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