The burden of proof: monitoring CCS in saline aquifers vs hydrocarbon reservoirs

Frequently asked questions

• What does “measurement, monitoring and verification (MMV)” mean in CCS, and why is it so important?

  MMV is the evidence package that shows injected CO₂ is behaving as predicted, staying contained and not creating unacceptable risks. For operators, it is the practical way to meet a high burden of proof: you must demonstrate storage performance not just during injection, but for the long term. MMV typically combines direct measurements, such as subsurface pressure and temperature data from wells, with wider-area surveillance, such as time-lapse (4D) seismic and modelling that forecasts plume movement. The value is not only reassurance; it enables early warning and faster response if behaviour deviates. MMV also supports permitting and regulatory compliance, helps protect people and ecosystems from leakage impacts, and underpins investor confidence in CCS as a credible decarbonisation tool.

• Which monitoring technologies are most commonly used to detect CO₂ movement and potential leaks?

  CCS monitoring usually relies on a layered set of tools because no single technology provides a complete picture. In wells, high-resolution pressure and temperature sensors give continuous insight into how the store is responding to injection, while acoustic fibre-optic sensing can detect subtle changes linked to flow or integrity issues. Chemical tracers can help confirm CO₂ presence and movement. For a site-wide view, 4D seismic is widely used to map plume migration over time by comparing repeated surveys. In saline aquifers, electrical resistivity tomography (ERT) can also be valuable because it detects changes as conductive brine is displaced by relatively non-conductive CO₂. The most robust approach typically fuses sensor data with reservoir models, improving confidence, narrowing uncertainties and helping pinpoint where to investigate if anomalies appear.

• Why do depleted hydrocarbon reservoirs and saline aquifers need different CCS monitoring strategies?

  The geology and operating conditions differ in ways that change what you can measure and how clearly you can interpret it. Depleted reservoirs often start with a major advantage: decades of seismic and subsurface data gathered during exploration and production, plus existing wells that can sometimes be repurposed for monitoring. However, they may have complex stress histories and, critically, legacy wells that can become leakage pathways if cement degrades or sealing is inadequate. Saline aquifers, by contrast, usually have less historical data and infrastructure, making proof of storage harder to assemble. Pressure management is also more sensitive because aquifers are already brine-filled at hydrostatic pressure, so monitoring focuses heavily on pressure changes and possible surface deformation, alongside plume tracking.

• Is 4D seismic the gold standard for proving CO₂ storage?

  4D seismic is often described as a gold standard because it can provide a field-wide picture of plume evolution, which is difficult to achieve with point measurements alone. It tends to perform particularly well in saline aquifers, where contrasts between brine and CO₂ and simpler pressure behaviour can make changes easier to image. That said, it is not a universal solution. Cost can be high, repeatability can be challenging, and in depleted reservoirs the seismic response may be patchy, with some areas falling below detection thresholds even after years. In practice, 4D seismic is most persuasive when integrated with other monitoring streams, such as downhole pressure and temperature, tracers and modelling, so that limitations in one dataset are compensated by strengths in another.

• Who is responsible for monitoring CCS, and what role do regulators play?

  Operators carry the core responsibility for monitoring, assessing leakage risk and demonstrating ongoing containment, because they control injection, wells and data collection. This is why well integrity and performance monitoring are treated as central, especially given that wells can be both essential infrastructure and the most likely leakage pathway. Regulators increasingly shape what “good” looks like by requiring a monitoring plan, reviewing models and assumptions at the permitting stage, and retaining the right to inspect monitoring reports and equipment. In the UK, for example, the regulator must be satisfied there is no significant leakage risk and agrees an ongoing verification plan. Elsewhere, government involvement varies widely, from strong policy frameworks to gaps or nascent regulation, which can affect investor confidence and the consistency of long-term assurance.