<p>Natural gas pipelines in mountainous areas are particularly vulnerable to mechanical damage because of complex terrain, frequent geological hazards such as landslides and rockfalls, and disturbances from external construction activities. Once leakage occurs, emergency response is more challenging and the potential hazards are more severe. Therefore, investigating the diffusion characteristics of pipeline leakage under such terrain conditions is of great engineering significance and practical urgency. This study investigates the diffusion characteristics of natural gas pipeline leaks across various terrains, focusing on a section of the Sichuan Gas Eastward Transmission Project in a mountainous area. Models for three geographical environments flat ground, hillside, and mountain foot were constructed using Design Modeler. The Detached Eddy Simulation (DES) method was employed to investigate the impact of different wind speeds on leakage dispersion under the condition of small-orifice leakage in natural gas pipelines. The analysis, based on methane gas cloud range and the Ω criterion, reveals distinct diffusion patterns across terrains. In flat terrain at low wind speeds, methane gas clouds within the explosion limit range accumulate extensively at high altitudes due to vortex formation at the gas cloud boundary. Hillside gas jets demonstrate minimal wind speed influence, while at the mountain foot, turbulent vortices entrain leaked gas, causing accumulation. Increasing wind speeds in flat terrain decrease methane gas cloud height but lead to significant near-ground accumulation within the explosion limit range, presenting extreme risks. On hillsides, leaked gas spreads upward along the slope, forming spherical methane clouds near the leakage point. At the mountain foot, higher wind speeds intensify turbulent vortices within the accumulated methane gas cloud, gradually diminishing the concentration gradient until it disappears.</p>