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  • Elsevier  (7)
  • AGU (American Geophysical Union)  (3)
  • 2020-2024  (10)
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  • 1
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    Greenhouse gas emissions from marine decommissioned hydrocarbon wells: leakage detection, monitoring and mitigation strategies (2020)
    Elsevier
    Details
    Publication Date: 2023-02-08
    Description: 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.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Seismic imaging of an active fluid conduit below Scanner Pockmark, Central North Sea (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Highlights • Unprecedented dense coverage of ocean-bottom seismometer data reveals seismic velocity variations within a vertical fluid pathway. • There are zones of both positive (faster) and negative (slower) velocity within the fluid pathway compared to the background formation velocities. • Velocity reductions are related to free gas in the fluid pathway, while the reason for velocity increases is unclear but potentially caused by cementation. Abstract Subsurface CO2 storage is a key strategy to reduce greenhouse gas emission, but leakage of CO2 along natural fluid pathways may affect storage formation integrity. However, the internal structure and the physical properties of these focused fluid conduits are poorly understood. Here, we present a three-dimensional seismic velocity model of an active fluid conduit beneath the Scanner Pockmark in the Central North Sea, derived from ocean-bottom seismometer data. We show that the conduit, which manifests as a pipe structure in seismic data, is separated into two parts. The upper part, extending to 260 m depth, i.e. 110 m below the seafloor, is characterised by seismic velocities up to 100 m/s slower than the surrounding strata. The deeper part is characterized by a 50 m/s seismic velocity increase compared to background velocity. We suggest that the upper part of the pipe structure represents a network of open fractures, partly filled with free gas, while the reason for the velocity increase in the lower part remains speculative. These observations suggest that active pipes can be internally heterogeneous with some intervals probably being open fluid pathways and other intervals being closed. This study highlights the complexity in evaluating focused fluid conduits and the necessity of their detailed assessment when selecting CO2 storage sites.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Thermal state of the Guaymas Basin derived from gas hydrate bottom simulating reflections and heat flow measurements (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: Seafloor heat flow provides information about the thermal evolution of the lithosphere, the magnitude and timing of volcanic activity, and hydrothermal circulation patterns. In the central Gulf of California, the Guaymas Basin is part of a young marginal spreading rift system that experiences high sedimentation (1–5 km/Myr) and widespread magmatic intrusions in the axial troughs and the off-axis regions. Heat flow variations record magmatic and sedimentary processes affecting the thermal evolution of the basin. Here, we present new seismic evidence of a widespread bottom-simulating reflection (BSR) in the northwestern Guaymas Basin. Using the BSR depths and thermal conductivity measurements, we determine geothermal gradient and surface heat flow variations. The BSR-derived heat flow values are less than the conductive lithospheric heat flow predictions for mid-oceanic ridges. They suggest that high sedimentation (0.3–1 km/Myr) suppresses the lithospheric heat flow. In the central and southeastern regions of the basin, the BSR-derived geothermal gradient increases as the intruded magmatic units reach shallower subsurface depths. Thermal modeling shows that recent (〈5000 years) igneous intrusions (〈500 m below the seafloor) and associated fluid flow elevate the surface heat flow up to five times. BSR-derived geothermal gradients correlate little with the depth of the shallowest magmatic emplacements to the north, where the intrusions have already cooled for some time, and the associated hydrothermal activity is about to shut down. Key Points - A regional bottom-simulating reflection (BSR) in the Guaymas Basin indicates a widespread occurrence of gas hydrates - The BSR derived thermal gradients show wavy patterns farther away from the spreading centre, indicating strong lateral heat flow variations - High sedimentation suppresses heat flow, while recent magmatic intrusion and fluid advection increase heat flow
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: We present porosity and free gas estimations and their uncertainties at anactive methane venting site in the UK sector of the North Sea. In the Scan-ner Pockmark area in about 150m water depth, we performed a multi-disciplinary experiment to investigate the physical properties of fluid flowstructures within unconsolidated glaciomarine sediments. Here we focus onthe towed controlled source electromagnetic (CSEM) data analysis with con-straints from seismic reflection and core logging data. Inferred backgroundresistivity values vary between 0.6–1 Ωm at the surface and 1.9–2.4 Ωm at150 mbsf. We calibrate Archie’s parameters with measurements on cores, andestimate porosities of about 50±10% at the seafloor decreasing to 25±3% at 150 mbsf which matches variations expected for mechanical compaction ofclay rich sediments. High reflectivity in seismic reflection data is consistentwith the existence of a gas pocket. A synthetic study of varying gas contentin this gas pocket shows that at least 33±8% of free gas are required to causea distinct CSEM data anomaly. Real data inversions with seismic constraintssupport the presence of up to 34±14% free gas in a 30–40 m thick gas pocketunderneath the pockmark within the stratigraphic highs of a till layer abovethe glacial unconformity in the Aberdeen Ground Formation.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Spontaneously exsolved free gas during major storms as an ephemeral gas source for pockmark formation (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-02-07
