Time-Lapse Modeling and Inversion of CO2 Saturation for Sequestration and Enhanced Oil Recovery. Quarterly Report for the Period January 1, 2005 through March 31, 2005.
In this quarter we have continued our development of the inversion procedure for Phase III, in which time-lapse changes in seismic attributes are inverted to yield changes in CO2 fluid properties over time. In order to extract seismic attributes from the Sleipner North Sea CO2 time-lapse data set, a new, detailed interpretation was performed and multiple horizons were picked for the 1994 and 2002 vintages. Traveltime difference maps were constructed at several levels within the Sleipner CO2 injection zone, and were quantitatively analyzed. No traveltime change was seen in the overburden, as expected, while significant changes were seen in the upper half of the CO2 injection zone. Evidence of thin-bed tuning and undershooting was also found. A new semi-automated, quantitative method for estimating time sag anomalies was developed, and was used to calculate the amount of time sag along a selected horizon. The resulting time sag estimates matched those seen in the traveltime difference maps. Such a method will be useful for obtaining rapid, accurate quantitative measurements of traveltime changes in the Sleipner data cubes. The traveltime changes will be combined with other attributes, such as amplitude and frequency changes, for input into the real-data inversion.
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