CO₂, the Subsurface, and Surveillance: Understanding Geological Storage Monitoring
Geological storage of CO₂ doesn't end once injection is complete. After capture, the CO₂ is directed into deep geological reservoirs, typically shielded by rock formations with low permeability. The challenge then becomes confirming that it stays confined, without any unwanted migration toward groundwater or the surface.
This verification is essential to the credibility of carbon capture and geological storage. According to the IPCC, a well-selected and well-managed site can retain more than 99% of injected CO₂ over long periods. [1]
This article outlines the principles that keep CO₂ trapped underground, along with the main tools used to track its behavior from the reservoir all the way to the surface.
What Keeps CO₂ Underground: The Mechanisms of Geological Trapping
Before looking at how stored CO₂ is monitored, it helps to understand why it tends to stay put. Geological storage relies on several trapping mechanisms that act simultaneously and whose effectiveness increases over time.
The Supercritical State and Injection Depth
CO₂'s physical behavior changes significantly depending on pressure and temperature. At depths generally greater than about 800 m, depending on the specific conditions at each site, CO₂ can reach what's known as a supercritical state. In this state it has a much higher density than in gaseous form, which allows a larger quantity of CO₂ to be stored within the same volume of rock. [1]
This state favors storage, but it isn't enough on its own to guarantee confinement. CO₂ generally remains less dense than the formation water, so it retains a natural tendency to migrate upward. Retention instead depends on a combination of several geological mechanisms: an effective caprock, residual trapping within the rock's pores, dissolution into formation fluids, and, over the longer term, mineralization. [1]
The Four Trapping Mechanisms
Geological CO₂ storage draws on several complementary processes whose relative importance shifts over time. Short-term mechanisms, such as structural trapping, are gradually taken over by more permanent processes like dissolution and mineralization. These unfold across a range of timescales, from relatively fast reactions to processes that can span hundreds or even thousands of years, depending on the minerals involved. [1]
Structural trapping (caprock): CO₂ is held beneath a very low-permeability cap rock, which limits its vertical migration into the formations above. This is the first mechanism to come into play after injection.
Residual trapping: CO₂ becomes immobilized as disconnected bubbles within the pores of the reservoir rock, held in place by capillary forces where it meets the formation water.
Dissolution trapping: CO₂ dissolves into the formation fluids, creating a slightly denser brine that tends to sink gradually. [1] This mechanism further reduces the risk of long-term migration.
Mineral trapping: over the long run, CO₂ reacts with the rock's minerals to form solid carbonates, locking it permanently within the rock matrix. [1]
Monitoring CO₂: Tools Deployed from the Reservoir to the Surface
Overseeing a geological storage site relies on several complementary tools. Some track the CO₂ plume at depth, while others watch well integrity, groundwater chemistry, or surface CO₂ flux. The goal isn't to depend on a single method, but to cross-reference several types of measurements to get a more reliable picture of the site.
4D Seismic: Imaging the CO₂ Plume at Depth
Time-lapse seismic, also called 4D seismic, is one of the main tools for visualizing how the CO₂ plume evolves spatially within the reservoir. It involves repeating three-dimensional seismic surveys at regular intervals, then comparing the resulting images to spot changes that have occurred underground between campaigns. These changes can reveal how CO₂ is progressing through the porous formation, or confirm that the caprock is fulfilling its role as a barrier.
The Sleipner site in the North Sea is one of the best-documented examples of 4D seismic surveillance. Repeated surveys there have made it possible to track how the CO₂ plume evolved within the Utsira Formation, gain a better understanding of its distribution in the reservoir, and compare field observations against simulation models. [2] [3] This kind of tracking helps confirm whether the CO₂'s actual behavior matches the projections made before and during injection.
Fiber Optics and Wellbore Monitoring
Injection wells are sensitive points within a geological storage site, since they connect the surface to the deep reservoir. Their integrity therefore needs close attention, both during injection and afterward. Distributed fiber-optic sensing systems offer a solution that complements more localized geophysical methods: a fiber installed along a well can act as a continuous sensor, capable of recording changes in temperature, vibration, or strain along its entire length. [6]
This approach is valuable because it allows the well to be watched at multiple depths, rather than at just a handful of isolated measurement points. It can help detect changes tied to injection conditions, cement and casing integrity, or possible fluid migration near the well. Fiber optics don't replace seismic surveys, geochemistry, or surface measurements, but they add a layer of continuous oversight right next to the injection infrastructure.
