The application of conventional acidification techniques confronts significant challenges in ultra-low porosity reservoirs. To resolve this issue, it is essential to develop an alternative acidification method. The present research utilized high-temperature and high-pressure experiments and molecular dynamics simulations to develop a three-dimensional model that describes acid gas diffusion behavior. The model quantifies the diffusion coefficient of acid gases within tight sandstone and analyzes the influence of various parameters such as pore tortuosity, pore geometry, and variations in temperature and pressure conditions on this process. Moreover, diffusion kinetics are explored through mean square displacement analysis, revealing the free diffusion mechanism of acid gas in tight rock porous media. Empirical results indicate that carbon dioxide (CO2) demonstrates the highest molecular diffusion. Meanwhile, simulations show that the combined hydrogen fluoride (HF) and CO2 molecules have the greatest diffusion coefficient. Experimental values were lower than those predicted by simulations. This deviation may be attributed to high tortuosity and complex flow around the core pores compared to the simplified structures used in simulations. The simulation results reveal that the pore tortuosity is the dominant factor influencing diffusion, while the impact of pore diameter effect is relatively negligible.

Conventional reservoir acidification technology is limited by the rheology of acidic fluid, and it is difficult to penetrate deep into tight reservoirs to achieve effective stimulation. In this study, it is proposed to use gas acid to transform tight sandstone reservoirs. By carrying out the dissolution experiment of gas acid on a tight sandstone surface, the dissolution characteristics of rock surface under the action of gas acid are systematically analyzed, the change of wetting behavior after gas acid dissolution is quantitatively characterized, and the dissolution mechanism of gas acid on tight sandstone is revealed. The experimental results show that under the erosion of gas acid, the cemented minerals on the rock surface gradually dissolve, resulting in the increase of surface roughness. Among them, hydrogen fluoride gas (HF) has the most prominent effect on improving rock surface roughness, and the growth rate of surface area can reach 34%. At the same time, the contact angle of the rock surface decreases by 23.47°. The numerical simulation further shows that the concentration of gas acid, initial injection rate, molecular size, and density all have significant effects on the acidification effect. The above research results provide new ideas and theoretical basis for the efficient development of tight reservoirs.