Carbon Capture Utilisation and Storage (CCUS) is transitioning from isolated demonstration projects to basin-scale storage developments involving multiple operators, shared reservoirs, and increasingly complex regulatory and commercial frameworks. Traditionally, storage site assessment has revolved around three fundamental criteria: capacity, injectivity, and containment. While these remain necessary conditions for successful storage, they may no longer be sufficient. As CCUS scales, stakeholders increasingly require confidence not only that storage systems will perform as predicted, but that deviations from expected performance can be detected and managed before material consequences emerge.
This paper argues that CCUS risk management should evolve from a philosophy centred on periodic verification towards one based on continuous assurance. Drawing upon concepts established within insurance engineering and industrial risk management, the paper introduces monitorability as a fourth pillar of storage project assessment. It proposes that the ability to identify leading indicators of changing reservoir behaviour may become as important as the ability to model reservoir behaviour itself. Attention is given to basin-scale pressure communication, conformance monitoring, and the distinction between leading and lagging indicators in CCUS operations.
Introduction
The success of geological carbon storage fundamentally depends upon confidence. Confidence that injected CO2 will remain contained. Confidence that reservoir performance will conform to expectations. Confidence that operators can identify and manage emerging risks before they become material liabilities. Historically, this confidence has been supported by three principal storage criteria:
- Capacity
- Injectivity
- Containment
Together these criteria form the basis of storage site qualification and regulatory approval across most jurisdictions. However, CCUS is entering a new phase of development. Across Europe, North America, and other emerging storage provinces in the Asia Pacific region, project developers are moving beyond individual storage projects towards regional storage hubs and basin-scale networks. Storage formations that were once evaluated independently are increasingly being viewed as shared subsurface resources. Pressure footprints may overlap. Infrastructure may be shared. Multiple operators may become technically interconnected despite holding separate licences.
In this environment, new forms of uncertainty emerge that cannot be addressed solely through static characterisation or reservoir modelling. The challenge is no longer simply: Can we model the future? Increasingly, the challenge becomes: Can we recognise when the future is beginning to diverge from our model?
This paper proposes that monitorability should be considered a fourth pillar of geological carbon storage assessment, alongside capacity, injectivity and containment. The proposed evolution of storage assessment criteria is illustrated schematically in Figure 1. While the first three criteria describe whether a storage resource can theoretically function, monitorability determines how effectively operators can demonstrate that it is functioning as intended throughout its lifecycle. This challenge is amplified by the unusually long-time horizons associated with geological storage, where confidence may need to extend across decades of operation and much longer periods of post-closure stewardship.
The limits of predictive confidence
Modern CCUS projects are supported by increasingly sophisticated technical assessments. Project developers routinely employ: static geological models, dynamic flow simulation models, geomechanical analyses, fault seal assessments, pressure propagation modelling and probabilistic risk evaluations. These tools are essential. Without them, storage permitting and investment decisions would be impossible. Yet all models operate under a common constraint: they represent interpretations of reality rather than reality itself.
Subsurface systems contain: geological heterogeneity, unresolved fault structures, legacy well uncertainties, long-term geochemical processes, pressure interactions at scales larger than individual projects. As a result, uncertainty can never be eliminated entirely. This observation is not unique to CCUS. Oil and gas reservoirs, geothermal systems, underground gas storage facilities, and geological disposal repositories all exhibit similar limitations.
Experience from these industries consistently demonstrates that confidence is greatest when observed performance repeatedly confirms model predictions. The distinction between prediction and validation is important. Models create confidence before investment. Observed performance creates confidence after investment.
However, predictive confidence has limits. Models may continue to reproduce observed behaviour even as uncertainty in the underlying interpretation increases. History matching can support multiple plausible explanations, creating the risk of confidence in the wrong model. The key challenge is therefore not whether a model matches the data, but whether deviations from expectation can be recognised early enough to permit corrective action. As projects scale, that challenge becomes increasingly important.
