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Pressure and Geomechanical Response to CO2 Injection at Snøhvit, Norway

Abstract

An unexpected pressure rise during CO2 injection into the Tubåen Fm. at Snøhvit led to a considerable decrease in the estimated total capacity and to the final abandonment of injection operations in this particular reservoir. This pressure build up and the fluvial nature of the reservoir suggests a strong stratigraphic compartmentalization. However the complex fault configuration in the area also suggests a potential structural contribution to the compartmentalization. Nevertheless, the pressure behavior after injection stopped indicates that the compartment is not completely closed. In this work we perform wellbore pressure and reservoir hydromechanical analyses to understand the geomechanical behavior of the Tubåen Fm. and shed light on the mechanisms of fluid migration outside the main compartment. The Snøhvit gas field is located offshore Norway in the Barents Sea. CO2 is separated from the produced gas and, until 2011, it was stored underground in the Tubåen Fm. at approximately 2600 m depth. Since then, injection has moved to a different horizon. The Tubåen Fm. corresponds to a delta plain environment dominated by fluvial distributary channels and some marine-tidal influence. It is separated from the producing gas reservoir (Stø Fm.) by the Nordmela 1 and 2 Fms. that contain wide shale layers expected to act as flow barriers. Structurally this area is extensively faulted, characterized by a dominant east-west-trending fault system, where the majority of the faults dip toward the basin axis and define typical horsts and graben geometry. However, it also present faults at high angles to this trend, leading to complex fault interactions. Given the reservoir characteristics, injection rates and pressures, 4D seismic observations, and available in situ stress tensor estimates, we use a coupled hydromechanical approach to understand the geomechanical response of the system to the CO2 injection, focusing on addressing the CO2 distribution and migration outside of the main compartment.