Pressure effects caused by CO sub(2) injection in the Tubaen Fm., the Snoehvit field

1.6 Mt of CO sub(2) was injected in the Tubaen Fm. at the Snoehvit field from April 2008 to April 2011. Injection was associated with a pressure build-up, and following a well intervention operation, the Stoe Fm. was used for further CO sub(2) injection, avoiding the pressure limitations in the Tubaen Fm. The pressure build-up in Tubaen Fm. was mainly caused by a limited reservoir volume and heterogeneities in the formation. Salt precipitation was evaluated as a possible source that reduced the near wellbore injectivity with associated pressure build-up. Weekly injection of a 90:10 mixture of methyl ethyl glycol and water (MEG) was implemented to mitigate salt precipitation. This paper evaluates the causes for the pressure build-up in more details. Analysis of bottom-hole-pressure measurements (BHP) from periods of shut in indicates a low permeable zone surrounding the well at early stage of injection. The low permeable zone may be caused by precipitated salt reducing the injectivity. Salt precipitation is also observed to reduce the porosity and permeability on core plugs from the injection well in laboratory measurements. The Tubaen Fm. comprise of 4 sandstone units (Tubaen 1-4) separated by interbedded shale units. Comparison between synthetic seismograms and time-lapse seismic data indicate that CO sub(2) propagated into the three lowermost Tubaen sandstone units (Tubaen 1-3), where the pressure also increased. The shale unit above Tubaen 3 sandstone unit appears sealing from the synthetic seismograms. Tubaen 1 sandstone unit has exceptionally good injectivity. This may have a negative impact on the overall storage capacity by channeling the CO sub(2) and increase the pressure locally. A dimming of the time-lapse seismic amplitude is observed that may be associated with the injected MEG.