## 4 Effect of Oil-Water Infiltration on Rock Mechanical Properties
### AE Characteristics of Damaged Rock Samples with Different Liquid Contents  
During the uniaxial compression test, various internal events occur within the rock specimens, including the compaction of pre-existing cracks, frictional slip between cracks, and the initiation and propagation of new cracks. Each of these events generates AE signals. The variations in stress, AE ringing count rate, and cumulative ringing count over time for damaged rock specimens subjected to different soaking times in uniaxial compression tests are shown in Figure 11.  
Figure 11: Stress, AE ringing count rate, and cumulative ringing count variation curves over time for damaged rock specimens subjected to different soaking times: (**a**) 0 h, (**b**) 8 h, (**c**) 16 h, (**d**) 24 h.  
Figure 10: (**a**) Relationship between compressive strength and soaking time, (**b**) relationship between peak strain and soaking time. From the stress-time curves in Figure 11, it can be observed that the stress change of each rock sample can be divided into four stages: compaction stage (I), elastic stage (II), yielding stage (III), and failure stage (IV). These stages are consistent with the conventional classification of uniaxial compression curves for rocks. However, due to the varying soaking times of the damaged rock samples, the AE characteristics observed during each stage are different. The following is a comprehensive analysis of the AE characteristics of the rock samples at each stage, based on the evolution of stress, AE ringing count rate, and cumulative ringing count. (1) Compaction stage (I): In this stage, the stress-time curve exhibits a concave shape, with the growth rate increasing progressively. Under the stress, the pre-existing microcracks in the damaged rock specimens become compacted. The AE ringing count rate and cumulative ringing count in dry specimens are significantly higher than those in soaked specimens. Additionally, as the oil-water soaking time increases, the proportion of time spent in this stage increases to 26%, 32%, 43%, and 40%, respectively, with a maximum increase of 65.4%. This shows that under the erosive effects of oil-water soaking, the number of cracks and pores in damaged rock specimens increases, requiring more time to compact the internal voids. Due to the presence of extensive cracks and pores, its ability to resist external forces is weakened, so the internal structure is more likely to be damaged and compacted without obvious acoustic emission signals. (2) Elastic stage (II): In this stage, stress increases linearly over time, while the AE ringing count rate remains stable and the cumulative ringing count grows linearly. As the oil-water soaking time increases, both the frequency and amplitude of the AE ringing count rate gradually decrease, and the proportion of time spent in this stage also decreases, with values of 57%, 39%, 43%, and 29%, respectively. In dry specimens and those soaked for 8 h, a slight increase in the AE ringing count rate is observed upon transitioning from the compaction to the elastic stage, accompanied by a distinct inflection point in the cumulative ringing count curve. However, this phenomenon is absent in specimens soaked for 16 and 24 h. These characteristics indicate that the crack propagation in rock specimens during the elastic stage is relatively stable, but the elastic properties of the rock gradually weaken under the erosive effects of oil-water soaking. (3) Yield stage (III): In this stage, the growth rate of the stress curve decreases, while the cumulative ringing count curve exhibits an accelerated increase, indicating a significant rise in both the number and propagation rate of internal cracks. As the soaking time increases, the duration of the yield stage also extends, suggesting that the specimens increasingly exhibit plastic deformation characteristics. (4) Failure stage (IV): At the moment of failure, the cumulative ringing count curve exhibits an almost 90\({}^{\circ}\) increase, while the AE ringing count rate surges sharply near the stress peak.