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Seismic
Seismic Imaging
(CO2SINK Work Package 2.3)
The objective of the seismic imaging exercise within the CO2SINK project 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. The seismic imaging exercise will attempt to resolve the structural geometry of the reservoir using three-dimensional investigation tools and imaging techniques.
The geological features that determine reservoir properties often have complex geometries and their dimensions can encompass several orders of magnitude. As a result, the project team will need diverse survey layouts, and measuring and imaging techniques, all of which can investigate and represent the reservoir in three-dimensions.
Following CO2 injection, the project team anticipates an upward migration of the CO2 that is associated with the increased compressibility and the decreased density of the reservoir along the flow paths. With time, dissolution of CO2 will reverse the migration direction, while various chemical processes start to compete in increasing and decreasing the rigidity of the reservoir. In the short-term, local variations of acoustic impedance and seismic velocities are expected to reach around 10%, and these variations should be detectable by both large and small scales seismic methods, i.e. by surface referenced and down-hole surveys, respectively. Conversely, the long terms effects would produce variations of the seismic attributes approximately two orders of magnitude smaller. Within the life span of the CO2SINK project, the large, short-term phenomena will prevail.
Three-dimensional (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, and the wave propagation time becomes the third dimension of the data set. The temporal dimension can be transformed to true depth by means of a velocity model generally obtained from well measurements. If a 3D seismic survey is repeated at several time intervals, this historical time (as opposed to the wave propagation time) becomes a fourth dimension, hence the name 4D or time-lapse seismic.
The 4D approach allows a detailed description of the evolution of the reservoir, but the overall cost of the survey escalates rapidly by repeating conventional 3D surveys. Alternative layouts with 3D-imaging capabilities include seismic receiver arrays deployed in the wells. These well-based investigation methods are generally less extensive and more focused on regions of the reservoir where changes with time are likely to occur. As an additional benefit, the down-well arrays are positioned far from the disturbing influence of the surface and three-component receivers can be used effectively to resolve geological features with complex 3D geometries.
The Seismic Project at Ketzin
A baseline 3D seismic investigation will be performed to ensure that the dimensions, geometry and properties of the reservoir, and especially of the reservoir seal, are well understood before injection. The investigations are expected to cover an area of approximately 15 km2 , to a depth of at least 1 km, with a bin size of 10 x 10 m and a minimum fold of 25. These parameters may, however, be modified according to results of pre-investigation pilot tests and examination of other site data, as they become available.
Non-explosive, high-frequency signal sources will be used, a time-distributed type being preferred. The energy is built up from a large number of impacts with a typical energy of 500-2000 J/impact at a rate of 7-15 impacts/second, bringing the energy per record to 1000-2000 kJ. The intention is also to evaluate other type of sources, including swept-frequency vibrators and drop weights, in order to compare and tune up the energy and frequency of the main source, which will be used for the investigations.
The source will be mounted on an industrial or agricultural vehicle, which would ensure its mobility through potentially difficult terrain while limiting environmental impact. Maintaining the highest possible source frequency content is vital for fulfilling the project's high-resolution imaging objectives. The high-frequency content at the receiver end may, however, be significantly reduced due to specific site conditions. The response of the site to the techniques and methods envisaged will be evaluated before the investigations begin. The project plan includes a pilot survey along two 2D seismic lines. This pilot survey will indicate whether the data that can be obtained in the given conditions would offer sufficient resolution to meet the project objectives. The pilot survey will also include the use of down-well sensors.
The current plan is to monitor the evolution of fluid movement within the reservoir by performing 2D surveys along six lines, approximately 12-18 months after the baseline survey. The lines will probably be arranged in two orthogonal sets; each set comprising three lines and each line being 3-5 km long. The source for these later surveys will be the same as that used for the baseline survey, and down-well three-component receivers will also be used. The seismic team will perform multi-azimuth vertical seismic profile (VSP) and cross-hole measurements to detect and characterise steeply dipping features and to obtain more detailed images of small-scale features in the vicinity of the injection and observation wells.
A second set of time-lapse surveys and a second 3D survey conducted at a late stage of the project would provide a more complete image of fluid movement through the reservoir with time , across various scales and potentially varying geometries. These late-stage investigations are, however, provisional and not part of the current schedule .
One of the observation wells will have permanent seismic sensors installed. Ten three-component sensors, spaced at 100 m, are planned. Their main role is to help reduce the acquisition footprint for the seismic phase of the project.
The integration of 3D seismic data with time-lapse 2D, VSP and cross-hole surveys will help the team to integrate seismic results with geological, geophysical, geomechanical and hydraulic well data over scales of investigation that cover several orders of magnitude.
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