Identification of CO₂ Leakage in an Active CO₂-EOR Field, Songliao Basin, China

CO2-enhanced oil recovery (CO2-EOR) not only helps to recover additional oil but also sequesters CO2 in the subsurface. However, any leakage of the injected CO2 may place the environment and ecosystems at risk. In this work, we measured the near-surface atmospheric CO2 concentrations (pCO2) and ground CO2 fluxes in and around the S16 block of the Yushulin oil field, Songliao Basin, China, to identify CO2 leakage induced by CO2-EOR activities. The measurements were performed at Type I, II, III, and IV sites, targeting the background area, the oil block, large faults, and the peripheries of injection wells, respectively. The mapped distributions of pCO2 and ground CO2 fluxes exhibit spatially heterogeneous. The peaks on the CO2 flux map are spikier in contrast to those on the pCO2 map, which indicates that the CO2 flux measurements are more powerful to locate leakage sites. Type IV sites have anomalously high CO2 fluxes, suggestive of significant leakage in the peripheries of the injection wells, likely due to well integrity failure and high injection pressure. The CO2 escaped from wellbore flaws could migrate laterally after reaching shallow aquifers, then to soils, and finally to the atmosphere. Hence, elevated CO2 fluxes are observed in some Type II sites away from the wells. The baseline of the CO2 fluxes is 14.8 g m–2 d–1, determined by a combination of a box plot and a probability plot. The calculated leakage rate of the entire block is 35.2 t d–1. Wellbore-peripheric leakage, despite the high CO2 fluxes, is only 0.2 t d–1 due to the limited area. This work provides a practical method for the identification and quantification of CO2 leakage in geological storage projects and highlights the importance of well integrity in CO2 sequestration.