## 4 Effect of Oil-Water Infiltration on Rock Mechanical Properties
### Stress-Strain Curves of Damaged Specimens with Different Liquid Contents  
The uniaxial compression stress-strain curves of damaged specimens with different soaking times are shown in Figure 9, with the stress-strain curve of the intact dried specimen (black) included for comparison. First, the mechanical differences between the intact dried specimen and the damaged dried specimen were analyzed. The red curve in the figure represents the uniaxial compression curve of the damaged dried specimen. Compared to the intact dried specimen, the strain required during the compression phase before the peak is noticeably higher. The intact dried specimen ends the compaction phase at a strain of 0.37%, whereas the damaged dried specimen ends at 0.49%. The peak strength also decreases from 53.1 MPa to 50.4 MPa, a reduction of 5.1%. The post-peak stress curves of both specimens drop nearly vertically, but the damaged dried specimen exhibits more pronounced plastic characteristics, with a residual strength of 35.8 MPa, lower than the 43.6 MPa of the intact dried specimen. These results indicate that the loading and unloading cycles alter the internal structure of the rock and weaken its mechanical properties. Under  
Figure 8: Uniaxial compressive strength of complete rock sample.  
stress, new fractures develop inside the intact rock, and the presence of these cracks reduces the compressive strength, residual strength, and other mechanical properties, which is unfavorable for the stability of oil storage spaces.  
Regarding the impact of soaking time on the mechanical properties of damaged rock, Figure 9 shows that before failure, the stress-strain curves of all specimens exhibit similar trends. The curves of the samples were different after failure. Specimens soaked for less than 8 h exhibit a nearly vertical stress drop upon failure, indicating a distinct brittle failure mode. Specimens soaked for 16 h and 24 h show a gradual post-peak stress decline and exhibit significant residual strength, reflecting pronounced plastic characteristics. This indicates that an increase in soaking time enhances the plastic behavior of the rock specimens.  
Each group of tests was carried out three times, and the analysis diagram with an error bar was drawn. The following analysis is based on the error bar base point data. As depicted in Figure 10a, the compressive strength of damaged specimens decreases with increasing soaking time, exhibiting a negative correlation. From the dry state to a soaking time of 24 h, the compressive strength of damaged specimens decreases from 50.4 MPa to 39.8 MPa, representing a reduction of 21%. This indicates that oil-water soaking significantly weakens the strength of the specimens. Figure 10b illustrates that the peak strain of the compressive strength follows a parabolic trend, initially decreasing and then increasing. According to the fitted curve, the peak strain of the compressive strength reaches its minimum at approximately 16 h of soaking. As the soaking time continues to increase, the peak strain gradually rises. From the dry state to 24 h of soaking, the peak strain decreases from 0.94 to 0.63, before increasing to 0.7, showing a downward trend with a decrease of 25.5%.  
Figure 9: Uniaxial compressive stress–strain curves of damaged specimens with different soaking times.