Numerical simulation of long-term storage of CO2 in Yanchang shale reservoir of the Ordos basin in China

Abstract

Shale reservoirs have attracted increasing attention in recent years as a new and promising geological repository for carbon dioxide (CO2) storage. However, their long-term storage mechanism is unclear and the interactions between CO2 adsorption to shale and the CO2-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 CO2 storage. The model is used to study the CO2-water-rock reactions coupled with gas adsorption and to identify the long-term trapping mechanisms of CO2 in shale gas. Results show that in short (several decades) to middle term (hundreds of years) CO2 storage in a shale reservoir, >60% CO2 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 CH4 with CO2 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 CH4 and CO2 increase due to pressure build-up, but due to low concentration of CO2 displacement does not occur at this stage; and the displacement stage during which the CO2 concentration is high enough to displace the CH4 from the host rock. Also, higher pressure facilitates the displacement of CH4 by CO2 in this stage. Study of the interaction between gas adsorption and CO2-water-rock reactions shows that by buffering the reservoir pressure during CO2 injection into the subsurface, gas adsorption can impact the solubility trapping process so as to interfere with the CO2-water-rock reactions. Furthermore, this interference is intensified with reduction in CO2 concentration in the reservoir over time. Sensitivity analysis indicates that variations in the CO2 adsorption isotherm, abundance of smectite and chlorite, reaction rate and reactive surface area of smectite can significantly affect the CO2 storage by different trapping mechanisms. The high CO2 adsorption capacity of shale is very beneficial for CO2 injection into a shale reservoir; the amount of adsorbed CO2 is proportional to the CO2 adsorption isotherm, while CO2 in other phases (gas/supercritical phase, dissolved phase and mineral phase) is inversely related to the CO2 adsorption isotherm. Smectite and chlorite are the key minerals that influence the CO2-water-rock reactions in Yanchang shale.