Post-Stack 4D Seismic Inversion for Improved Monitoring at Sleipner Field

More than 20 million tons of CO2 has been injected into the Utsira formation at Sleipner field since 1996. The injected CO2 in the subsurface is characterized on the post-stack seismic images as a stack of high-amplitude bright spots which caused interference effects in the seismic response of CO2-saturated thin beds. In this study, a dedicated time-lapse (4D) inversion workflow is proposed, where the estimated time-strain will play the role of the low-frequency initial model for the model-based inversion as there are no additional independent measurements or repeated well logs available in the provided dataset for the Sleipner area. The purpose of the workflow is to improve the confidence in 4D post-stack inversion without using any additional data to the seismic traces. Application of the proposed workflow proves to be effective for what it is designed for i.e. for time lapse inversion of the Sleipner data. The use of time-strain was particularly useful for the construction of the low frequency initial model for the model-based inversion. It helped the model-based inversion to drastically enhance the results in terms of impedance change values of the CO2 plume and to eliminate most of the incorrect signals.