Compliance Monitoring for Carbon Sequestration: Webinar by Battelle and Hart Energy, August 20, 2025: Review and Summary

  

     This webinar covered the multiple types of monitoring and monitoring wells required for carbon sequestration wells. It was presented by Jason Vanderkooi, Division Manager, Carbon Services, for Battelle.

     Sequestration wells must be approved as Class VI injection wells. Generally, it takes 18-24 months for Class VI approval, depending on the state and circumstances. The five phases of a Class VI injection well are shown below.

     Sequestration projects typically begin with geo-modeling, including seismic interpretations. Along with the CO2 injection wells, monitoring wells are required. The injection wells may receive CO2 for 30 years – the same amount of time Battelle has been studying carbon sequestration. The full period of monitoring during and after injection can be 80-130 years, with post-injection monitoring being 50-100 years.

     One important parameter to monitor is the critical pressure front, which could potentially lift fluids. Both injection zone and out-of-zone monitoring wells are required. Typically, the zone above the caprock or confining layer is monitored for any CO2 leakage. Local groundwater monitoring wells are also required. Seismicity may also be monitored.

     As detailed below, there are three types of compliance monitoring for carbon sequestration wells: 1) assurance monitoring, 2) operational monitoring, and 3) verification monitoring. It is also critical to get baseline monitoring from all relevant zones prior to injection. He notes that seasonal variation can affect monitoring and baseline monitoring.

     Groundwater monitoring wells monitor for any changes in groundwater that could be caused by leaking CO2 or land changes as a result of injection. An increase or change in total dissolved solids TDS can indicate leaking or fluid movement into groundwater. An increased concentration of CO2 is an obvious indicator. A decrease in groundwater pH can also indicate leakage. The chain of custody of sampling is critical.

     Above confinement zone monitoring well and injection zone monitoring well. Pressure and temperature are continuously monitored in injection wells and out-of-zone monitoring wells. Monitoring wells may be perforated so a sample can be taken. This is not required as there are other ways to get samples. Injection zone monitoring wells should see a change in pH, TDS, and CO2 concentration. Soil gas or surface air may also be required to be monitored. For this type of monitoring, it is important to avoid other sources of surface CO2, like feed lots. Battelle developed with the National Science Foundation what they call the NEON monitoring plan.

     Operational monitoring is done while injecting. Annulus pressure and temperature are monitored as well. Mechanical integrity testing (MIT) of injection wells is very important. MIT involves pressuring up and holding pressure. There can be internal MIT and external MIT (annulus). Acoustic and temperature logs may also be run in the well to search for and detect fluid movement.

     Verification monitoring involves comparing direct monitoring to simulations over time. Verification monitoring may be direct measurement monitoring or indirect monitoring, which includes passive seismic monitoring. USGS seismometers can provide background seismic monitoring. Passive seismic monitoring involves installing geophones in wellbores or shallow holes. Active seismic monitoring is 2D/3D reflection seismic surveys.

     When CO2 displaces water, it should show up on active seismic. Equinor’s long-running Sleipner Field sequestration project in the North Sea is a good example, where 9 seismic lines have been shot over the past 24 years. 

     Vertical seismic profiles (VSPs), or borehole seismic, can be used to get a 2D slice through the reservoir. It may be convenient to leave geophones downhole, but that is often not practical. Fiber optics can be installed for continuous listening, much like a geophone. Surface orbital vibrators can be used as a signal source for additional data. 

    The EPA may require more indirect surveying or more often. Time-lapse gravity monitoring can show gravitational changes. InSAR, satellite-based radar, can measure very small land changes. CO2 injection is known to alter land surfaces, often by small but measurable amounts. Any potential fluid movements, surface release of CO2, or any USDW source danger must be reported to the EPA. Leaks and any dangers to drinking water must also be reported to emergency response officials. State regulations must also be followed. Additional monitoring may be a requirement for approval of additional state credits. California, in particular, has such requirements. States may add additional monitoring requirements. Louisiana requires cross-well electromagnetic monitoring to help find old wells that could be affected. Regulations continue to change and grow as more projects come online.

     In the Q&A section, there was a question about monitoring costs. He replied that there were no rules of thumb yet for the cost of monitoring. Currently, there are no requirements for corrosion monitoring, but it can be very important to limit corrosion. It is an economic issue. When CO2 mixes with water, it can become very corrosive. Another question involved Class VI preparation. Regional data is often used. Battelle likes to drill a test well. Another question involved casing grades. EPA wants higher-quality casing in some areas. Other places are OK with EOR casing grades. Purer streams of CO2 require higher grades of casing due to higher corrosion likelihood.