Mass balance monitoring of geological CO2 storage with a superconducting gravimeter — A case study

Abstract Although monitoring of geological carbon dioxide (CO 2 ) storage is possible with a number of geophysical and geodetic techniques (e.g., seismic survey), gravimetric monitoring is known to be the most accurate method for measuring total mass changes. Therefore, it can be used for detection of storage pore space content changes and migration of CO 2 plumes. A superconducting gravimeter (SG) installed on the Earth's surface provides precise and continuous records of gravity variations over time for periods from minutes to decades, which are required for monitoring subsurface CO 2 storage. Due to the fact that gravimeter records combine the gravity effects of surface displacement and subsurface mass change, these two effects must be separated properly for observing CO 2 mass balance. The Newtonian attraction gravity effect of stored CO 2 is modeled as a function of reservoir depth and CO 2 mass for different locations of the gravimeter over the reservoir. The gravity effect of the surface deformation is considered according to the modeled and measured displacement above the CO 2 reservoir at the gravimeter's position. For estimation of the detection threshold, an assessment is carried out for the gravity corrections, which must be subtracted from the raw gravity data before obtaining the gravity signal of the stored CO 2 . A CO 2 signal larger than about 0.5 μGal can be detected with an SG's continuous recordings. A measured gravity profile along the reservoir can support the continuous measurements. For providing objective evidence of a CO 2 stored gravity signal, real measured raw SG gravity data of the MunGyung site in Korea were superimposed with an artificial uniformly continuous gravity signal up to 1.7 μGal, representing a gravity signal from a CO 2 storage site with increasing injections up to about 105 kt at a depth of 600 m. These data were analyzed, and the CO 2 storage signal could be clearly identified.