Application of mercury intrusion method and digital image analysis in quantitative analysis of micro-scale pores in tight sandstone reservoirs: a case study of X block in Wuqi Oil Field, Ordos Basin

Abstract

In order to investigate the pore structures of the tight sandstone reservoirs in the 4+5th and 6th members of the Triassic Yanchang Formation (Chang4+5 and Chang6, respectively), and the 9th and 10th members of the Jurassic Yan'an Formation (Yan9 and Yan10, respectively) in the X block of Wuqi Oil Field, Ordos Basin, 12 samples were collected to analyze reservoir properties with the approaches of scanning electron microscope observation, X-ray diffraction and high pressure mercury intrusion. We also quantitatively characterized the pore parameter and fractal dimension of the tight sandstones by the using of digital image analysis and fractal geometry. In addition, we discussed the relationship between fractal dimension and sample properties (porosity, permeability), pore structure parameter (average pore-throat radius, sorting coefficient), pore geometric parameters (dominant pore size, perimeter over area, and pore body-to-throat ratio). The influence of sedimentary facies and diagenetic environment on pore structures were also quantitatively analyzed. Results show that the pore structure fractal dimension ranges from 2.164 to 2.895, with an average value of 2.395. Fractal dimension is negatively correlated to permeability, porosity and average pore-throat radius, and positively related to sorting coefficient. Tight sandstones in the study area generally show properties of low dominant pore size, high perimeter over area, lower body-to-throat ratio, and high dimensions. The fractal dimension is positively related to body-to-throat and perimeter-to-area ratio, and negatively related to pore size. It is indicated that the pore structure of the samples is relatively complex and has strong heterogeneity. Depositional environment affects the compositional maturity and structural maturity of reservoir.

Keywords

• tight sandstone
• pore structure
• fractal dimension
• mercury injection method
• digital image analysis
• ordos basin