Bubbling methane around subsea infrastructure is an alarming discovery ahead of decommissioning – but is it a leak from the reservoir, or harmless shallow gas?

During abandonment planning on a North Seafield, ROV inspection found gas in multiple annulus void spaces across around a dozen wells. APT combined gas and fluid geochemistry – composition and stable carbon and hydrogen isotopes (δ¹³C & δD) – with wellsite gas interpretation in its Girasol software to establish the gas type, source and whether it posed a problem.

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Scope of Work

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Following ROV inspection that identified gas in the B and C annuli across around a dozen wells, APT designed and delivered an integrated geochemical well-integrity evaluation.

• Gas composition and stable isotope (δ¹³C & δD) analysis of annulus gases.
• Provenance assessment against benchmark reservoir/injection gases – biogenic vs thermogenic vs corrosion.
• Fluid finger printing on recovered annulus liquids – whole-oil GC, biomarkers and isotopes – compared to reservoir oil and drilling fluids
• Interpretation of wellsite gas data through the overburden using APT's Girasol software
• Integration of all datasets to identify gas source(s) and inform the remediation programme

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“Bubbling methane around infrastructure can be a concerning discovery – but if geochemical analyses quantifiably confirms the source as recent, near-surface gas, rather than a reservoir leak, follow up considerations are very different. Gas geochemistry is inexpensive, rapid and delivers the quantifiable information you need to be sure about.”
– Craig D. Barrie,Senior Geochemist, APT

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Key Highlights

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• Gas in all but one annulus was diagnosed as shallow biogenic gas (methane-dominated, dryness >99%, methane δ¹³C heavier than −65‰) – recent sediment gas permeating in, not a deep reservoir leak.
• A single B-annulus gas carried a thermogenic signature (enriched methane, lower dryness), showing that one annulus had gained access to the reservoir via leaking casing, corrosion or a poor cement seal.
• Elevated hydrogen with corrosion-consistent deuterium isotopes flagged active corrosion in several annuli.
• Are covered annulus liquid was resolved as mostly drilling fluid with a minor reservoir-oil component - independently confirming reservoir access on that well.
• Girasol interpretation of the wellsite gas data mapped biogenic, thermogenic and drilling-influenced intervals through the overburden and across cement plugs, complementing the annulus results.

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The first line of defence. Gas geochemistry targets exactly the component you need to be sure isn't leaking — and it is inexpensive and rapid relative to intervention. Diagnosing bubbling methane as recent, near-surface gas rather than a deep reservoir leak lets operators report with confidence.

Conclusion

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By fingerprinting the gases and fluids themselves, APT resolved a critical well integrity/decommissioning question: almost all of the annulus gas was shallow, recent and unrelated to the deep reservoir, with a single reservoir-connected annulus isolated for targeted remediation. Integrating Girasol wellsite gas interpretation with annulus geochemistry gave a full picture of where hydrocarbons sit in the wellbore and overburden. Delivered from targeted analyses at a fraction of the cost of intervention, the workflow gives operators defensible answers – and is set to be increasingly central to safe, cost-effective decommissioning.