Estimation of CO₂ Storage Capacity in a Depleted Gas Field on the Korean Continental Shelf

The onshore geological sequestration of carbon dioxide (CO2) in Korea is unlikely as the onshore basins are tight and shallow. Moreover, when the 5.4 magnitude earthquake struck Pohang City in the largest onshore basin of Korea in November 2017, fluid injection activities, including those for geothermal and CO2, had to be suspended. Therefore, the ideal location for storing CO2 in Korea would be in offshore basins, such as the Ulleung, Gunsan, and Jeju Basins. These basins contain thick and young clastic sediments, with the Ulleung basin in particular consisting of over 11 km of late Oligocene and younger, dominantly marine sediments. More importantly, several structures containing a small amount of natural gas have been discovered in the last two decades. The Donghae-1 gas field in the southern Ulleung basin margin is the only producing field in South Korea, at the moment. Since its inception in 2004 up until the end of 2016, a total of 169 billion cubic feet of natural gas and 3.2 million barrels of ultra-light crude from the field was produced. This iconic gas field is scheduled to shut down soon due to production cutbacks. In this study, we carried out numerical simulations of injecting CO2 into a depleted gas field to estimate its storage capacity. Prior to this injection, the natural gas of the field was produced up to 90% of the original gas in place. With dynamic modeling of CO2 injection, it was confirmed that the potential storage capacity of depleted gas reservoirs easily exceeds saline aquifers. In addition, more active utilization of underlying aquifers or brine extraction for pressure management, to maximize storage capacity was investigated.