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Seismics
Seismic Imaging
(CO2SINK Work Package 2.3)
The objective of the seismic measurements is to provide an understanding of the structural geometry for flow pathways within the reservoir and to evaluate their evolution as the reservoir is developed as a CO2 storage facility.
Three-dimensional (3D) seismic measurements
3D seismic surveys utilise 2D grids of seismic sources and receivers deployed on the ground surface. The seismic response of the ground is recorded as a time series corresponding to each source-receiver pair. The temporal dimension can be transformed to true depth (the 3rd dimension) by means of a velocity model generally obtained from well measurements. By repeating the 3D survey, time-lapse (4D) seismic can be obtained which are necessary for investigations of the reservoir evolution during CO2 injection.
A baseline 3D seismic investigation was performed in autumn 2005 to ensure that the dimensions, geometry, and properties of the reservoir, especially of the reservoir seal, are well understood before injection. The investigations cover an area of approximately 14 km2, to a depth of at least 1 km, with a bin size of 12 m x 12 m and an average fold of 25. As a non-explosive, high frequency source, an accelerated weight drop was used for the complete area. Additionally, seven profiles around the injection point ( " star " ) were shot with a swept impact source. These profiles will be repeated after the injection start in order to obtain a time-lapse data set.
Vertical Seismic Profiling (VSP), Moving Source Profiling (MSP), and Crosshole Tomography measurements
While the 3D seismic survey from the ground survey provides data over a large area but with relatively low resolution, borehole seismic measurements focus on a smaller volume around the injection point with higher spatial resolution. Additionally, these measurements need less logistic and financial effort so that they can be repeated easily.
The VSP measurements are carried out with a swept impact seismic source at the ground surface, located near the well head of an observation borehole. Seismic receivers are placed in the borehole. The measurements provide the base for velocity-depth profiles and they can also be used for imaging steeply dipping reflectors (e.g., faults). MSP measurements also use seismic receivers in a borehole, but the seismic source moves to different locations on the ground surface. The result of these measurements is a high resolution 3D image around the borehole.
Crosshole tomography uses two boreholes. The non-explosive seismic source excites seismic waves at different depth levels in one borehole and the seismic response is recorded at different depth levels in the other borehole. The seismic data are used to derive a high resolution model of wave propagation velocity.
All these borehole-based seismic measurements will be repeated several times after the injection start in order to monitor the reservoir near the injection point.
Pictures of the seismic measurement campaign in September / October 2009
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