Anthropogenic emissions of greenhouse gases from the seafloor into the North Sea

The present thesis provides the first methane (CH4) emissions measurements from offshore abandoned wells in the North Sea and found that boreholes constitute unrecognized, but important conduits for the release of biogenic CH4 originating from shallow gas accumulations (<1,000 m sediment depth) in the overburden of deep reservoirs. Such kind of leakages is not currently considered in any regulatory framework or greenhouse gas emission inventory. In the North Sea and in other hydrocarbon-prolific areas of the world shallow gas pockets are frequently observed in the sedimentary overburden above the deep hydrocarbon reservoirs and aggregate emissions along numerous wells may be significant. This conclusion also has important implications for Carbon Capture and Storage (CCS) since it implies that leakage from a carbon dioxide (CO2) storage site can potentially occur along any type of well (production, exploration, or abandoned), as long as it penetrates the subsurface CO2 plume. The second part of this thesis focuses on the Sleipner CO2 storage site in the Central North Sea and investigates hypothetic, but probably realistic leakage of CO2 along a well that penetrates the subsurface CO2 plume and leaks into the ~80 m deep water column, using a combination of experimental field data and numerical modelling. Small footprints of CO2 leakage in seawater imply that the environmental consequences of a single well leaking CO2 are insignificant and finding those leaks may pose challenging. Considering the millions of oil and gas wells drilled world-wide and the prospective implementation of CCS at a scale that would have significant impact on global CO2 emissions, this thesis stresses that pressure-based testing of well integrity is not sufficient for identifying and quantifying gas emissions (CH4 or CO2) along hydrocarbon wells. Improving our surveying and monitoring efforts and adapting the respective regulatory frameworks (national and international) is important

Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane

Source at https://doi.org/10.1073/pnas.1618926114.Continued warming of the Arctic Ocean in coming decades is projected to trigger the release of teragrams (1 Tg = 106 tons) of methane from thawing subsea permafrost on shallow continental shelves and dissociation of methane hydrate on upper continental slopes. On the shallow shelves (<100 m water depth), methane released from the seafloor may reach the atmosphere and potentially amplify global warming. On the other hand, biological uptake of carbon dioxide (CO2) has the potential to offset the positive warming potential of emitted methane, a process that has not received detailed consideration for these settings. Continuous sea−air gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO2 uptake rates (−33,300 ± 7,900 μmol m−2⋅d−1) twice that of surrounding waters and ∼1,900 times greater than the diffusive sea−air methane efflux (17.3 ± 4.8 μmol m−2⋅d−1). The negative radiative forcing expected from this CO2 uptake is up to 231 times greater than the positive radiative forcing from the methane emissions. Surface water characteristics (e.g., high dissolved oxygen, high pH, and enrichment of 13C in CO2) indicate that upwelling of cold, nutrient-rich water from near the seafloor accompanies methane emissions and stimulates CO2 consumption by photosynthesizing phytoplankton. These findings challenge the widely held perception that areas characterized by shallow-water methane seeps and/or strongly elevated sea−air methane flux always increase the global atmospheric greenhouse gas burden

Footprint and detectability of a well leaking CO2 in the Central North Sea: Implications from a field experiment and numerical modelling

Highlights • CO2 gas bubbles are completely dissolved within 2 m above the seabed. • CO2 is not emitted into the atmosphere but retained in the North Sea. • Dissolved CO2 is rapidly dispersed by tidal currents in the North Sea. • Harmful effects on benthic biota occur in the direct vicinity of the leak. • Monitoring has to be performed at the seabed and close to the leak. Abstract Existing wells pose a risk for the loss of carbon dioxide (CO2) from storage sites, which might compromise the suitability of carbon dioxide removal (CDR) and carbon capture and storage (CCS) technologies as climate change mitigation options. Here, we show results of a controlled CO2 release experiment at the Sleipner CO2 storage site and numerical simulations that evaluate the detectability and environmental consequences of a well leaking CO2 into the Central North Sea (CNS). Our field measurements and numerical results demonstrate that the detectability and impact of a leakage of <55 t yr−1 of CO2 would be limited to bottom waters and a small area around the leak, due to rapid CO2 bubble dissolution in seawater within the lower 2 m of the water column and quick dispersion of the dissolved CO2 plume by strong tidal currents. As such, the consequences of a single well leaking CO2 are found to be insignificant in terms of storage performance. Only prolonged leakage along numerous wells might compromise long-term CO2 storage and may adversely affect the local marine ecosystem. Since many abandoned wells leak natural gas into the marine environment, hydrocarbon provinces with a high density of wells may not always be the most suitable areas for CO2 storage

