Characterization of pore structure and the impact of key pore nodes on seepage in coral reef limestones

Abstract

To accurately characterize the intrinsic relationship between pore structure and seepage behavior in coral reef limestone (CRL), this study investigates four types of CRL with varying degrees of cementation. We employed CT scanning, three-dimensional (3D) reconstruction techniques, and equivalent pore network modelling to statistically analyze pore characteristic parameters. Pore network modelling (PNM) was used to conduct a network analysis of the internal pore structure of reef limestone, identifying key pore nodes that govern seepage characteristics. The findings reveal that CRL exhibits high porosity and significant non-homogeneity, with pore radius, throat size, and coordination number conforming to a log-normal distribution. CRL samples exhibiting low degrees of cementation contain extensive interconnected pore clusters, which are characterised by short average percolation paths, high clustering coefficients, and robust connectivity. In contrast, increased cementation significantly diminishes pore connectivity. Topological analysis has revealed pore nodes with exceptionally high values of degree centrality (DC), betweenness centrality (BC), and closeness centrality (CC). Additionally, a method for identifying critical pore nodes has been proposed, based on betweenness centrality and global efficiency within the context of network analysis. Blocking critical pore nodes leads to a substantial reduction in permeability, with the maximum decrease reaching 98.88 %. A comparative analysis between simulation and experimental results indicates that the porosity error ranges from −4.94 % to 1.33 %, while the relative error in permeability falls between 1.15 % and 4.44 %. This validates the accuracy and reliability of the constructed PNM and network analysis methods in characterizing pore structures and simulating seepage. This study not only demonstrates the applicability of the proposed approach for the quantitative characterization and response analysis of seepage behavior at the pore scale but also reveals the regulatory mechanism by which the heterogeneous microscopic pore structures of CRL influence macroscopic seepage behavior.