of Pore-throat of Tight Gas Reservoirs and Its Relation with Percolation:A case from He 8 Member of the Permian Xiashihezi Formation in Ordos Basin

选取鄂尔多斯盆地盒8段16块致密砂岩样品进行恒速压汞测试,结合同位样品核磁共振实验,分析了致密气储层孔喉分布特征;在此基础上,运用分形几何原理和方法,开展了致密气储层孔喉分形研究,并表征了分形与储层渗流特征和孔隙结构参数的关系。结果表明:致密气储层有效孔隙被亚微米—微米级孔喉所控制,其中孔隙主要为大孔和中孔,喉道由微喉道、微细喉道和细喉道所组成;致密气储层孔隙分布不具分形特征,而孔喉整体和喉道则符合分形结构,且分别对应分形维数D1和D2;基于储层孔喉分形结构与其渗流特征,将盒8段致密气储层孔喉分形结构划分为2种类型:Ⅰ型表现为阶段式分形特征,以进汞压力1 MPa为界,大于1 MPa孔喉具有分形特征,且储层阶段进汞饱和度主要由喉道贡献,反之,孔喉不符合分形特征,其进汞饱和度增量由孔隙贡献;Ⅱ型为整体式分形,进汞饱和度几乎全由喉道贡献。储层孔喉分形维数与渗透率、平均喉道半径和主流喉道半径存在较好的负相关性,与微观非均质系数呈现较明显的正相关性,而与孔隙度、平均孔隙半径和平均孔喉半径比之间没有明显的相关性。

Constant-rate mercury intrusion （CRMI） was performed on 16 samples from the He8 Member tight sand-stone of the Permian Xiashihezi Formation in Ordos Basin, and combined with the NMR experiment, the pore （pore and throat） structure features were investigated finely. Based on the research of the pore structure features, fractal characteristics of pore （ pore and throat） in tight sandstone reservoir were carried out, and the relationship between pore fractal and percolation characteristics and pore structure parameters were quantitatively characterized. The results show that the effective pores and throats of tight gas reservoir are controlled by submicron and micron scale pore- throats. The pores are mainly composed of macrospores and mesopores, and the throats consist of micro throats, micro-fine throats and fine throats. In the light of effective pores and throats recognized by CRMI and NMR, it is suggested that both pore-throat and throat distribution conform to the fractural structure with dimension value D1 and D2, respec-tively, while pore distribution does not. Based on the characteristics of pore fractal structure and percolation, pore fractal structures of He 8 Member tight gas reservoir are divided into two categories ： type Ⅰ has the staged fractal char-acteristic , i . e., there is a clear inflection point with about 1 MPa pressure of mercury injection （ MIP）. When the MIP is greater than 1 MPa, the reservoir pore-throat has fractal characteristic and the throats contribute to mercury satura-tion increment （MSI）. On the contrary, the MSI is almost entirely contributed by the pores. Type Ⅱ is the integral fractal, and the MSI is almost all contributed by the throats. The fractal dimensions of tight gas reservoir pore which has a certain correlation with pore micro geometrical parameters where as its relationship with permeability, average throat radius and mainstream throat radius are obvious negative correlation. The fractal dimensions show a positive lin-ear correlation with micro heterogeneity coefficient, while no direct or weak relationships with porosity, average pore radius and average pore-throat radius ratio.