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Simulation Study of Sleipner Plume on Entire Utsira Using A Multi-Physics Modelling Approach

Sleipner CO 2 storage project offshore Norway is a pioneering industrial-scale CCS project in operation since 1996. CO 2 injection is done in the Utsira Formation (part of the Sleipner hydrocarbon reservoir overburden) in a depth-range from 800 to 1000 m using a single, long-reach deviated well [1]. The project operator Equinor has released substantial field data related to the project including several seismic surveys, well data, and benchmark models. Using the available data, several studies have been reported to match simulated plume movement to 4D observations [2]. Most of the published studies have focused on plume development in the so-called Sand Wedge, containing CO 2 in Layer 9, for the simple reason that a full-field Utsira model including all sequences from the injection point close to the bottom of the reservoir to the top-most layer has not been available. One of the main shortcomings of such an approach is the uncertainty in the amount of CO 2 that has arrived in Layer 9. Moreover, as both the arrived volume and the CO 2 plume print in Layer 9 depend on the same seismic data, there exists interdependency between the match parameter and the match reference (4D seismic boundary) if the history-matching is done in this way. At the bottom-hole condition, the injected CO 2 is in dense phase but due to the relatively shallow depth of the formation, it is safe to assume that some degree of phase change would occur in the CO 2 mobile phase, especially near the top of the Utsira formation as the reservoir condition is very close to the CO 2 critical point. This might considerably affect the plume movement due to both the phase expansion and CO 2 phase density changes close to the critical point. A valid history-matching approach would then involve predicting the phase properties as a function of reservoir condition. Shortcomings of isothermal approaches are consequently evident because modifications of static properties have been used to In this study, we report the results of simulation studies using a recently developed reservoir simulation model that includes all vertical sequences of the Utsira Formation. The base geo-model used in creating the simulation model includes communication pathways such as chimney and feeder that have resulted in the characteristic movement of CO 2 in the storage reservoir as observed from the 4D data. Using thermal reservoir simulation, the effects of temperature gradient and injection induced temperature variations on phase properties of CO 2 have been modelled. By applying ultra-high-resolution mesh, onset of the convective movement of CO 2 dense aqueous phase is estimated. The extent of convective dissolution is calibrated against the observed data from natural CO 2 reservoirs and enhanced dissolution is consequently considered in the history matching process. The results explain the distinctive pattern of movement of CO 2 plume in Utsira and provide a plausible description of further plume movement. An analysis of the importance of various storage mechanisms and their contribution to total capacity of the Sleipner CCS project will also be also provided.