Once CO2 has been stored in the subsurface, scientists and regulators need to ensure it remains safely and permanently stored in the target Area of Review (AoR) and that all underground sources of drinking water (USDWs) are protected. While the evidence attained from enhanced oil recovery (EOR), research, and current commercial-scale carbon capture, utilization and storage efforts strongly suggests that CO2 storage is safe and sustainable, sites still need to be well-selected, well-designed, and operated appropriately. Risks associated with these large-scale projects must be identified, quantified and closely monitored throughout any project’s lifecycle.
Monitoring Tools
Monitoring is an important part of every carbon capture and storage project, occurring before (baseline), during and after the injection phase. Operators monitor to fulfill obligations such as: locating the injected CO2 plume, re-evaluating AoR, and ensuring that potable water sources and ecosystems are protected throughout the lifecycle of the storage project.
How and where does monitoring occur? Tools are typically categorized as atmospheric, near-surface or subsurface.
Atmospheric Monitoring Tools
Atmospheric monitoring involves testing at the surface and in the atmosphere to identify and quantify possible releases associated with carbon storage operations. The monitoring plan must specify a strategy for detecting and quantifying surface release of CO2 and an approach for establishing baselines for monitoring CO2 surface releases. A reliable, above-ground monitoring system can detect elevated levels of atmospheric CO2 that may have been released from wellbores, faults or other conduits. For example, optical CO2 sensors may be deployed aboveground to monitor potential release of CO2 to the atmosphere.1DOE. (2017). Best practices: Monitoring, verification, and accounting (MVA) for geologic storage projects (revised edition). U.S. Department of Energy. DOE/NETL-2018/1847.
Near-surface Monitoring Tools
Near-surface monitoring involves testing in the vadose zone (above the water table) and groundwater sources to identify and quantify possible releases associated with carbon storage operations. With this type of monitoring, scientists are looking to identify if CO2 has leaked from deep geologic storage reservoirs into the shallow subsurface. For example, scientists may perform geochemical sampling of shallow groundwater above the CO2 storage reservoir to demonstrate isolation of the reservoir from a USDW.2DOE. (2017). Best practices: Monitoring, verification, and accounting (MVA) for geologic storage projects (revised edition). U.S. Department of Energy. DOE/NETL-2018/1847.
Subsurface Monitoring Tools
Subsurface monitoring involves testing to locate CO2 in the target and surrounding storage formations. Objectives include monitoring the evolution of the dense-phase CO2 plume, assessing the area of elevated pressure caused by injection, and determining that both pressure and CO2 are within the expected and acceptable areas and migrating in a way that does not damage resources or compromise subsurface integrity. Tracking the movement of an injected CO2 plume in a deep geologic formation can include defining the lateral extent and boundaries of the plume as expected by the U.S. Environmental Protection Agency under Class VI rules to show that the plume remains in the AoR. For example, fluid sampling can be conducted at wells distant from the Class VI injection well to assess movement of CO2 in the subsurface.3DOE. (2017). Best practices: Monitoring, verification, and accounting (MVA) for geologic storage projects (revised edition). U.S. Department of Energy. DOE/NETL-2018/1847.
Case Study: Subsurface Monitoring at Bell Creek Oil Field, Montana
Denbury (operator), the Plains CO2 Reduction (PCOR) Partnership, and the U.S. Department of Energy are studying associated CO2 storage incidental to a commercial enhanced oil recovery operation at the Bell Creek oil field.
Associated CO2 Storage
Over years of operating an EOR project, there are many cycles of CO2 injection. With each cycle, additional injected CO2 becomes permanently stored in the oil reservoir through a combination of several trapping mechanisms. At the end of the EOR project, nearly all of the CO2 purchased for injection remains naturally trapped in the reservoir.4University of North Dakota Energy & Environmental Research Center. (2017). Fact Sheet: Bell Creek Project – Enhanced Oil Recovery Resulting
in Associated CO2 Storage. https://undeerc.org/pcor/images/resources/Bell%20Creek_CO2%20Sequestration%20Monitoring.pdf
The following video demonstrates the installation of a well casing-conveyed permanent downhole monitoring system developed by the University of North Dakota’s Energy and Environmental Research Center. This type of monitoring technology that can be used at geologic storage and enhanced oil recovery sites for continuous pressure and temperature monitoring of the producing or storage reservoir and the overlying rock layers. This monitoring system provides real-time information to support decision making and reservoir performance evaluations.5NETL. (n.d.). Monitoring, Verification, and Accounting (MVA) Focus Area. National Energy Technology Laboratory. Retrieved June 21, 2021, from https://netl.doe.gov/coal/carbon-storage/core-storage-research-development/monitoring-verification-and-accounting-focus-area
The results of the research at Bell Creek are expected to benefit both dedicated CO2 storage—where eliminating emissions of anthropogenic CO2 is the primary purpose of underground injection—and associated storage of either anthropogenic or geologic CO2 that occurs as a natural part of CO2-EOR operations.6University of North Dakota Energy & Environmental Research Center. (2017). Fact Sheet: Bell Creek Project – Enhanced Oil Recovery Resulting
in Associated CO2 Storage. https://undeerc.org/pcor/images/resources/Bell%20Creek_CO2%20Sequestration%20Monitoring.pdf