Three types of measurement are commonly associated with these systems:
DTS (Distributed Temperature Sensing): DTS measures temperature along the fiber. Unusual variations can point to a shift in injection conditions, fluid movement near the well, or a thermal anomaly linked to cement or casing integrity. This measurement is especially useful for tracking the well's thermal behavior and flagging signals that warrant closer analysis. [5]
DAS (Distributed Acoustic Sensing): DAS turns the fiber into a distributed acoustic sensor. It records vibrations and seismic signals along the well, which can support seismic monitoring, injection-profile analysis, or the detection of certain events linked to fluid flow and well integrity. [5]
DSS (Distributed Strain Sensing): DSS measures mechanical strain along the fiber. It can provide information on wellbore deformation and, combined with geomechanical interpretation, help assess stresses and mechanical responses within the well-reservoir system. [6]
In practice, the main value of fiber optics lies in their ability to deliver continuous, localized, and complementary coverage. That said, the resulting data needs to be interpreted alongside the site's other observations, since a change in temperature, vibration, or strain doesn't automatically mean there's a leak. It's better understood as a signal to be examined within the project's geological and operational context.
Groundwater Geochemistry and Surface Monitoring
Geochemical monitoring aims to catch changes in groundwater composition that could signal CO₂ or deep fluids migrating toward aquifers above the reservoir. It's not just about looking for CO₂ directly; it also involves watching the chemical reactions that can occur as water, rock, and formation fluids interact.
A monitoring program can track several geochemical parameters:
pH and water chemistry: a drop in pH, along with shifts in the concentration of certain dissolved elements, can accompany the arrival of CO₂ in groundwater;
dissolved elements: elements such as Na, K, Ca, Sr, and Si can change in response to interactions between dissolved CO₂, water, and the aquifer's minerals;
isotopic tracers: the isotopic composition of dissolved inorganic carbon (δ¹³C) and the ⁸⁷Sr/⁸⁶Sr ratio can help track the movement of a CO₂ plume and shed light on the geochemical reactions that accompany it. [7]
In an experimental study where dissolved CO₂ was introduced in a controlled way into an aquifer, Newell et al. found that the plume's arrival at the monitoring wells coincided with a drop in pH and shifts in the concentration of several dissolved elements. Changes in δ¹³C and ⁸⁷Sr/⁸⁶Sr also made it possible to track the plume's movement and certain reactions between the CO₂, groundwater, and aquifer minerals. [7] These findings highlight the benefit of combining multiple geochemical indicators to detect and interpret potential changes in groundwater.
Surface monitoring rounds out this approach by observing what happens at ground level and in the air above the site. The goals differ but complement one another:
accumulation chambers measure the CO₂ flux emitted from the soil at specific points;
eddy covariance measures CO₂ exchange between the ground and the atmosphere across a wider area;
atmospheric remote sensing, particularly with tools like DIAL, can be used to map abnormally high CO₂ concentrations. [9]
These methods still call for careful interpretation, since soils naturally produce CO₂ through root respiration and the breakdown of organic matter. These emissions vary with the seasons, humidity, temperature, and local terrain characteristics. Baseline measurements taken before injection are therefore essential for establishing the natural background level and telling a normal variation apart from an unusual signal. Geochemical and surface monitoring thus fit into a multi-layered program, where each measurement helps confirm whether the CO₂ remains confined and whether the site is evolving as expected.
Table I: Key Technologies for Monitoring Stored CO₂ by Depth and Objective
| Technology | Zone Monitored | Measurement Type | Objective |
|---|---|---|---|
| 4D Seismic (time-lapse) | Deep reservoir | Indirect (geophysical) | Plume tracking, caprock verification |
| DAS / DTS / DSS (fiber optics) | Well (surface → reservoir) | Distributed: acoustic/seismic, temperature, strain | Well integrity and behavior, injection profiles, geomechanical signals |
| Groundwater geochemical monitoring | Intermediate aquifers | Direct (chemical) | Detection of migration toward groundwater |
| Soil flux / accumulation chambers | Surface | Direct (gas flux) | Detection of surface leaks |
| DIAL / eddy covariance | Atmosphere | Optical / micrometeorological | Concentration mapping and estimation of atmospheric fluxes |
| InSAR (satellite) | Surface | Indirect (deformation) | Surface deformation; inference of pressure changes and geomechanical behavior |
| Microseismicity | Reservoir / caprock | Indirect (acoustic) | Detection of fault slip, caprock behavior |
InSAR and Microseismicity: Monitoring the Site's Geomechanical Integrity
Beyond tracking CO₂ directly within the reservoir, a monitoring program also needs to watch how the subsurface responds mechanically to injection. Rising pressure can cause slight deformation, alter stresses around the reservoir, or reactivate small geological discontinuities. These signals don't directly measure the presence of CO₂, but they help assess the stability of the reservoir, the caprock, and the surrounding structures.
