Effects of micropores on geometric, topological and transport properties of pore systems for low-permeability porous media

Abstract

Pore systems become very complex in heterogeneous reservoirs such as low-permeability and tight sandstones for macropores and micropores both exist in such reservoirs. Although the large pores may control the reservoir quality and rock properties, micropores always can affect the geometric and transport properties of the pore systems. In this paper, thus, such an effect is studied comprehensively. As such, low- and high-resolution computed tomography (CT)scanning experiments were carried out on the same low-permeability sandstone in order to obtain the low- and high-resolution digital rock (LRDR and HRDR)images, respectively. The impacts of micropores on properties of pore systems were investigated based on digital rock analysis of the LRDR and HRDR images for two samples. In this study, the micro-porosity is defined as the pores and throats which can be covered by the HRDR images but cannot be covered by the LRDR images. The results show that there are better connectivity, smaller tortuosity and higher heterogeneity for the HRDR image, compared to the LRDR image of the same rock. The fractal analysis of pore space indicates that the surface of the pore space with micropores becomes rougher. Moreover, we found that the micropore space plays a more significant role in the throats than the pores by comparing the pore and throat size distributions of the LRDR and HRDR images. In addition, the micropores can lead to a decrease in the average shape factor and smoothness of the boundary of pores and throats. It was also found that micro-porosity not only increases the average coordination number but also increases the number of isolated pores according to the coordination number distributions. Finally, the results of single- and two-phase flow simulation show that the absolute permeability and formation factor become larger and smaller, respectively, for the pore systems with micropores. This result reveals that micropores can improve the transportability and connectivity of the pore systems.