Geophysical Monitoring for Geologic Carbon Storage. Группа авторов

1 Evaluating Different Geophysical Monitoring Techniques for Geological Carbon Storage

       Lianjie Huang1 and Xianjin Yang2

       1 Geophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, USA

       2 Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, Livermore, California, USA

      ABSTRACT

      Various geophysical techniques can be used to monitor geologic carbon storage and ensure that it is safe in the long term. Seismic methods are often used to monitor CO₂ migration in the deep regions of a geologic carbon storage site while nonseismic methods provide complementary monitoring from shallow subsurface to the surface. This chapter gives an overview of the geophysical monitoring methods presented in this book including Interferometric Synthetic Aperture Radar (InSAR), frequency modulated spectroscopy, induced microseismic monitoring, time‐lapse active seismic monitoring, gravity, electrical and electromagnetic techniques, controlled‐source electromagnetic method, electrical resistivity tomography (ERT), and self‐potential measurements.

1.1. INTRODUCTION

      Geologic carbon storage, or geologic carbon sequestration, is an emerging technology to permanently store or sequester separated and captured anthropogenic carbon dioxide (CO₂) from industrial sources into deep geologic formations. Some large‐scale anthropogenic CO₂ sources include coal‐fired or gas‐fired power plants, oil and gas refineries, steel mills, and cement plants. The purpose of geologic carbon storage is to mitigate the rising CO₂ concentration in Earth's atmosphere and to substantially reduce its impact on the global warming.

      Geophysical monitoring is crucial for ensuring safe, long‐term geologic carbon storage. A geologic carbon storage project requires site characterization before CO₂ injection to evaluate if the site is suitable for geologic carbon storage, and monitoring of CO₂ migration during and after CO₂ injection. Various geophysical monitoring techniques can remotely track subsurface CO₂ plumes and provide crucial information to mitigate potential leakage risks. A geologic carbon storage project should integrate complementary geophysical monitoring techniques to form a comprehensive monitoring plan because various geophysical monitoring techniques have their own advantages and limitations. Joint analyses of information from different geophysical monitoring techniques can increase the monitoring confidence. Monitoring plans must be adaptable during different phases of a geologic carbon storage project from site characterization to injection to postinjection site care. The type of monitoring technique that should be deployed also depends on the monitoring targets, such as the atmosphere, drinking water aquifers, cap rock, and storage formation.

This book provides a comprehensive reference to different geophysical techniques currently used and being developed for monitoring geologic carbon storage, and 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