Shallow gas migration along hydrocarbon wells – An unconsidered, anthropogenic source of biogenic methane in the North Sea

Shallow gas migration along hydrocarbon wells constitutes a potential methane emission pathway that currently is not recognized in any regulatory framework or greenhouse gas inventory. Recently, the first methane emission measurements at three abandoned offshore wells in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic methane originating from shallow gas accumulations in the overburden of deep reservoirs were released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets in 3D seismic data of the CNS indicating that about one third of the wells may leak, potentially releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant contribution to the North Sea methane budget. A large fraction of this gas (~42 %) may reach the atmosphere via direct bubble transport (0-2 kt yr-1) and via diffusive exchange of methane dissolving in the surface mixed layer (1-5 kt yr-1), as indicated by numerical modeling. In the North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are frequently observed in the sedimentary overburden and aggregate leakages along the numerous wells drilled in those areas may be significant

Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection, monitoring and mitigation strategies

Highlights • Gas release from wells may counteract efforts to mitigate greenhouse gas emissions. • An approach for assessing methane release from marine decommissioned wells. • This gas release largely depends on the presence of shallow gas accumulations. • Methane release from hydrocarbon wells represents a major source in the North Sea. Abstract Hydrocarbon gas emissions from with decommissioned wells are an underreported source of greenhouse gas emissions in oil and gas provinces. The associated emissions may partly counteract efforts to mitigate greenhouse gas emissions from fossil fuel infrastructure. We have developed an approach for assessing methane leakage from marine decommissioned wells based on a combination of existing regional industrial seismic and newly acquired hydroacoustic water column imaging data from the Central North Sea. Here, we present hydroacoustic data which show that 28 out of 43 investigated wells release gas from the seafloor into the water column. This gas release largely depends on the presence of shallow gas accumulations and their distance to the wells. The released gas is likely primarily biogenic methane from shallow sources. In the upper 1,000 m below the seabed, gas migration is likely focused along drilling-induced fractures around the borehole or through non-sealing barriers. Combining available direct measurements for methane release from marine decommissioned wells with our leakage analysis suggests that gas release from investigated decommissioned hydrocarbon wells is a major source of methane in the North Sea (0.9-3.7 [95% confidence interval = 0.7-4.2] kt yr−1 of CH4 for 1,792 wells in the UK sector of the Central North Sea). This means hydrocarbon gas emissions associated with marine hydrocarbon wells are not significant for the global greenhouse gas budget, but have to be considered when compiling regional methane budgets

Simulating and Quantifying Multiple Natural Subsea CO2 Seeps at Panarea Island (Aeolian Islands, Italy) as a Proxy for Potential Leakage from Subseabed Carbon Storage Sites

Carbon dioxide (CO2) capture and storage (CCS) has been discussed as a potentially significant mitigation option for the ongoing climate warming. Natural CO2 release sites serve as natural laboratories to study subsea CO2 leakage in order to identify suitable analytical methods and numerical models to develop best-practice procedures for the monitoring of subseabed storage sites. We present a new model of bubble (plume) dynamics, advection-dispersion of dissolved CO2, and carbonate chemistry. The focus is on a medium-sized CO2 release from 294 identified small point sources around Panarea Island (South-East Tyrrhenian Sea, Aeolian Islands, Italy) in water depths of about 40–50 m. This study evaluates how multiple CO2 seep sites generate a temporally variable plume of dissolved CO2. The model also allows the overall flow rate of CO2 to be estimated based on field measurements of pH. Simulations indicate a release of ∼6900 t y–1 of CO2 for the investigated area and highlight an important role of seeps located at >20 m water depth in the carbon budget of the Panarea offshore gas release system. This new transport-reaction model provides a framework for understanding potential future leaks from CO2 storage sites