Study on the Evolution Law of Pore Structure and Mechanical Properties of Mudstone Under the Effect of Water-Rock Chemistry

Abstract

The influence of different pH and concentration of water chemical solutions on the mechanical properties of mudstone is studied. The triaxial compression test and nuclear magnetic resonance (NMR) test of mudstone are carried out, and the root cause of the deterioration of mechanical properties of mudstone during the hydration damage process is explored, combining macroscopic and microscopic. The results show that hydrolysis and chemical ion exchange are responsible for the deterioration of mudstone shear strength parameters. The essence of mudstone hydration damage is that water chemistry changes the original pore space structure and pore size distribution inside. Part of the minerals dissolve and fall off under the influence of the water chemical solution, which expands the pore size and increases the porosity. The connection force between mineral particles decreases, macroscopically, the mudstone undergoes a softening evolution from brittleness to ductility, and the shear strength parameters deteriorate. The results show that mudstone is more sensitive to acidic solution than neutral and alkaline solution. The higher the pH of the alkaline solution or the higher the concentration of the neutral solution, the more significant the deterioration of mudstone. Under the influence of hydrochemical solution, the fractal dimension of mudstone gradually tends to 2 from 3, which indicates that the process of hydration damage reduces the complexity of mudstone pore structure. On this basis, the damage variable D<sub>n</sub> defined based on the change of porosity before and after hydration damage of mudstone, to a certain extent, quantitatively reflects the law of mudstone shear strength parameters accompanied by microscopic pore structure evolution. The shear strength parameters of mudstone decrease significantly with the increase of damage variable D<sub>n</sub>. The test analyzes the root cause of the deterioration of the macroscopic mechanical properties of mudstone during the hydration damage process from a microscopic point of view, and the conclusions obtained provide a good reference for the quantitative study of the effect of water chemistry on the mechanical properties of mudstone.

Study of the Influence of Damage Structures in Coal Seam Floors on the Damage of Small Hidden Faults

“…The schematic diagram of the water breakout of the small hidden faults is shown in Figure 4. The rift ends of small hidden faults in the rock system are very prone to stress concentration [30][31][32]. The extended damage and the fracture extension zone of the small hidden fault connects to the damaged structure in the rock below, which in turn connects to the damaged zone of the coal seam floor and the zone of the pressurized water conductivity, and causes uplift.…”

“…In this section, we reproduce the whole process of a water breakout in the coal seam floor by using the numerical simulation method of FLAC to study the process of small hidden faults and investigate the influence of the damage structure on the water damage law of the small hidden faults. The rift ends of small hidden faults in the rock system are very prone to stress concentration [30][31][32]. The extended damage and the fracture extension zone of the small hidden fault connects to the damaged structure in the rock below, which in turn connects to the damaged zone of the coal seam floor and the zone of the pressurized water conductivity, and causes uplift.…”