Research and analysis of the impact of the pore structure on the mechanical properties and fracture mechanism of sandstone

Abstract

Rock pore structure, including porosity, pore size, and particle size, plays a crucial role in determining rock mechanical properties. This study investigates the impact of pore structure on rock mechanical properties through laboratory experiments and numerical simulation methods on sandstones with varying porosity and pore size. The pore structure characteristics of sandstone were determined using XRD, SEM, and NMR microscopic techniques. Additionally, the influence of rock porosity on rock mechanical properties was analyzed by combining Uniaxial compression test with DIC technology, revealing the fracture mechanism caused by porosity in rocks. Numerical models were constructed to simulate sandstones with different porosities and pore diameters, followed by uniaxial compression simulation tests to explore the influence of varying pore diameters on rock mechanical properties and the fracture mechanism. The study reveals the presence of four distinct pore types in the sandstone: micropores in the matrix (0–0.002 µm), intergranular pores (0.002–0.2 µm), dissolution pores (0.2–4 µm), and fracture pores (4–30 µm). Among these, intergranular pores and dissolution pores are the predominant types, accounting for approximately 92–96% of the total pore volume in the sandstone. It is observed that an increase in porosity leads to a decrease in the mechanical strength of sandstone and an increase in Poisson's ratio while keeping the pore size constant. Similarly, when the porosity remains constant, an increase in pore size results in a decrease in the mechanical strength and an increase in Poisson's ratio. Additionally, the study also finds that as the porosity and pore diameter of sandstone increase, the area of internal equivalent stress expands, leading to a shorter crack initiation time and accelerated crack penetration speed, ultimately resulting in a decline in the mechanical properties of sandstone. Numerical simulation experiments further demonstrate that large pores exhibit higher stress concentration compared to small pores, thereby exerting a more significant influence on the mechanical properties of sandstone.