Lessons from insurance engineering
The insurance industry offers a useful perspective on managing complex industrial risks. Few industrial hazards can be eliminated entirely. Instead, insurers have historically focused on improving: risk visibility, early warning capability, intervention effectiveness and loss mitigation. Examples are everywhere: fire detection systems, gas sensors, process alarms, automatic shutdown systems and sprinkler systems. The purpose of these systems is not to prevent every incident. Rather, their purpose is to ensure abnormal conditions are identified and corrected before they develop into catastrophic losses.
This philosophy creates an important distinction between: Lagging Indicators – indicators that become apparent only after consequences occur. Examples include: Structural damage, leakage events, regulatory breaches, and environmental impacts.
Leading Indicators on the other hand are indicators that signal changing risk conditions before material consequences emerge. Examples may include: unexpected pressure responses, changes in injectivity performance, abnormal plume migration, reservoir deformation and deviations from expected monitoring signatures.
The insurance industry has repeatedly demonstrated that leading indicators provide significantly greater value than post-event analysis. In simple terms, early warning often matters more than perfect hindsight.
Among the variables that govern storage performance, pressure occupies a unique position because it influences injectivity, capacity utilisation, geomechanical stability and, ultimately, containment confidence. Unlike many subsurface parameters, pressure can respond relatively quickly to changing reservoir conditions and may therefore provide one of the earliest measurable indicators that system behaviour is diverging from expectation. For this reason, effective pressure surveillance may become a cornerstone of future risk-management strategies.
As CCUS transitions towards basin-scale development, pressure itself may become a managed resource. Bump and Hovorka (2024) describe pressure space as a key subsurface commodity, highlighting the need to move beyond project-by-project optimisation towards regional resource stewardship. In this environment, monitorability becomes particularly important because pressure communication risks cannot be effectively managed if they cannot be observed, quantified and attributed.
Reframing MMV
Measurement, Monitoring and Verification (MMV) is frequently presented as a compliance requirement. While regulatory obligations remain important, this framing understates its true value. MMV should be considered a risk-management framework. Its primary function is not data collection, but uncertainty reduction. Just as storage capacity, injectivity and containment are assessed during the evaluation phase, the feasibility and effectiveness of monitoring should also be considered during site selection. A site that cannot reliably generate detectable monitoring signals may present a fundamentally different risk profile from a site where pressure, plume movement and conformance can be observed with confidence (Gendrin et al., 2014).
Traditionally, monitoring programs have often focused on generating periodic evidence that storage systems remain compliant. Increasingly, however, storage projects require something more dynamic. They require systems capable of: identifying changing conditions, validating assumptions, triggering investigation and supporting intervention. In this context, monitoring success should not be defined by the quantity of data produced. Instead, success should be measured by the ability to detect meaningful departures from expected behaviour. The future of MMV may therefore depend less upon producing additional data and more on generating timely and actionable information.
Information generates value only when it informs a decision. An effective MMV programme therefore requires not only data acquisition, but also predefined thresholds, interpretation frameworks and response pathways. Monitoring that simply demonstrates compliance satisfies regulatory obligations. Monitoring that supports timely intervention reduces risk. As storage projects mature, the distinction between compliance-driven monitoring and action-oriented monitoring may become increasingly important for operators, regulators and insurers alike.
Monitorability: The Fourth Pillar of Storage Assessment
Historically, site evaluation has focused upon: Capacity, Injectivity and Containment. All remain necessary. As CCUS scales, a fourth criterion deserves formal recognition: Monitorability, which can be defined as: The ability to detect, verify, interpret and respond to deviations from expected storage system behaviour within a time frame sufficient to avoid unacceptable consequences. As illustrated in Figure 1, monitorability complements rather than replaces the traditional storage criteria of capacity, injectivity and containment by introducing a framework for continuous assurance throughout the project lifecycle.
Importantly, monitorability should not be viewed solely as an operational consideration. It should form part of storage site screening and project design from the outset. Just as capacity, injectivity and containment are evaluated during site selection, the ability to observe and verify reservoir behaviour should also influence project development decisions. In many cases, monitorability is partially determined by geological and geographical conditions that cannot easily be modified once a project is underway.
