Effects of Crude Oil Intrusion on Mechanical Damage Characteristics and Acoustic Emission Behavior of Coal

The weakening of coal strength caused by crude oil intrusion serves as a primary trigger for frequent disasters in coal–oil symbiotic mines. Understanding the degradation characteristics and underlying mechanisms of crude oil intrusion into coal is crucial for safe mining operations. This study examined the uniaxial strength of coal following varying periods of crude oil saturation. Comparative analyses were performed on the evolution of compressive strength, elastic modulus, strain, and damage coefficient. By employing acoustic emission monitoring, we investigated crack propagation patterns and failure mode transitions, thereby elucidating the degradation mechanism of crude oil–intruded coal. The results demonstrated that with increasing crude oil intrusion duration, strength and elastic modulus progressively decreased whereas strain and damage coefficient exhibit gradual increased. Concurrently, the compaction stage (OA), yield stage (BC), and postpeak failure stage (CD) became prolonged, whereas the elastic stage (AB) shortened. Notably distinct postfailure stress drop patterns were observed: original coal exhibited a direct stress drop; coal samples intruded for 7 and 14 days displayed single-step stress drops; and those exposed for 30 and 60 days manifested multistep stress drop characteristics. The cumulative AE count exhibited an exponential functional relationship with crude oil intrusion duration. Compared with original coal samples, those subjected to crude oil intrusion for 7, 14, 30, and 60 days demonstrated reductions in cumulative AE counts of 43.25%, 52.46%, 73.83%, and 77.91%, respectively. As the coal intrusion duration increases, the failure mode of the coal become increasingly complex, with crack types transitioning from shear-dominated to tensile-dominated. The ratios of tensile cracks for invasion durations of 0, 7, 14, 30, and 60 days were 12.87%, 69.63%, 77.03%, 80.74%, and 84.36%, respectively. Macroscopically, this evolution was characterized by a transition from shear failure to tensile-shear composite failure. As the coal intrusion duration increases, the fractal dimension of the fragmented coal gradually decreases, the mass fraction of larger-sized fragments progressively declines, and the mass proportions of fragments in other size ranges generally increase. These findings provide valuable insights for the stability assessment in coal–oil symbiotic mining systems.