## 3 Mechanical Sealing and Thermal Expansion Sealing Mechanism  
### Mechanical Sealing Performance  
(1) Compaction response  
The proposed sealing should be pre-achieved by compressing the sealing groups. To study its compressive sealing performance, the compressive loading response and deformation of sealings were directly tested using an electronic servo control testing machine (model MTS E45 (MTS Systems Corporation, Eden Prairie, MN, USA)) with a compressive displacement rate of 0.2 mm/min (Figure 3a). Two sealing samples (coated with lubricant and without lubricant) were tested to compare the friction effect of interlock surfaces of sealing components.  
Figure 2: Sealing structures and sealed rocks. (**a**) Sealing structure and processes; (**b**) sealing in rocks.  
The compression loading response of the seal shows an overall upward trend with increasing compressive displacement (Figure 3b). Generally, three stages can be determined. First, the compression load increases sharply with displacement from 0 to 0.6 mm due to the initial contact friction during sealing-surface interlocking. As the compressive displacement increases, the compression load grows steadily, despite the loading fluctuation caused by the interlocking or biting of sealing edges. The steady increase in the loading stage ceases when the compression displacement reaches about 6 mm. Therefore, the compression displacement should be maintained at 2-6 mm during sealing. Finally, as the upper part of the seal is fully nested into the lower part, the compression load-displacement curve rises sharply due to the load-bearing capacity of the sealing components as a whole. Sealing will fail if the sealing processes are not achieved before the third compressive stage, and the dimensions of the borehole and sealings should be carefully redesigned. For seals with lubricant, the loading-displacement curve is lower than that of seals without surface treatment.  
(2) Deformation of copper sealing  
The deformations of sealings are shown in Figures 3 and 4. The initial dimensions of the coated sealing were \(\varphi 18\times 21\) mm. After the sample was compressed to 17.7 mm (a compression ratio of 15.7%), the sample was laterally expanded by 1.8 mm. With a completely nested displacement of 15 mm (a compression ratio of 28.6%), a lateral expansion of 3.0 mm was achieved. Therefore, the gap between the borehole and the outer surface of the seals should be smaller than 3.0 mm and a gap of about 1.8 mm is suggested.  
Figure 3: Test setups and materials. (**a**) Test setups; (**b**) load versus displacement curves of copper sealings; (**c**) deformation of sealing components. I: Intact; II: Compacted with a small loading; III: Compacted with a higher loading.  
The injection pipe is located in the borehole and is locked by the inner hole surface of the seal. When the seal was compressed to 17.7 mm, the inner diameter of the upper seal component was reduced from 12 mm to 10 mm (Figure 3c). Therefore, the gap between the injection pipe and the inner surface of the seals should be less than 2.0 mm; specifically, after the installation of the injection pipe, the internal deformation of the seal is limited. Due to the thermal expansion of the injection pipe, sealings and rocks would be further sealed.