    Description: Abrupt fluid emissions from shallow marine sediments pose a threat to seafloor installations like wind farms and offshore cables. Quantifying such fluid emissions and linking pockmarks, the seafloor manifestations of fluid escape, to flow in the sub-seafloor remains notoriously difficult due to an incomplete understanding of the underlying physical processes. Here, using a compositional multi-phase flow model, we test plausible gas sources for pockmarks in the south-eastern North Sea, which recent observations suggest have formed in response to major storms. We find that the mobilization of pre-existing gas pockets is unlikely because free gas, due to its high compressibility, damps the propagation of storm-induced pressure changes deeper into the subsurface. Rather, our results point to spontaneous appearance of a free gas phase via storm-induced gas exsolution from pore fluids. This mechanism is primarily driven by the pressure-sensitivity of gas solubility, and the appearance of free gas is largely confined to sediments in the vicinity of the seafloor. We show that in highly permeable sediments containing gas-rich pore fluids, wave-induced pressure changes result in the appearance of a persistent gas phase. This suggests that seafloor fluid escape structures are not always proxies for overpressured shallow gas and that periodic seafloor pressure changes can induce persistent free gas phase to spontaneously appear. Key Points - Storm-induced pressure changes can lead to spontaneous appearance of free gas phase near the seafloor - This process is driven by pressure-sensitive phase instabilities - This mechanism could help explain elusive gas sources in recently observed pockmarks in the North Sea
    Type: Article , PeerReviewed
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Assuring the integrity of offshore carbon dioxide storage (2022)
    Elsevier
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    Publication Date: 2024-02-07
    Description: Highlights • An artificial CO2 release demonstrated MMV techniques for offshore CCS. • Detection of leakage was demonstrated using acoustic, chemical and physical approaches. • Attribution of leakage was proved possible using artificial and natural tracer compounds. • Leakage quantification was possible using approaches not previously applied to CCS studies. • Non-catastrophic leaks were detected at levels below those that would cause environmental harm. Carbon capture and storage is a key mitigation strategy proposed for keeping the global temperature rise below 1.5 °C. Offshore storage can provide up to 13% of the global CO2 reduction required to achieve the Intergovernmental Panel on Climate Change goals. The public must be assured that potential leakages from storage reservoirs can be detected and that therefore the CO2 is safely contained. We conducted a controlled release of 675 kg CO2 within sediments at 120 m water depth, to simulate a leak and test novel detection, quantification and attribution approaches. We show that even at a very low release rate (6 kg day−1), CO2 can be detected within sediments and in the water column. Alongside detection we show the fluxes of both dissolved and gaseous CO2 can be quantified. The CO2 source was verified using natural and added tracers. The experiment demonstrates that existing technologies and techniques can detect, attribute and quantify any escape of CO2 from sub-seabed reservoirs as required for public assurance, regulatory oversight and emissions trading schemes.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Towards improved monitoring of offshore carbon storage: A real-world field experiment detecting a controlled sub-seafloor CO2 release (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Carbon capture and storage (CCS) is a key technology to reduce carbon dioxide (CO2) emissions from industrial processes in a feasible, substantial, and timely manner. For geological CO2 storage to be safe, reliable, and accepted by society, robust strategies for CO2 leakage detection, quantification and management are crucial. The STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project aimed to provide techniques and understanding to enable and inform cost-effective monitoring of CCS sites in the marine environment. A controlled CO2 release experiment was carried out in the central North Sea, designed to mimic an unintended emission of CO2 from a subsurface CO2 storage site to the seafloor. A total of 675 kg of CO2 were released into the shallow sediments (~3 m 49 below seafloor), at flow rates between 6 and 143 kg/d. A combination of novel techniques, adapted versions of existing techniques, and well-proven standard techniques were used to detect, characterise and quantify gaseous and dissolved CO2 in the sediments and the overlying seawater. This paper provides an overview of this ambitious field experiment. We describe the preparatory work prior to the release experiment, the experimental layout and procedures, the methods tested, and summarise the main results and the lessons learnt.