In a recent publication and press release, researchers from the German GEOMAR Helmholtz Centre for Ocean Research in Kiel reported on the presence of seabed methane leaks in the UK Central North Sea. The leaks are thought to be related to abandoned wells and the conclusion was that the gas does not originate from the reservoir targeted by these wells, but from gas pockets at much shallower depths. The gas is thought to migrate to surface along the outside of the well bore.
During expeditions with RV POSEIDON in 2017 and 2019, the researchers were able to detect gas leakage at 28 of 43 directly investigated wells. “The propensity for such leaks increases the closer the boreholes are located with respect to shallow gas pockets, which are normally uninteresting for commercial use. Apparently, however, the disturbance of the overburden sediment by drilling process causes the gas to rise along the borehole,” explains Dr. Matthias Haeckel from GEOMAR, who led the study.
This find highlights an important aspect of the oil and gas industry that will need to be addressed by both the regulators and the operating companies.
Expronews spoke to Francis Buckley, who has spent many years identifying and interpreting shallow gas hazards for the oil and gas industry. Although Buckley admits that shallow gas may be a source for gas leaks, he does reiterate that the only way to find out whether the gas is of biogenic (shallow) or thermogenic (deep) origin is isotopic analysis.
“The industry has always been acutely aware of the hazards shallow gas pockets present and that’s the reason why the overwhelming majority of wells drilled in the North Sea scrupulously avoid them,” Buckley adds. He is therefore curious to see follow-up research that is able to confirm the shallow gas origin of these leaks.
There have been several recent instances of North Sea operators reporting the development of shallow gas anomalies around well casing shoes, suggesting possible leakage from the well and therefore a thermogenic source for the gas. Furthermore, leaking wellheads have long been a source of concern for operators
There have been a few incidents related to drilling through shallow gas pockets in the North Sea. One that recently received attention is the 22/4b blowout in 1990. Greenpeace reported on 14 August this year that the craters caused by the blow-out are still emitting large quantities of methane to this day.
This has long been known, as the area has been under surveillance ever since the incident. The researchers from GEOMAR also report on this blow-out and the associated release of methane. In fact, the release of methane from this single event exceeds the estimated release of methane from all other wells in the study area, which only emphasises the work that Buckley and others in the geohazards sector have been doing for a long time.
However, there are countless shallow gas accumulations documented in this part of the North Sea and natural seepage may also play a significant part in methane release to the atmosphere as do drilling-related disturbances or leaking well completions, so more research on this phenomenon is probably needed.
Shallow gas is not always considered a drilling hazard; in a few cases it is being commercially exploited. For instance, the Aviat field in the UK Central North Sea produces biogenic gas from a Lower Pleistocene sand at a depth of around 800 m. The gas is used as fuel gas to power the Forties field power turbines.
In the Dutch sector, a number of shallow gas fields in the northernmost offshore also produce gas from Pleistocene unconsolidated sands. In the Norwegian North Sea, a good example of a shallow gas accumulation that is considered for commercial exploitation is the Peon field in Quadrant 35. The Pleistocene reservoir of this field lies at a depth of only 210 m below the sea floor, which poses challenges to the way the field can be developed.
Böttner, C., M. Haeckel, M. Schmidt, C. Berndt, L. Vielstädte, J. A. Kutsch, J. Karsten, T. Weiß (2020): Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection, monitoring and mitigation strategies. International Journal of Greenhouse Gas Control, https://doi.org/10.1016/j.ijggc.2020.103119