Subsurface Monitoring

The Subsurface Monitoring key technology covers tools and techniques used to map 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 geologic storage reservoir and to detect faults, fractures, and any seismic activity that may be present in the injection zone and adjacent confining zones.

Description

Development and advancement of subsurface monitoring tools is a key research area. Typically used 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 plume

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 (via logging and downhole sensors), but can become more challenging (and possibly expensive) when performing measurements over the large area typical of a geologic storage project.

Research Agenda and Challenges

Major challenges in subsurface monitoring include the need to monitor large areas and to distinguish between the pressure front and the injected CO₂ plume front. Additionally, tools for subsurface placement must withstand high pressures and temperatures and potentially deep, corrosive environments over very long periods. Many subsurface monitoring techniques were originally designed for oil and gas exploration and development and have been adapted for use in CO₂ storage applications. Some technologies, such as well logging and reflection seismic imaging, have reached a highly sophisticated level as a result of decades of utilization in the petroleum industry. An initial focus of R&D activities for MVA of geologic carbon storage has been to adapt these methods to the specific requirements of CO₂ injection, storage, and long-term monitoring. Some of these subsurface monitoring technologies are now available commercially and are being utilized in injection projects to identify formation characteristics and track CO₂ migration and pressure changes in the reservoir.

Many subsurface monitoring techniques do not directly measure CO₂; rather, they detect changes in another property, such as seismic velocity, electrical resistivity, or chemical changes in formation fluids, which may then be interpreted to provide relevant information about CO₂.
 
NETL’s research pathways for new subsurface monitoring research include:

• By 2020: Develop methods for improved geophysical imaging and detection of CO₂ plumes, specialized subsurface sensors, and methods to distinguish between pressure front, brine front, and plume front. The development of long-term monitoring tools that can withstand long-term exposure to subsurface conditions is also included in this research area.
   
• By 2030: Develop and test novel geophysical approaches that utilize long-term and permanent tools for direct and indirect measurements of CO₂ saturation and employ high-resolution and cost-effective monitoring. Projects to develop advanced data analysis and modelling methods for distinguishing pressure front, brine front, and CO₂ plume front are also included in this research area.

The figure below shows an approximate timeline for technology development along both pathways.

NETL-Supported Subsurface Research

NETL supports projects that are addressing research challenges within the Subsurface Monitoring key technology area. For instance, projects supporting this key technology area have included: improvement of seismic data interpretation; novel approaches to tracking CO₂ plumes; integration of monitoring data into simulation efforts; development of remote sensing capability for assessment over large areas; and development of monitoring tools to detect potential CO₂ leakage events. 
 
The MVA webpage  offers links to detailed information on projects performing research in this area.