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.1National Energy Technology Lab. (n.d.). Near-surface Monitoring. U.S. National Energy Technology Laboratory. Retrieved July 1, 2021, from https://netl.doe.gov/node/5872
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.2National Energy Technology Lab. (n.d.). Near-surface Monitoring. U.S. National Energy Technology Laboratory. Retrieved July 1, 2021, from https://netl.doe.gov/node/5872
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.3National Energy Technology Lab. (n.d.). Near-surface Monitoring. U.S. National Energy Technology Laboratory. Retrieved July 1, 2021, from https://netl.doe.gov/node/5872
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 CO2 that 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.4National Energy Technology Lab. (n.d.). Surface Monitoring. U.S. National Energy Technology Laboratory. Retrieved July 1, 2021, from https://netl.doe.gov/node/5873
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 plume5National Energy Technology Lab. (n.d.). Surface Monitoring. U.S. National Energy Technology Laboratory. Retrieved July 1, 2021, from https://netl.doe.gov/node/5873
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.6National Energy Technology Lab. (n.d.). Surface Monitoring. U.S. National Energy Technology Laboratory. Retrieved July 1, 2021, from https://netl.doe.gov/node/5873