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Multiscale characterisation of chimneys/pipes: Fluid escape structures within sedimentary basins (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Evaluation of seismic reflection data has identified the presence of fluid escape structures cross-cutting overburden stratigraphy within sedimentary basins globally. Seismically-imaged chimneys/pipes are considered to be possible pathways for fluid flow, which may hydraulically connect deeper strata to the seabed. These fluid migration pathways through the overburden must be constrained to enable secure, long-term subsurface carbon dioxide (CO2) storage. We have investigated a site of natural active fluid escape in the North Sea, the Scanner Pockmark Complex, to determine the physical characteristics of focused fluid conduits, and how they control fluid flow. Here we show that a multi-scale, multi disciplinary experimental approach is required for complete characterisation of fluid escape structures. Geophysical techniques are necessary to resolve fracture geometry and subsurface structure (e.g., multifrequency seismics) and physical parameters of sediments (e.g., controlled source electromagnetics) across length scales (m to km). At smaller (mm to cm) scales, sediment cores were sampled directly and their physical and chemical properties assessed using laboratory-based methods. Numerical modelling approaches bridge the resolution gap, though their validity is dependent on calibration and constraint from field and laboratory experimental data. Further, time-lapse seismic and acoustic methods capable of resolving temporal changes are key for determining fluid flux. Future optimisation of experiment resource use may be facilitated by the installation of permanent seabed infrastructure, and replacement of manual data processing with automated workflows. This study can be used to inform measurement, monitoring and verification workflows that will assist policymaking, regulation, and best practice for CO2 subsurface storage operations.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 metres below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Volcanic flank collapse, secondary sediment failure and flow-transition: multi-stage landslide emplacement offshore Montserrat, Lesser Antilles (2024)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2024-07-02
    Description: Volcanic flank collapses, especially those in island settings, have generated some of the most voluminous mass transport deposits on Earth and can trigger devastating tsunamis. Reliable tsunami hazard assessments for flank collapse-driven tsunamis require an understanding of the complex emplacement processes involved. The seafloor sequence southeast of Montserrat (Lesser Antilles) is a key site for the study of volcanic flank collapse emplacement processes that span subaerial to submarine environments. Here, we present new 2D and 3D seismic data as well as MeBo drill core data from one of the most extensive mass transport deposits offshore Montserrat, which exemplifies multi-phase landslide deposition from volcanic islands. The deposits reveal emplacement in multiple stages including two blocky volcanic debris avalanches, secondary seafloor failure and a late-stage erosive density current that carved channel-like incisions into the hummocky surface of the deposit about 15 km from the source region. The highly erosive density current potentially originated from downslope-acceleration of fine-grained material that was suspended in the water column earlier during the slide. Late-stage erosive turbidity currents may be a more common process following volcanic sector collapse than has been previously recognized, exerting a potentially important control on the observed deposit morphology as well as on the runout and the overall shape of the deposit. Key Points Landslide emplacement offshore Montserrat included volcanic flank collapses, sediment incorporation, and a late-stage erosive flow Highly erosive flows are likely to be common processes during volcanic flank collapse deposition Pre-existing topography plays a major role in shaping flank collapse-associated mass transport deposits Plain Language Summary Disintegration of volcanic islands can cause very large landslides and destructive tsunamis. To assess the tsunami hazard of such events, it is crucial to understand the processes that are involved in their formation. We present new insights from seismic data and drill cores from a landslide deposit offshore Montserrat, a volcanic island in the Lesser Antilles Arc in the Caribbean. Our analysis reveals the emplacement of landslide material in several stages, including multiple volcanic flank collapses, incorporation of seafloor sediments and an erosive flow that carved channels into the top of the deposit right after its emplacement. We suggest that highly erosive flows are a common process during volcanic flank collapse deposition and that they play a significant role in the shaping of the deposit's appearance.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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