Natural CO2 leakage and responses of shallow aquifers in the southern Xining Basin

Abstract

Objective Geological storage of CO2 is an important technology for reducing CO2 emissions, and the assessment of CO2 leakage risk is the key to its implementation. Research on natural CO2 leakage is an important means to obtain key information for leakage assessment. Methods The source of CO2, the characteristics of CO2 leakage, and the relevant response of shallow aquifers to the CO2 leakage have been analysed through field investigations, on-site measurements, and sampling and testing of water, gas and rock. Results A number of natural CO2 leakages have been discovered in the southern Xining Basin, including CO2-rich springs, CO2-driven cold-water geysers from abandoned wells, and CO2 blowouts, as well as large-scale travertine associated with them. CO2 is the dominant component in the gas phase, and the abundance of carbon isotope of CO2 is between -2.5‰ and -0.4‰, indicating that the leaked CO2 comes from a deep inorganic origin, leaks into shallow confined aquifers through deep faults, and flows and discharges with the groundwater or accumulates secondarily in shallow formations. The concentration of soil 222Rn in areas of CO2 leakage is abnormal (over 9 000 Bq/m3), which can be used as an important method of identifying hidden leakage channels. The groundwater has a pronounced response to CO2 leakage, including a unique phenomenon of intermittent eruptions (eruption for 200 s and incubation for 130 s), changes in groundwater hydrochemical characteristics (e.g., a decrease in pH, an increase in conductance and HCO3- and Ca2+ concentrations, and a drift in oxygen isotopes), and travertine composed mainly of calcite with a bubble structure at the surface. The natural CO2 leakage characteristics at this site are highly similar to those in Utah, USA. Conclusion The results of this study not only provide knowledge of natural analogous for leakage risk assessment of CO2 geological storage, but also contribute to the understanding of geological activity in the deep earth.

Keywords

• natural co2 leakage
• shallow aquifer
• response
• co2 geological storage
• xining basin