Investigations into Variations in Meso- and Macro-Physicomechanical Properties of Black Sandstone under High-Temperature Conditions Based on Nuclear Magnetic Resonance

The high-temperature environment has a significant deterioration effect on the meso- and macro-physicomechanical properties of rocks. Black sandstone in Chuxiong Yunnan was chosen as the test sample to perform a high-temperature test, a nuclear magnetic resonance (NMR) test, and a uniaxial compressive test (UCT). After high-temperature heating (25 degrees C-1,200 degrees C), the thermally damaged samples were subjected to NMR tests in both saturated and centrifugal states. The transverse relaxation time T-2 spectrum distribution and T-2 spectrum area were subsequently obtained. The temperature effect of the mesopore structure was investigated by analyzing the changes in porosity. The T-2 spectrum cutoff value was determined, and the temperature effect of the movable fluid migration law for rocks was evaluated. The thermal damage constitutive relationship with porosity as the damage variable was determined, and the evolution law of mesopore structure parameters and macroscopic mechanical parameters was discussed. UCT results showed that the mechanical characteristics of black sandstone were obviously degraded by high temperature, and especially the uniaxial compressive strength (UCS) decreased sharply after the temperature exceeded 900 degrees C. NMR results identified 900 degrees C as an inflection point for the mesostructure change. For temperatures lower than 900 degrees C, the improvement of pore connectivity was not obvious. For temperatures exceeding 900 degrees C, the porosity increased rapidly. The porosity had a great influence on UCS, and there was a clear exponential function relationship between the two. The larger the porosity, the smaller the UCS. It showed that it was an effective angle to study the mesopore characteristics and mechanical properties of high-temperature rocks based on the NMR technique.