Monitoring Of CO2 Leakage Using High-Resolution 3D Seismic Data – Examples From Snøhvit, Vestnesa Ridge And The Western Barents Sea

Source at https://doi.org/10.3997/2214-4609.201802965.Injection of CO2 in subsurface reservoirs may cause overburden deformation and CO2 leakage. The aim of this study is to apply technologies for detection and monitoring of CO2 leakage and deformation above the injection reservoirs. The examples of this study include data from the Vestnesa Ridge natural seep site, the Snøhvit gas field and CO2 storage site region, and the Gemini North gas reservoir. Reprocessing of existing 3D high-resolution seismic data allows resolving features with a vertical and lateral resolution down to c. 1 m and c. 5 m respectively. The current acquisition systems could be modified to image structures down to one meter in both the vertical and horizontal directions. We suggest a monitoring workflow that includes baseline and time-lapse acquisition of highresolution 3D seismic data, integrated with geochemical, geophysical, and geotechnical seabed core and watercolumn measurements. The outcome of such a workflow can deliver reliable quantitative property volumes of the subsurface and will be able to image meter-sized anomalies of fluid leakage and deformation in the overburden

Multiscale characterisation of chimneys/pipes: Fluid escape structures within sedimentary basins

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. The properties of 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., multi-frequency seismics) and physical parameters of sediments (e.g., controlled source electromagnetics) across a wide range of 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

Sand waves and sediment transport on the SW Barents sea continental slope

I study a sand-wave field in ~600 meters water depth on the continental slope offshore Northern Norway. Using multibeam bathymetry data from 2008 and 2011 and P-Cable high-resolution 3D seismic data from 2011, I characterize the field. Sand waves reach up to 6.6 m in height and have wavelengths as large as 140 m. They are mostly asymmetric in shape with the steepest side dipping to the northwest, indicating that current flow over the field is predominantly to the northwest. Larger sand waves (>2 m in height, >100 m wavelength) are observed on topographic highs in the sand-wave field, whereas smaller sand waves (<2 m in height, <100 m wavelength) are present in topographic lows. These topographic lows occur where three ~1-2-km-wide channels cut down the continental slope through the sand-wave field. Seismic data reveal that there are no buried sand waves beneath the seafloor, suggesting that the sand waves are being continually eroded and redeposit at the seabed. Seismic data reveal that the depositional environment over the last ~1 Ma has been largely controlled by debris flows during the glaciations and melt-water plumes and channel formation during the glaciations. High-resolution imaging of the first few meters below the seabed shows that winnowing and associated sand-wave migration is currently the dominant sedimentary process. Data across the study area show that there are no buried sand waves beneath the seafloor. This suggests that the sand waves are being continually eroded and redeposited at the seabed. By measuring the offset of the crest of sand waves in the 2008 and 2011 bathymetry data, I calculate that sand waves migrate from 0 to 3.3 m/yr and have an average migration rate of 1.6 m/yr to the northwest. This migration direction which I directly observe in the bathymetry data is in agreement with the migration direction that I infer from the asymmetry of the sand waves. Integrating these migration rates over the cross section of the sand-wave field, I estimate that sand is transported along the continental slope at a rate of 22.3-118x106 m3/yr. These results provide hard constraints for numerical sand-wave migration models trying to identify the link between ocean currents and sand-wave migration. Furthermore, I show that sand-wave migration has the potential to rapidly move large volumes of sand across the deep water. This movement of sand can complicate drilling and production procedures in the energy industry and may affect slope stability on continental margins around the world

3D and 4D seismic investigations of fluid flow and gas hydrate systems

Using the state-of-the-art high resolution P-Cable 3D seismic system, in this thesis we (1) study the shallow strata (<1km below seabed) and describe the geological controls and driving mechanisms for fluid leakage at two sites in the northern Barents Sea, and (2) introduce a new time-lapse seismic method for high-frequency (~30-350 Hz) P-Cable seismic data. The study areas are interesting as they are located close to the upper termination of the gas hydrate stability zone and may experience ongoing or past growth and decomposition of gas hydrates. Bjørnøyrenna area hosts over 100 km-wide craters – a possible result of methane blowouts in the past. In Storfjordrenna, methane venting associates to gas hydrate bearing mounds (pingos). The northern Barents Sea is an underexplored area compared to the southern Barents Sea open for petroleum exploration. Therefore, our studies provide unique insight into the architecture and nature of shallow methane accumulations, fluid flow dynamics and gas hydrate inventories connected to thermogenic gas reservoirs that are deemed to occur elsewhere in the region. Our results point towards different geological controls on fluid flow. In Storfjordrenna, methane from Paleocene strata migrates along permeable beds and extensional faults linked to the regional Hornsund Fault Complex, accumulates under Quaternary glacial tills and locally forms gas hydrate chimneys. The Bjørnøyrenna lacks a glacial cover and the craters are incised in lithified, yet fractured, Triassic bedrocks. The source and reservoir of methane here is shallow Triassic clinoforms widespread across the Barents Sea. Furthermore, using 6 P-Cable 3D seismic datasets from three areas with and without known active fluid flow, we test seismic repeatability at various geological setting and develop an optimal workflow for 4D seismic approach. The results show high potential of such high-resolutions time-lapse seismic studies to reveal natural fluid flow dynamics on a yearly time scale

