Identification of CO<sub>2</sub> Leakage in an Active CO<sub>2</sub>-EOR Field, Songliao Basin, China
Manping Yang, Yu Qi, Tianshuo Shao, Ming Gao, Kaixue Ma, Lan Meng, Bingfan Li, Yanjun Lü, Hongjian Zhu, Keying Wang
Abstract
CO 2 -enhanced oil recovery (CO 2 -EOR) not only helps to recover additional oil but also sequesters CO 2 in the subsurface. However, any leakage of the injected CO 2 may place the environment and ecosystems at risk. In this work, we measured the near-surface atmospheric CO 2 concentrations ( p CO 2 ) and ground CO 2 fluxes in and around the S16 block of the Yushulin oil field, Songliao Basin, China, to identify CO 2 leakage induced by CO 2 -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 p CO 2 and ground CO 2 fluxes exhibit spatially heterogeneous. The peaks on the CO 2 flux map are spikier in contrast to those on the p CO 2 map, which indicates that the CO 2 flux measurements are more powerful to locate leakage sites. Type IV sites have anomalously high CO 2 fluxes, suggestive of significant leakage in the peripheries of the injection wells, likely due to well integrity failure and high injection pressure. The CO 2 escaped from wellbore flaws could migrate laterally after reaching shallow aquifers, then to soils, and finally to the atmosphere. Hence, elevated CO 2 fluxes are observed in some Type II sites away from the wells. The baseline of the CO 2 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 CO 2 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 CO 2 leakage in geological storage projects and highlights the importance of well integrity in CO 2 sequestration.