Understanding the near-field characteristics of leakage is essential for safety distance calculations and risk assessment of emergency response to a pipeline leakage. This paper presents a small-scale experiment and computational fluid dynamics model designed to investigate the transient characteristics of near-field parameters. The study also analyzes the effects of leakage orifice diameter on the transient characteristics of the near-field. The results show that a pipeline leak starts with the formation of a compression wave, which causes the air at the front of the stream to be compressed, heated and pressurized. CO2 is then released at high pressure, creating a strong expansion wave that keeps the fluid pressure below local atmospheric levels. For full-hole and large-hole leaks, the temperature drop and rate show a negative linear correlation with the size of the leak hole. Conversely, for small hole leaks with the ratio of hole diameter to pipe diameter values below 0.2, the temperature drop shows an opposite trend. As the ratio of hole diameter to pipe diameter values increases, the maximum diameter of the barrel drum-shaped disk gradually increases, but the maximum expansion angle of the jet decreases. It is hoped that this work will contribute to the improvement of research models that assess the consequences of potential high-pressure pipeline rupture scenarios.