Pore structure, fractal characteristics and permeability prediction of tight sandstones: A case study from Yanchang Formation, Ordos Basin, China

Abstract

Many fractal models have been proposed based on mercury intrusion porosimetry (MIP) for investigating the fractal property of porous materials. However, results calculated from different fractal models may be not always consistent according to previous works. In view of this, a combination of techniques including routine core analysis, MIP analysis and images analysis (thin sections and BSE) was used to study the pore structure and petrophysical properties of tight sandstones. The results of pore structure and petrophysical properties were used to check whether the fractal dimension values from five popular MIP fractal models can represent the heterogeneity of tight sandstones.It turned out that different MIP fractal models usually give different results. A single fractal dimension value representative of the entire range of pore sizes can be obtained by either Zhang and Li's model or Shen and Li's model. As fractal dimension increases, pore structure becomes more complex and permeability decreases. However, permeability or pore structure shows no significant correlation with fractal dimensions obtained from other models (Neimark's, Li's and Friensen and Mikula's model) by zone analysis. Nevertheless, two fractal regions can be identified by Neimark's or Li's model and clear inflection points could be identified on the fractal curves. The inflection points are found to be almost equal to the apex determined by Pittman's method, and could be regarded as good indicators for permeability prediction. The results obtained from Friensen and Mikula's model is more complex, and not very satisfactory to describe the heterogeneity for tight sandstones.