Geophysical Monitoring for Geologic Carbon Storage. Группа авторов

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Geophysical Monitoring for Geologic Carbon Storage - Группа авторов


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number of seismic stations. They design an optimal microseismic monitoring network based on widely accepted guiding principles, and the relationship between the location accuracy of microseismic events and the total number of seismic stations. The method is based on the trade‐off curve between the mean location accuracy and the number of seismic stations. In practical applications, three‐component geophones can provide more useful information of shear‐wave signals to improve microseismic monitoring compared with one‐component geophones.

      Time‐lapse 3D seismic monitoring, or 4D seismic monitoring, is considered as the most effective tool for 3D subsurface monitoring of CO2 injection and migration. However, time‐lapse 3D seismic surveys and data processing are costly and time consuming. For the similar purpose of optimal design of a microseismic monitoring network, Gao et al. in Chapter 7 introduce a numerical method for designing optimal time‐lapse seismic monitoring surveys by analyzing sensitivities of elastic waves in isotropic and anisotropic media with respect to reservoir geophysical property changes. Conventional seismic surveys are designed based on seismic‐wave illumination of the entire subsurface imaging region, and require a large number of sources and receivers to produce high‐resolution images of the subsurface. By contrast, time‐lapse seismic monitoring is not designed to image the entire subsurface region, but only the target monitoring regions, such as the CO2 storage reservoir, caprock, and faults. Therefore, time‐lapse seismic monitoring needs only seismic information from such regions, rather than from the entire subsurface region. The optimal design of time‐lapse seismic surveys is based on elastic‐wave sensitivity analysis, that is, numerical modeling of elastic‐wave changes with respect to changes of geophysical properties within target monitoring regions. The method numerically solves the elastic‐wave sensitivity equations obtained by differentiating the elastic‐wave equations with respect to geophysical parameters, such as density, compressional‐ and shear‐wave velocities, and saturation parameters, in isotropic and anisotropic media. Receivers should be placed in surface regions for surface seismic surveys or borehole locations for vertical seismic profiling (VSP) surveys with significant values of elastic‐wave sensitivity energies. The number of receivers needed for cost‐effective time‐lapse seismic monitoring is only a fraction of a regular 3D seismic survey.

      The 3D surface seismic monitoring has the advantage of monitoring a large subsurface area to track CO2 migration in the 3D space. However, seismic imaging/monitoring resolution decreases with the depth, particularly for CO2 storage at geologic formations at several kilometers in depth. Compared with surface seismic monitoring, VSP monitoring improves seismic imaging/monitoring resolution in the deep region when receivers are placed in the deep region of the subsurface. The image resolution of VSP monitoring is usually twice that of surface seismic monitoring. The limitation of VSP monitoring is that the lateral monitoring range is smaller than surface seismic monitoring.

      VSP surveys use active seismic sources at various offset locations (offset VSP), or along various walkway lines from the monitoring well (walkaway VSP), or using a 2D surface source distribution (3D VSP). Offset VSP monitoring uses only a few offset source points, and has the lowest cost among the three different types of time‐lapse VSP survey. However, offset VSP can monitor only in the sparse azimuthal directions along a monitoring well to offset source directions. 3D VSP monitoring is the most expensive among the three VSP monitoring approaches, with the highest spatial coverage of the monitoring region. The walkaway VSP monitoring is the trade‐off between the offset VSP monitoring and 3D VSP monitoring.

      In Chapter 9 on walkaway VSP monitoring, Wang et al. apply reverse‐time migration (RTM) to time‐lapse walkaway VSP data acquired at the SACROC CO2‐EOR field in Scurry County, Texas, USA, to reveal changes in the reservoir caused by CO2 injection and migration. Before they apply RTM to the data, they perform statics correction and amplitude balancing to the time‐lapse walkaway VSP data sets. To mitigate the image artifacts caused by the limited subsurface seismic illumination of the walkaway VSP surveys, they analyze and process the RTM images in the angle domain to greatly improve the image quality.

      Because of limited seismic illumination of VSP surveys, migration imaging of VSP data often contains significant image artifacts, which can be alleviated using an angle‐domain imaging condition. This alleviation is tedious if not impossible for 3D VSP data. 3D least‐squares reverse‐time migration (LSRTM) is an alternative approach to addressing such a problem. To demonstrate the improved imaging capability of 3D LSRTM of 3D VSP data, Tan et al., in Chapter 10, apply the method to a portion of the 3D VSP data acquired at the Cranfield CO2‐EOR field in Mississippi, USA, for monitoring CO2 injection to obtain a high‐resolution 3D subsurface image. LSRTM solves


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