The stable protection of the walls of high-temperature geothermal wells is a challenging issue for sustainably exploiting geothermal resources. However, the cement stone filling layer of the cemented portion of the well deteriorates gradually during geothermal mining due to the dry-wet cycles of the saline geothermal water, reducing the service life of the geothermal well. For this, this paper presented five groups of cement stone cylinders with salt contents of 0%, 1%, 6%, and 11%, which were subjected to heating to 300 degrees C and 1-5 dry-wet cycles. Nuclear magnetic resonance (NMR) and nonmetallic detection were used to test and analyze the porosity and wave velocity. Additionally, the damage evolution induced by dry-wet cycles was captured based on acoustic emission (AE) data. The experimental results indicated that the heating process primarily resulted in mineral and salt crystal expansion, which in turn caused damage. The damage threshold due to the salt content was found to be 6%. The sudden increase in the thermal stress caused by cooling and deterioration of the tensile strength of the cement column were the key factors in the damage during the cooling process. As the number of cycles increased, the accumulated AE energy moved forward and backward, with decreasing and increasing temperature, respectively. The threshold of signal mutation in the heating process is 200 degrees C, and the accumulated AE energy decreases by 11.7%. When the salt content was 0%, 1%, 6% and 11%, the wave velocity decreased by 19%, 27.3%, 35.5% and 35.9%, respectively. This study also proposed a damage model, which could provide theoretical support for long-term health monitoring and safety protection of geothermal wells. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The strain paths of cement stone in the deviatoric and meridian planes under the constant Lode angle loading path (true triaxial stress state) are analyzed. The amount of volumetric and shear strains first increases and then decreases with the intermediate principal stress coefficient. Owing to the generation of plastic volumetric strain and plastic shear strain in the direction of deviatoric stress, the strain path exhibits nonlinearity in the meridian planes. The deviation of the strain path from the constant Lode angle arises from the accumulation of plastic shear strain along the Lode angle direction. In the framework of fractional plasticity, a three-dimensional elastoplastic constitutive model incorporating Lode angle is proposed, including yield function, potential function, and fractional flow rule. The yield surface evolves in both meridian and deviatoric planes, allowing the yield function to precisely characterize the stress state. Since the plus-minus sign in the flow direction of the yield surface is opposite to that in the flow direction of cement stone, a simple elliptic function incorporating Lode angle serves as the potential function. The procedure for the determination of fractional order based on the entirety of the deformation process is proposed, including variable and constant fractional order. The comparison between the experimental result and the analytical solution of constitutive model confirms its accuracy and validity. Furthermore, the difference between variable and constant fractional order on deformation is analyzed. The comparison results indicate that the variable fractional order can provide a more accurate description of deformation than the constant fractional order.