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Shale reservoirs have attracted increasing attention in recent years as a new and promising geological repository for carbon dioxide (CO 2 ) storage. However, their long-term storage mechanism is unclear and the interactions between CO 2 adsorption to shale and the CO 2 -water-rock reactions have not been investigated. A 2-D simplified reactive transport model is developed in this paper based on the properties of a shale reservoir in the Ordos basin in China which has been identified as a promising location for shale gas production and CO 2 storage. The model is used to study the CO 2 -water-rock reactions coupled with gas adsorption and to identify the long-term trapping mechanisms of CO 2 in shale gas. Results show that in short (several decades) to middle term (hundreds of years) CO 2 storage in a shale reservoir, > 60% CO 2 is trapped in the supercritical and adsorbed phases, but in the long term (thousands of years), mineral trapping gradually plays a dominant role among other storage mechanisms. The displacement of CH 4 with CO 2 during injection is also analyzed. Results show that this process can be divided into two stages: the pre-displacement stage during which the adsorbed concentrations of both CH 4 and CO 2 increase due to pressure build-up, but due to low concentration of CO 2 displacement does not occur at this stage; and the displacement stage during which the CO 2 concentration is high enough to displace the CH 4 from the host rock. Also, higher pressure facilitates the displacement of CH 4 by CO 2 in this stage. Study of the interaction between gas adsorption and CO 2 -water-rock reactions shows that by buffering the reservoir pressure during CO 2 injection into the subsurface, gas adsorption can impact the solubility trapping process so as to interfere with the CO 2 -water-rock reactions. Furthermore, this interference is intensified with reduction in CO 2 concentration in the reservoir over time. Sensitivity analysis indicates that variations in the CO 2 adsorption isotherm, abundance of smectite and chlorite, reaction rate and reactive surface area of smectite can significantly affect the CO 2 storage by different trapping mechanisms. The high CO 2 adsorption capacity of shale is very beneficial for CO 2 injection into a shale reservoir; the amount of adsorbed CO 2 is proportional to the CO 2 adsorption isotherm, while CO 2 in other phases (gas/supercritical phase, dissolved phase and mineral phase) is inversely related to the CO 2 adsorption isotherm. Smectite and chlorite are the key minerals that influence the CO 2 -water-rock reactions in Yanchang shale. [ABSTRACT FROM AUTHOR]