Numerical simulation of atmospheric diffusion of CO₂ and optimization of monitoring points in Yanchang Oil Field

Due to the complex local meteorological conditions in Northern Shaanxi, the diffusion direction of CO₂ concentration is uncertain. Additionally, the rugged and undulating topography of the oil field in the loess plateau makes it difficult to arrange CO₂ atmospheric monitoring points. In this paper, Fluent software is used to simulate the diffusion patterns of CO₂ during flooding and storage leakage in different leakage scenarios. The monitoring point layout is optimized, and monitoring practices are conducted. The research results indicate that as wind speed increases, the concentration of the CO₂ cloud decreases longitudinally, but the lateral diffusion distance of the CO₂ cloud shows an initial increase followed by a decrease. When the wind speed is 2.0 m/s, the maximum diffusion distance of the CO₂ cloud is 47 m, the cloud diffusion distance of the largest concentration of the CO₂ cloud is 15 m. With an increasing leakage rate, the concentration of the CO₂ cloud near the ground surface increases both longitudinally and laterally. When the leakage rate is less than 0.4 m/s, the diffusion distance of the CO₂ cloud increases rapidly with increasing leakage rate. When the leakage rate exceeds 0.4 m/s, the rate of increase in the diffusion distance of the CO₂ cloud slows down. Taking into account the simulation results, the meteorological conditions in Northern Shaanxi, the direction of fissure development, the preferred direction of gas injection, and the height of near-surface vegetation, the layout of the monitoring points is determined as follows: a northeast-southwest direction; a downwind distance of 15 m from the wellhead in the horizontal plane; a sampling height of 50 cm in the vertical direction. The monitoring results of CO₂ concentration in the atmosphere indicate that no leakage has occurred.