Near-Surface Monitoring

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.

Description

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. 

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. 

Research Agenda and Challenges

Research is needed to develop near-surface monitoring tools for detecting possible releases of CO₂ in the vadose zone and in shallow groundwater and surface waters. Its important to note that near-surface measurements complement atmospheric measurements, because natural variations in CO₂ levels in the near-surface ecosystem are minimal. Shallow groundwater monitoring is obviously important for protection of underground sources of drinking water. Techniques are needed for monitoring large areas associated with CO₂ storage projects.

Pathways for near-surface monitoring research include:

• By 2020: Develop real-time monitoring systems for measuring CO₂ at the surface, in the unsaturated zone, and in shallow groundwater. Measurement of gas concentrations and biological changes at the surface and in the shallow subsurface, using geophysical methods and other approaches, are included in this research area. Techniques to measure land surface deformation that may result from CO₂movement in the subsurface are also included. 
   
• By 2030: Develop advanced systems for measuring near-surface manifestations of CO₂ movement and release.

NETL-Supported Near-Surface Monitoring Research

NETL supports projects that are addressing research challenges associated with Near-Surface Monitoring. Examples of projects supporting this key technology include: (1) development of low-cost, autonomous hardware which will detect CO₂ in groundwater and automatically process data; (2) development of a network of shallow subsurface sensors to monitor statistical fluctuations of CO₂concentrations to detect any excess emissions indicating leakage from a storage site; and (3) development of a procedure for ¹⁴C tagging CO₂ streams prior to injection into a geologic formation as a method for leakage detection. 

The MVA webpage offers links to detailed information on projects performing research in this area.