Subsurface and Near-Surface

Near-Surface monitoring tools enable both direct and indirect detection of CO₂ above underground storage reservoirs. These tools include geochemical monitoring in the soil and vadose zone, geochemical monitoring of shallow groundwater or surface waters, surface displacement monitoring, and ecosystem stress monitoring. The purpose of these monitoring approaches is to detect near-surface manifestations of CO₂that may be leaking from geologic storage.¹

Geochemical monitoring in the soil and vadose zone involves direct sampling of CO₂ and its reaction products, as well as sampling for tracers that were injected into underground storage along with the CO₂. Geochemical monitoring of groundwater involves installation of shallow monitoring wells for detecting any changes in groundwater chemistry related to CO₂ injection. Such geochemical sampling techniques provide valuable direct measurements of CO₂ and associated indicators, but characterizing a large area requires many individual data collection points.²

Surface displacement measurements are designed to detect uplift of the land surface that may have been caused by CO₂ injection within the storage formation. Ecosystem stress monitoring is aimed at mapping vegetative stress that may have resulted from elevated CO₂ levels near the soil/atmosphere interface. Remote sensing data can provide highly precise surface displacement measurements and indications of vegetative stress over a large area. However, the data can be a challenge to interpret where site conditions are complex.³

Suburface Monitoring

Subsurface monitoring tools and techniques enable scientists to track the CO₂ plume, document physical property changes deep within the subsurface, and identify potential migration pathways in the geologic carbon storage reservoir. Subsurface monitoring tools are used to detect and quantify CO₂that has been injected into a reservoir and to detect faults, fractures, and any seismic activity that may be present in the injection zone and adjacent confining intervals.⁴

Typical subsurface monitoring tools include well logging tools, downhole monitoring tools, subsurface fluid sampling, tracer analysis, seismic-imaging methods, high-precision gravity methods, and electrical techniques. Such tools are needed to:

Track the movement of the injected CO₂ plume through the storage reservoir
Define the lateral extent and boundaries of the plume
Track associated pressure changes and other physical property changes in the reservoir
Identify possible release pathways
Demonstrate long-term stability of the CO₂ plume⁵

Most techniques and tools used for subsurface monitoring are also used to characterize the geologic framework and rock and fluid properties of the storage reservoir. Monitoring is relatively straightforward near wells, but can become more challenging, and possibly expensive, when obtaining measurements over the large area typical of a carbon storage project.⁶