Creep Behaviors in Red Sandstone Subjected to Different Deviatoric Stresses under Thermohydromechanical Conditions Using Ultrasonic Technology and Nuclear Magnetic Resonance Technology

A series of triaxial creep tests were carried out on red sandstone under the conditions of high temperature, high pore pressure, and high confining pressure, and the creep mechanism was analyzed in terms of the velocity and energy variations in the ultrasonic waves emitted during the creep test. Changes in porosity and pore distribution in the samples were investigated by nuclear magnetic resonance (NMR) for different creep stages, and the evolution of the internal microstructure of the samples was examined. The experimental results showed that shear microcracks were mainly initiated during the primary creep stage, these microcracks coalescing to form macroshear cracks in the accelerated creep stage. In addition, the changes in porosity exponentially increased with increased deviatoric stress, with small pores developing into medium pores, while the number of large pores was basically stable.