Synthetic Time-Lapse Seismic Study of CO₂ Injected into a Marine Aquifer - The North Sea Sleipner Field

Khanh Duc Nguyen
University of The Faroe Islands, Torshavn, Faroe Islands

CO₂ separated from produced natural gas in the Sleipner West field is being injected into sands of the Utsira Formation to prohibit emission to the atmosphere. Time-lapse seismic surveying has proved to be a suitable geophysical technique for monitoring CO₂ injection into a saline aquifer. The effects of the CO₂ on the seismic data are large both in terms of reflection amplitudes and also in the time delays observed.
The former effect was clearly observed in the different data between pre and post-injection time lapse surveys. By conventional post-stack analysis we can see different amplitudes are as large as three times the initial reflections from this area. The same comparison using pre-stack analysis only gives the different amplitude of up to two times as large.
The latter effect has been used to estimate the total volume and mass of CO₂ in place under reservoir conditions and the estimates tend to be too large favouring lower velocities in the CO₂ saturated sand. One of the most uncertain direct data source causing the difference is the time delays estimated from the post-stack time lapse seismic data. In another study, we introduced the method for estimation of relative velocity changes within a reservoir layers from pre-stack time lapse seismic data and then compared with the results of conventional post-stack analysis. Again, the pre-stack method gave smaller velocity changes when estimated between two key horizons.
A synthetic time lapse seismic study that incorporates time lapse seismic modeling, time lapse seismic processing and time lapse seismic analysis has been performed to understand the aforementioned differences between pre-stack and post-stack 4D response in the real seismic data of the Sleipner field.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece