On seismic monitoring of the scope of CO₂ storage in the seabed saline aquifers: Taking the Sleipner CCS project as an example

CO₂ saline aquifer sequestration accounts for 98% of the total sequestration potential. In the past, most of the studies on 4D seismic monitoring of the CO₂ seabed saline aquifer spread range were qualitatively analyzed by the variability of time-delayed seismic data, which lacked the constraints of well-logging data. Therefore, seismic monitoring methods for the spread range of CO₂ seabed saline aquifer storage based on the logging and 4D seismic data collected by the Sleipner Saline Aquifer CO₂ Sequestration Project in Norway were investigated. Based on the logging and 4D seismic data collected in the project, the anisotropic response characteristics caused by the change of the CO₂-saline two-phase medium in the process of CO₂ injection were studied by rock physics modelling, the technique of well control seismic attribute analysis was applied, the seismic attributes that are sensitive to the change of the saturation degree of CO₂ were selected, and the seismic forward and inverse analysis were combined to better understand the time-shifted CO₂ saline aquifer spread range. Results show that the bulk modulus, bulk density, primary wave velocity, and shear wave velocity of the saturated rocks decreased with the increase of CO₂ saturation, the overall amplitude increased in the forward simulation results, the amplitude changes decreased with the increase of CO₂ injection, and the root-mean-square (RMS) amplitude attribute was the most sensitive to the change of CO₂ saturation. During the injection period, CO₂ was mainly transported along the SSW-NNE and accumulates in the higher part of the tectonic structure. Vertically, CO₂ was transported from the injection point to the upper layer, and the lower layer reached the maximum spread range earlier than the upper layer. Combined with the nature of the reservoir and the structural interpretation results, the spreading range of CO₂ in the reservoir was controlled by mainly the anisotropic permeability and the structural high or low levels.