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

Key words: 
• pore structure /
• fractal dimension /
• percolation characteristic /
• tight gas reservoir /
• constant-rate mercury intrusion

Abstract: Constant-rate mercury intrusion (CRMI) was performed on 16 samples from the He8 Member tight sandstone 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, respectively, 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:typeⅠhas the staged fractal characteristic, 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 saturation 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 linear correlation with micro heterogeneity coefficient, while no direct or weak relationships with porosity, average pore radius and average pore-throat radius ratio.