Two approaches are particularly relevant for this type of oversight:
InSAR, to detect surface deformation: satellite radar interferometry compares radar images taken at different times to measure extremely small surface movements, sometimes down to the millimeter scale. In the context of geological CO₂ storage, this deformation can point to pressure changes at depth. InSAR therefore offers an indirect way to follow the geomechanical behavior of the reservoir and caprock, provided the results are interpreted using suitable models. [4]
Microseismicity, to listen to the subsurface's response: microseismic monitoring picks up small microseismic events tied to pressure changes and deformation in or around the reservoir. Locating and characterizing these signals can shed light on the rock's mechanical behavior, the presence of active faults, or how stresses are evolving. These events shouldn't automatically be read as signs of a leak; their meaning depends on the geological setting, the rock type, and the injection history.
These methods complement seismic surveys, geochemistry, and surface measurements. Their main value is providing an indirect read on the site's geomechanical integrity, helping flag changes that might call for closer analysis.
Conclusion
Monitoring stored CO₂ relies on a combination of natural geological mechanisms and complementary surveillance technologies. From the deep reservoir to the surface, these tools make it possible to track CO₂ behavior, assess caprock integrity, and catch potential anomalies. According to the IPCC, for well-selected and well-managed sites, it's very likely that more than 99% of injected CO₂ will be retained over the first 100 years, and likely that retention will stay above 99% over 1,000 years. [1]
This oversight also addresses a matter of trust. In a landscape where emissions reductions must be measured, verified, and accounted for rigorously, demonstrating that CO₂ stays confined becomes essential to the credibility of geological storage projects. Every site has its own characteristics, which calls for an approach tailored to local conditions and potential risks.
Geological CO₂ storage thus illustrates just how central subsurface understanding is to the energy transition. Geology, engineering, and regulation all need to work together to meet objectives that are technical, environmental, and climatic at once.
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Note: The information presented in this article is general and informational in nature. It does not constitute technical, regulatory, or operational advice, and should not be interpreted as representing any specific projects, fieldwork, or commitments made by Squatex in the CCUS space.
References
[1] Intergovernmental Panel on Climate Change. "Underground Geological Storage." Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, 2005, pp. 195–265. IPCC, https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapter5-1.pdf.
[2] Corlay, Q., L. Tortarolo, and L. Souche. "4D Seismic Monitoring of Sleipner Field Leveraging a New Quantitative 4D Time-Shift Estimation Workflow." World CCUS Conference 2025, European Association of Geoscientists & Engineers, Sept. 2025, vol. 2025, pp. 1–5. EarthDoc, https://www.earthdoc.org/content/papers/10.3997/2214-4609.202522040.
[3] Fornel, Alexandre, and Audrey Estublier. "To a Dynamic Update of the Sleipner CO₂ Storage Geological Model Using 4D Seismic Data." Energy Procedia, vol. 37, 2013, pp. 4902–4909. Elsevier, https://www.sciencedirect.com/science/article/pii/S1876610213006449.
[4] Bohloli, Bahman, et al. "Monitoring CO₂ Storage Sites Onshore and Offshore Using InSAR Data and Strain Sensing Fibre Optics Cables." Proceedings of the 16th Greenhouse Gas Control Technologies Conference (GHGT-16), 2022. SSRN, https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4286039.
[5] Syed, Talib. Fiber-Optic Monitoring of Well Integrity of CO₂ Storage Wells. Ground Water Protection Council, 2023. https://www.gwpc.org/wp-content/uploads/2023/01/CCS_TalibSyed.pdf.
[6] Macquet, Marie, Dana Jurick, Don Lawton, and Artur Guzik. "Using Continuous Fiber Optic for Carbon Storage Monitoring: DSS and DAS Applications from the CMC-CaMI Newell County Facility." GeoConvention 2024, GeoConvention, 2024. https://geoconvention.com/wp-content/uploads/abstracts/2024/103990-using-continuous-fiber-optic-for-carbon-storage-mo.pdf.
[7] D.L. Newell, T.E. Larson, G. Perkins, J.D. Pugh, B.W. Stewart, R.C. Capo, R.C. Trautz, Tracing CO2 leakage into groundwater using carbon and strontium isotopes during a controlled CO2 release field test, International Journal of Greenhouse Gas Control, Volume 29, 2014, Pages 200-208, https://doi.org/10.1016/j.ijggc.2014.08.015.
