assesses their advantages and limitations. Part I introduces two techniques for geodetic and surface monitoring. Part II is on subsurface monitoring using seismic techniques. Part III focuses on subsurface monitoring using nonseismic techniques. Part IV presents five case studies of geophysical monitoring at different worldwide geologic carbon storage sites.
The field of geophysical monitoring for geologic carbon storage is rapidly growing. Many new technologies are being developed. This book does not aim to include all possible geophysical monitoring technologies but rather presents an overview of current techniques and their applications, drawing on examples from geologic carbon storage sites across the world.
1.2. GEODETIC AND SURFACE MONITORING
Geodetic monitoring, including global positioning system (GPS) monitoring, tilt and Interferometric Synthetic Aperture Radar (InSAR), measures displacements and strains, both on the surface and within the interior of the Earth. Space‐based InSAR is perhaps the most cost‐effective geodetic technique for remote monitoring of land‐based geologic carbon storage sites.
CO2 injection might cause Earth's surface to deform. Geodetic monitoring is a cost‐effective approach to monitoring reservoir integrity and detecting possible CO2 leakage. The technique involves repeated measurements of the deformation of Earth's surface. In Chapter 2, Vasco et al. present a geodetic monitoring technique using InSAR. InSAR, which provides high spatial resolution and broad surface coverage, is particularly suitable for monitoring large‐scale geologic carbon storage. Multi‐temporal analysis can improve the accuracy of surface displacement measurements. Data interpretation and inversion techniques may be used to relate the observed surface displacements to the CO2 injection‐induced volume change at depth. Some advantages of geodetic monitoring include: (1) observations are usually frequent in time, from every few minutes to every few months; (2) geodetic measurements are often conducted remotely, simplifying data collection and enabling cost‐effective monitoring; (3) geodetic observations are sensitive to fluid volume and pressure changes associated with geologic carbon storage; and (4) geodetic monitoring may be able to detect CO2 leakage and the upward migration of fluid under pressure because the magnitude of surface displacement increases dramatically when the fluid‐injection‐induced deformation approaches the surface. InSAR monitoring has been successfully used at a gas storage site at In Salah, Algeria, where it has been determined that the flow in the reservoir was influenced by large‐scale fault/fracture zones. InSAR monitoring at the Aquistore CO2 storage project in Canada and the Illinois Basin Decatur Project in the United States indicates no major surface deformation that might be attributed to stored carbon dioxide. InSAR can possibly monitor ground deformation with an accuracy of 0.5 cm. InSAR data quality may be compromised by diverse land surface environments and unfavorable site conditions, such as mining and construction activities, groundwater recharge, swelling clays, and slope instabilities.
Surface monitoring is used to detect CO2 on the surface when some of the injected CO2 migrates to the surface. Most surface monitoring techniques involve monitoring absolute changes in bulk CO2 concentration, which is complicated by the diurnal cycle. In Chapter 3, Clegg et al. present a surface monitoring technique using frequency modulated spectroscopy, which uses changes in the carbon stable isotope ratio in CO2 to distinguish anthropogenic and natural CO2. Passive and active absorption spectroscopy can measure the absolute concentration of atmospheric CO2 and derive seepage from the sequestration site using changes from the background diurnal concentrations. Absorption spectroscopy has the advantage of both point source in situ analysis and wide area remote analysis of the area above a geologic carbon storage site.
1.3. SUBSURFACE SEISMIC MONITORING
Seismic monitoring can use active seismic surveys and/or microseismic events induced by CO2 injection and migration. Microseismic monitoring uses sensors/geophones deployed on the surface covering the monitoring region, and/or sensors/geophones in one or more boreholes to monitor induced microseismic events smaller than what surface seismic arrays can detect. For cost‐effective microseismic monitoring, we need to understand how many sensors/geophones are needed, and how to distribute them spatially to monitor targets of interest, such as the CO2 storage reservoir, caprock, faults, and so on. In Chapter 4, Chen and Huang present a methodology to determine the optimal number of geophones using a surface seismic monitoring network or a borehole geophone array for cost‐effective monitoring of target regions. They determine the optimal number of seismic stations based on the trade‐off curve of the event location accuracy versus the total 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.
Active seismic monitoring uses time‐lapse seismic reflection/transmission data. The underlining physical principle of this monitoring technique is based on the effects of (supercritical) carbon dioxide on subsurface elastic parameters. CO2 injection and migration alter elastic parameters such as compressional and shear velocities, density, and seismic attenuations in isotropic geologic formations, and also anisotropic parameters in anisotropic geologic formations. Studies of rock physics on the effects of CO2 injection and migration form the foundation of time‐lapse seismic monitoring for geologic carbon storage. In Chapter 5, Nakagawa and Kneafsey present laboratory measurements of the seismic response of fractured sandstone during geologic carbon sequestration. They employ a modified resonant bar technique to make laboratory measurements, and give the dynamic Young's modulus, shear modulus, and attenuation in core samples with supercritical CO2 injected, within a sonic frequency band of ~1 kHz to ~2 kHz. They use X‐ray CT to understand the distribution of supercritical CO2 injected into the core samples. Their laboratory experiment results show that changes in seismic‐wave velocities and attenuations strongly depend on the fracture orientation. In Chapter 6, Delaney et al. use laboratory ultrasonic experiments to study rock physics properties of rhyolite and carbonate samples, and the effects of pressure, temperature, porosity, and fluid saturation on their rock properties. In their experiments, they vary the pore‐filling fluids, effective pressures, and temperatures. They find that the framework composition, porosity, heterogeneities, temperature, effective pressure and pore‐filling fluids are first‐order controls on trends in elastic parameters and wave attenuation. Their findings could provide insight on using amplitude versus offset (AVO) for seismic monitoring. Their results show that the quality factor of compressional waves measuring compressional‐wave attenuation is inversely proportional to rock porosity and is weakly dependent on temperature. These results reveal the relationships of the ultrasonic velocity and the quality factor as a function of both temperature and effective pressure.
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