Consider two storage sites with similar storage capacity, injectivity and containment characteristics. Site A is located beneath an area where reservoir responses are difficult to observe directly due to limited well control, complex overburden conditions and restricted opportunities for surface deformation monitoring. Site B possesses comparable storage characteristics but benefits from favourable conditions for pressure surveillance, conformance verification and deformation monitoring. Although both sites may appear similarly attractive from a traditional storage assessment perspective, Site B offers a greater ability to detect and interpret deviations from expected behaviour. From a risk-management perspective, that enhanced monitorability may represent a significant advantage, directly influencing operator, investor, regulatory and insurance confidence. This concept becomes increasingly important as project portfolios and CCUS hubs expand.
Pressure communication: A new category of risk
The issue of pressure communication illustrates why monitorability matters. As multiple storage projects increasingly occupy interconnected formations, pressure footprints can extend well beyond licence boundaries. While pressure communication has always been an inherent characteristic of large storage systems, the growth of regional storage hubs increases both the likelihood and significance of these interactions. Pressure communication creates challenges including reduced storage capacity, pressure-space competition, injection-rate constraints, impacts on neighbouring operators, potential conflicts with other subsurface users and increased commercial and regulatory uncertainty.
Historically, industry discussions have focused on modelling pressure communication. While necessary, modelling alone does not manage the associated risk. What ultimately matters is understanding: how pressure evolves in reality, whether real behaviour matches forecasts, and when intervention becomes necessary.
The key question is therefore not: Can we model pressure propagation? Rather: Can we identify early indicators that pressure behaviour is diverging from expectation? In this respect, CCUS faces a challenge similar to other industrial sectors. The most effective systems are not those that explain failure after it occurs. They are those that recognise warning signals before failure develops.
From periodic verification to continuous assurance
Historical monitoring approaches often relied upon periodic surveys separated by months or years. While valuable, such approaches can leave substantial intervals during which evolving risks remain poorly understood. Emerging monitoring concepts suggest a different future. In these systems: baselines are established and leading indicators are continuously tracked. Decision thresholds are defined. Escalation procedures are pre-planned and detailed investigations are triggered only when warranted.
This philosophy represents a shift from periodic verification to continuous assurance. Periodic verification asks what happened? Continuous assurance asks what is changing? For long-life CCUS assets, continuous assurance offers a more robust framework for risk management, regulatory oversight and insurance support.
A useful way of framing this challenge is to ask what does the first 0.7 Mt of injected CO2 tell us about the confidence we should place in injecting 7 Mt? The value of early operational data is not simply that it confirms a project is working. Its greater value lies in identifying subtle deviations from forecast performance before they become material. From a risk-management perspective, the most important monitoring results may therefore be the earliest indicators that challenge long-term assumptions rather than the eventual confirmation that those assumptions were correct.
Conclusion
The long-term success of CCUS depends on more than storage capacity, injectivity and containment. It depends upon confidence. Confidence emerges when stakeholders can demonstrate that storage systems continue to behave as expected throughout their operational life. As CCUS evolves towards basin-scale deployment, monitoring will increasingly become a central component of risk management rather than simply a regulatory requirement.
This evolution supports the recognition of monitorability as a fourth pillar of storage project assessment. Monitorability shifts the focus from: predicting future behaviour towards Identifying changes in behaviour before material consequences occur.
Ultimately, CCUS must move beyond a philosophy of periodic verification and adopt one of continuous assurance. The projects most likely to attract regulatory approval, investment capital and insurance support may not necessarily be those with the largest storage resources. They may be those capable of demonstrating, continuously and transparently, that observed performance remains aligned with prediction.
In subsurface risk management, confidence is not created by the absence of uncertainty. Confidence is created by the ability to identify, understand and respond to uncertainty as it evolves. Capacity, injectivity and containment may determine whether a storage project is viable. Monitorability may determine whether it remains investable, insurable and scalable. In that sense, early warning is not merely desirable; it may become one of the defining requirements of commercial-scale CCUS.
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