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

Although monitoring of geological carbon dioxide (CO2) 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 CO2 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 CO2 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 CO2 mass balance. The Newtonian attraction gravity effect of stored CO2 is modeled as a function of reservoir depth and CO2 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 CO2 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 CO2. A CO2 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 CO2 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 CO2 storage site with increasing injections up to about 105 kt at a depth of 600 m. These data were analyzed, and the CO2 storage signal could be clearly identified.