Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 40

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 have the capability of distinguishing CO₂ seepage from natural emissions under ambient conditions and without the diurnal complications.

In this chapter, the current state‐of‐the‐art MVA techniques are outlined along with a brief description of the strengths and limitations. By far, most of the MVA techniques currently used to monitor sequestration sites do not exploit stable isotope ratios. This section will describe the virtues of in situ laboratory and field analysis versus remote methods capable of wide area monitoring. Finally, the chapter will describe frequency modulated spectroscopy (FMS) as a means of in situ and remote surface monitoring of geological sequestration sites with stable isotope sensitivity.

Figure 3.1 The CO₂ produced from several anthropogenic and natural processes fractionates the carbon isotope. Here, the range of δ¹³C produced from several processes is depicted (Fessenden et al., 2010).