Pore-structure and damage behaviors of cement stone subjected to dry‒wet cycles

Abstract

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 °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 °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.

Keywords

• Cement stone
• Dry-water cycle
• Porosity
• Wave velocity
• Fatigue damage