Long-term variations of CO2 trapped in different mechanisms in deep saline formations: A case study of the Songliao Basin, China

The geological storage of CO2 in deep saline formations is increasing seen as a viable strategy to reduce the release of greenhouse gases to the atmosphere. There are numerous sedimentary basins in China, in which a number of suitable CO2 geologic reservoirs are potentially available. To identify the multi-phase processes, geochemical changes and mineral alteration, and CO2 trapping mechanisms after CO2 injection, reactive geochemical transport simulations using a simple 2D model were performed. Mineralogical composition and water chemistry from a deep saline formation of Songliao Basin were used. Results indicate that different storage forms of CO2 vary with time. In the CO2 injection period, a large amount of CO2 remains as a free supercritical phase (gas trapping), and the amount dissolved in the formation water (solubility trapping) gradually increases. Later, gas trapping decrease, solubility trapping increases significantly due to the migration and diffusion of CO2 plume and the convective mixing between CO2-saturated water and unsaturated water, and the amount trapped by carbonate minerals increases gradually with time. The residual CO2 gas keeps dissolving into groundwater and precipitating carbonate minerals. For the Songliao Basin sandstone, variations in the reaction rate and abundance of chlorite, and plagioclase composition affect significantly the estimates of mineral alteration and CO2 storage in different trapping mechanisms. The effect of vertical permeability and residual gas saturation on the overall storage is smaller compared to the geochemical factors. However, they can affect the spatial distribution of the injected CO2 in the formations. The CO2 mineral trapping capacity could be in the order of 10 kg/m3 medium for the Songliao Basin sandstone, and may be higher depending on the composition of primary aluminosilicate minerals especially the content of Ca, Mg, and Fe.