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3 Surface Monitoring, Verification, and Accounting (MVA) for Geologic Sequestration Storage

       Samuel Clegg1, Kristy Nowak-Lovato1, Robert Currier1, Julianna Fessenden2, and Ronald Martinez1

       1 Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA

       2 Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, New Mexico, USA

      ABSTRACT

      Geologic sequestration of carbon dioxide (CO₂) is one of the immediate solutions to the permanent storage of greenhouse gases. Geologic storage of CO₂ requires monitoring, verification, and accounting (MVA) to assess the location of the sequestered material as well as track the plume movement. Surface MVA techniques have been developed to detect CO₂ emissions should some of the injected CO₂ migrate to the surface. Most of these techniques involve monitoring absolute changes in bulk CO₂ concentration, which is complicated by the diurnal cycle. Changes in the carbon stable isotope ratio in CO₂ has been shown to be a more sensitive diagnostic to distinguish anthropogenic and natural CO₂. Both cavity ringdown spectroscopy (CRS) and frequency modulated spectroscopy (FMS) are sensitive spectroscopic techniques that have been developed to measure these stable isotope ratios. While CRS is limited to analysis of point source emission samples, field experiments of FMS instruments have been demonstrated in both captured samples and in remote configurations. In this chapter, the application of FMS to the MVA of carbon dioxide is reviewed.

3.1. INTRODUCTION

      Methods to limit greenhouse gas emissions in an effort to arrest global warming are necessary and geological sequestration is immediately available. Geological sequestration involves pumping CO₂ into deep geological reservoirs such as depleted oil and natural gas reservoirs (Rodosta et al., 2014, Rodosta & Ackiewicz, 2014). Geological sequestration has the benefit of accommodating any source of CO₂ including any industrial process where the CO₂ is collected. The expectation is that the sequestered CO₂ will become mineralized over time resulting in the permanent CO₂ storage.

      Monitoring, verification, and accounting (MVA) is a fundamental requirement for geological sequestration sites to ensure the permanent storage as well as ensure public health and environmental safety (Rodosta & Ackiewicz, 2014). In order to pay for sequestration, there is an interest in the development of a carbon economy where those that sequester CO₂ would receive a financial gain. If a carbon economy is established, it is critical to verify that the CO₂ is permanently stored. Furthermore, MVA methods are required to ensure that the CO₂ or other hazardous gases within the reservoir are not mobilized beyond the reservoir into used water reservoirs or to the surface at dangerous concentrations.

Many MVA methods have been developed that have demonstrated many of the performance requirements in field tests. These MVA techniques must be capable of at least measuring the CO₂ flux and these techniques would detect seepage as a change in concentration above ambient conditions. These fundamental measurements are complicated or compromised by the diurnal CO₂ concentrations that raise the minimum detection limit. MVA techniques must also be capable of determining the location of the seepage at the surface. In collaboration with subsurface methods discussed in Part II of this volume, the pathway from the reservoir to the surface can be traced and the failure mechanism could be determined. Most critically, MVA techniques are required to identify the CO₂ seepage pathway and mechanism before the seepage becomes a catastrophic failure.

There is also a desire to identify seepage at the surface at concentrations that are at or below ambient CO₂ concentrations. Carbon stable isotope ratios enable one to distinguish the sequestered anthropogenic CO₂ from ubiquitous natural emissions as depicted in Figure 3.1 (Fessenden et al., 2010). Anthropogenic CO₂ has a carbon stable isotope signature (δ¹³CO₂) that ranges from ~‐23 to ‐37% (or per mil or parts per thousand) from petroleum burning to ~‐40 to ‐46% from natural gas combustion. These isotopic signatures differ significantly from natural ~‐7% CO₂ isotope ratios found in the atmosphere or other natural sources of CO₂ depicted in Figure 3.1. Consequently, MVA techniques with stable isotope sensitivities

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