## 3 Results
### Image Processing of CT Images without Confining Stress
From Figure 11, the fact of different image processing methods resulting in different CT image identification results is confirmed. Those intercepted CT images are located on the pixel number positions that range from 330-358, 454-528, to 588-626 in each CT image in the \(x\)-direction (i.e., original CT image, RG segmentation image, ED segmentation image), as presented in Figure 12a. Especially, the CT image difference is primely presented in the pixel number range from 454 to 528, and the original CT image shows that the low-density position exists in the range of 454-528. It is further illustrated through the difference of yellow profile line 514 in Figure 12b. The continuity of the rock voxel and the aperture closure in this position are supposed. This low-density position is segmented as an aperture open under the RG method. In contrast, the boundary of this low-density position is identified as a rock continuity using the ED method. Moreover, from the comparative result of line 514, it can be seen that in the distance of 70 to 80 in the original CT image in Figure 12c, the result shows that the CT gray value is relatively lower. It may correspond to the aperture open position. Compared with the gray value of the original CT image, both of the RG and ED images show that the gray values in the distance of 70 to 80 are equal to 0. Therefore, the aperture open position is verified. The RG method shows that the gray value position is relatively larger. However, the gray value under the ED method corresponds well with that in the original CT image. From observing 0 MPa Slice A, it is noted that different image processing methods are a main reason for influencing the calculation result, and the aperture value difference may easily occur on the aperture closed position. To further verify the image processing and the aperture difference, CT image analysis is separated with two steps: Firstly, several CT image slices are extracted and checked from different positions in rock sample CT images without confining stress. Secondly, the aperture closure influence under confining stress is also studied. In order to further demonstrate the image processing difference, the CT images of Slice B-E at 0 MPa are extracted and analyzed. The image segmentation results of each  
Figure 12: Comparison of different CT images in different positions. (**a**) Comparison of different positions in \(x\)-direction (i.e., positions at the pixel number of 330–358, 454–528, 588–626, respectively.) (**b**) Comparison of the profile line in different CT images (i.e., the profile line is picked up from the pixel number equal to 514). (**c**) Comparison of the value of 514. CT image are shown in Figure 13. The aperture boundary is successfully identified under these two methods. Compared with Slice A, fractures of Slice B-E at 0 MPa show that the plural aperture boundaries are recognized in the original CT image, which may result in an obvious image processing difference occurring on the segmented CT images. Moreover, the CT image shows that the fracture aperture boundary at different positions often has complicated shapes. The purpose of image processing is to auto-trace the aperture boundary, and the most difficult target is the aperture boundary extraction. The CT image segmentation algorithms generally depend on the CT gray values, and different gray values are used to judge the discontinuity and the similarity in a region. The image processing goal is to partition different regions in the CT image depending on the gray value abrupt variation. Results show that a bit difference occurs on each CT slice. Although the profile continuity under two methods is confirmed at each slice, several image identification differences also appear. The complicated shape of the aperture boundary leads to different image identification, especially on a relative aperture closed position, as presented in the comparative result of Slice D and Slice E in Figure 13. A relative aperture closed position is a difficult target on the boundary extraction.