Development of a Quantitative Wellbore Leakage Risk Assessment Tool for Screening Storage Projects (Q-WELLRATe) (Flexible Funding 2024)

Dr Aaron Cahill, Heriot-Watt University, was awarded funding in the UKCCSRC’s Flexible Funding 2024 call to look at “Development of a Quantitative Wellbore Leakage Risk Assessment Tool for Screening Storage Projects (Q-WELLRATe)”.

Decommissioned oil and gas wells may pose a hidden risk for safe CO₂ storage. That’s why we developed Q-WellRATE, a fast, physics-based model to help assess legacy well potential leakage and support the UK’s carbon storage ambitions.

Understanding the Challenge

As the UK advances its plans for large-scale carbon capture and storage (CCS), the safe containment of CO₂ underground is essential. A particular challenge for many North Sea storage sites is the presence of thousands of legacy wells, old oil and gas wells drilled and abandoned over previous decades. Even when sealed, these wells can potentially act as pathways for CO₂ leakage if plugs degrade or go undetected, threatening project success and public trust.

Traditional approaches to legacy well risk assessment are often slow and require detailed site-specific data, limiting their usefulness for screening large numbers of wells. Recognising this, our team set out to create a new tool that balances scientific rigour with speed and flexibility.

The Q-WellRATE Solution

Q-WellRATE is a simplified, physics-based model designed for rapid estimation of CO₂ leakage rates along compromised legacy wells. Developed at Heriot-Watt University with support from UKCCSRC, it allows users to:

• Simulate a wide range of geological and engineering scenarios, including different well designs and abandonment practices
• Incorporate depth-dependent temperature, pressure, and fluid properties for realistic non-isothermal flow
• Run thousands of simulations per hour to enable screening and spatial risk mapping across entire regions

The model aims to support both high-level regional studies and targeted analysis of individual wells, helping operators, regulators, and policymakers prioritise resources and interventions.

Key Insights and Results

In applying Q-WellRATE to North Sea scenarios, several important findings emerged:

• Cement plug integrity is critical: even minor defects can significantly increase leakage risk
• Plug length has diminishing returns: after a certain threshold, longer plugs provide little additional benefit, informing cost-effective design
• Site-specific factors matter: geology, temperature, and pressure influence risk, but wellbore condition remains the overriding control
• Fast, transparent assessment: the model’s efficiency allows for rapid “area of review” studies, supporting timely decision-making

Stakeholder Engagement

A key part of the project was collaboration with stakeholders, including the North Sea Transition Authority, Shell, and Storegga. Their technical feedback guided the model’s development, from simulating multi-modal well failures to integrating real-world abandonment designs. Engaging with both regulators and industry has helped Q-WellRATE address practical challenges and better meet potential user needs.

Looking Ahead

Our main technical output, i.e. a draft manuscript prepared for peer-reviewed publication, details the Q-WellRATE methodology, findings, and implications for CCS project safety. Looking forward, we plan to refine the model further, explore more complex scenarios, and share our work with the wider CCS community.

By providing a robust and rapid tool for legacy well risk assessment, Q-WellRATE supports the UK’s journey toward safe, large-scale carbon storage and helps turn old infrastructure from a challenge into an opportunity.

Schematic illustration of a legacy well (left) and CO2 injection well (right) with potential leakage pathways annotated. A: leakage via damaged casing, B: leakage via cement plugs and open hole, C: leakage via damaged annular cement

Read more on Aaron’s Flexible Funding 2024 project page.