Pore structure evolution of hydration damage of mudstone based on NMR technology

Experimental study on the hydration damage of rocks has been a hot topic of many scholars. It is common to study sedimentary rocks with high strength and large porosity, but the hydration damage of rocks with low strength and porosity needs to be supplemented. Based on this phenomenon, the authors took the mudstone as the experiment and research object and configured the chemical solution of different components, concentrations and pH values. A series of immersion experiments were carried out on the mudstones. Based on nuclear magnetic resonance (NMR) technique, the transverse relaxation time T2 spectrum distribution of mudstones at different soaking times was obtained, and the mass, porosity and pH value of mudstone samples were measured in different times. The results show that ① the mass, water absorption and porosity of each sample increase with the soaking time under the water-rock chemical interaction, so there is a good consistency. However, the increase range and rate of parameters in different chemical solutions are different. ② The results of NMR show that there are two peaks in the transverse relaxation time T2 spectrum distribution of mudstone under different chemical solutions. The first peak (representing the number of small pore) is obviously higher than the second peak (the number of large pore). Thus the main reason for the hydration damage of mudstone is the increase of the number of small pores. In addition, the damage degree of mudstone samples soaked in different aqueous chemical solutions is significantly different. In 0.01 mol/L NaCl solution, the degree of damage is: pH=2>pH=12>pH=7; under the condition of pH=7, there are distilled water>0.1 mol/L NaCl>0.01 mol/L NaCl. ③ In NaCl solution with pH=2, the hydration damage of mudstone is the most severe. With the increase of immersion time, the two peaks in the NMR T2 spectrum distribution curve both increase and shift to the right, reflecting the evolution characteristics of the pore structure during the hydration damage process of mudstone. The pore evolution process can be summarized as three stages: crack initiation stage, extended connection stage and crack penetration stage. ④ The measurement results of pH values of different aqueous chemical solutions show that with the extension of immersion time, the pH value increases gradually under acidic conditions and decreases under basic and neutral conditions, but the acid-alkalinity of different solutions tends to be neutral with the prolongation of time, the water-rock chemical interaction has self-balance. ⑤ According to the changes of mineral composition of rock sample and pH value of solution, the microscopic mechanism of hydration damage of mudstone in different chemical solutions is given. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.