Sand waves and sediment transport on the SW Barents sea continental slope

I study a sand-wave field in ~600 meters water depth on the continental slope offshore Northern Norway. Using multibeam bathymetry data from 2008 and 2011 and P-Cable high-resolution 3D seismic data from 2011, I characterize the field. Sand waves reach up to 6.6 m in height and have wavelengths as large as 140 m. They are mostly asymmetric in shape with the steepest side dipping to the northwest, indicating that current flow over the field is predominantly to the northwest. Larger sand waves (>2 m in height, >100 m wavelength) are observed on topographic highs in the sand-wave field, whereas smaller sand waves (<2 m in height, <100 m wavelength) are present in topographic lows. These topographic lows occur where three ~1-2-km-wide channels cut down the continental slope through the sand-wave field. Seismic data reveal that there are no buried sand waves beneath the seafloor, suggesting that the sand waves are being continually eroded and redeposit at the seabed. Seismic data reveal that the depositional environment over the last ~1 Ma has been largely controlled by debris flows during the glaciations and melt-water plumes and channel formation during the glaciations. High-resolution imaging of the first few meters below the seabed shows that winnowing and associated sand-wave migration is currently the dominant sedimentary process. Data across the study area show that there are no buried sand waves beneath the seafloor. This suggests that the sand waves are being continually eroded and redeposited at the seabed. By measuring the offset of the crest of sand waves in the 2008 and 2011 bathymetry data, I calculate that sand waves migrate from 0 to 3.3 m/yr and have an average migration rate of 1.6 m/yr to the northwest. This migration direction which I directly observe in the bathymetry data is in agreement with the migration direction that I infer from the asymmetry of the sand waves. Integrating these migration rates over the cross section of the sand-wave field, I estimate that sand is transported along the continental slope at a rate of 22.3-118x106 m3/yr. These results provide hard constraints for numerical sand-wave migration models trying to identify the link between ocean currents and sand-wave migration. Furthermore, I show that sand-wave migration has the potential to rapidly move large volumes of sand across the deep water. This movement of sand can complicate drilling and production procedures in the energy industry and may affect slope stability on continental margins around the world

Occurrence and Distribution of Bottom Simulating Reflections in the Barents Sea

The Barents Sea, located close to the Arctic Ocean, is a petroleum province featuring an extensive occurrence of gas hydrates and shallow gas in compacted sediments. Glacial erosion and uplift have contributed to the migration of gas originating from deeper rocks to the shallow sediments of this region, resulting in hydrates with higher-order hydrocarbons in addition to methane. This article documents reported gas hydrate indications and major controls on hydrate stability in the Barents Sea

High-resolution 3D seismic exhibits new insights into the middle-late Pleistocene stratigraphic evolution and sedimentary processes of the Bear Island trough mouth fan

Arctic Ocean trough mouth fans (TMFs) represent a valuable archive of glacial-interglacial sedimentary processes that are especially important when reconstructing pre-Weichselian glaciations that may lack distinct imprints on the shelves. In 2011, we acquired the first high-resolution 3D seismic cube (~3 m vertical and 6 m horizontal resolution) on the continental slope of the SW Barents Sea by use of a P-Cable 3D system, to study in detail the seismic stratigraphy and glacial depositional history of the Bear Island Trough Mouth Fan. This technology provides data with a resolution that, for the first time on the western Barents Sea slope, enables detailed mapping of deposits of different glacial cycles. The dataset provides entire spatially coverage, allowing us to reconcile multiple generations of glacigenic deposits and channel systems. High-resolution 3D seismic data is crucial to describe buried channels, glacial units, as well as low relief landforms such as sediment waves accurately. The 30 km2 seismic cube is located at the southern flank of the Bear Island TMF at water depths from 592 to 660 m where sandwaves dominate the present seafloor. The data covers the glacially derived stratigraphy in the uppermost ~700 m below the seafloor. We establish a robust stratigraphic framework by interpreting seismic reflectors along 2D tie-in lines to previously well-constrained seismic and well data. We find that our data provide a record of progradation of glacigenic debris flows (GDFs) since MIS 12 (0.5 Ma) to present. Horizon slices reveal a range of gullies and channels at different depths overlying the GDFs. We describe the paleoenvironment and sedimentary processes throughout this time-span (that covers seven glacial cycles) and discuss the impact of the Barents Sea Ice Sheet waxing and waning on erosion, sedimentation, and deposition along the continental slope. Abundant buried gullies were hitherto unknown at the Bear Island TMF, with previous work describing this succession as a debris-flow dominated unit where meltwater-related features are lacking, and interpreting this to represent low average temperatures. By use of the relatively small high-resolution 3D seismic dataset, we provide new evidence for the presence of gullies and channels indicating that periods of ice sheet melting and meltwater runoff existed throughout the middle-late Pleistocene succession. The work offers new insight into the stratigraphic evolution of a continental margin dominated by GDFs and demonstrates the value of high-resolution seismic, such as the P-Cable system, in resolving important details of paleo-slope-environments

Geological controls of giant crater development on the Arctic seafloor

Active methane seepage occurs congruent with a high density of up to 1 km-wide and 35 m deep seafloor craters (>100 craters within 700 km2 area) within lithified sedimentary rocks in the northern Barents Sea. The crater origin has been hypothesized to be related to rapid gas hydrate dissociation and methane release around 15–12 ka BP, but the geological setting that enabled and possibly controlled the formation of craters has not yet been addressed. To investigate the geological setting beneath the craters in detail, we acquired high-resolution 3D seismic data. The data reveals that craters occur within ~250–230 Myr old fault zones. Fault intersections and fault planes typically define the crater perimeters. Mapping the seismic stratigraphy and fault displacements beneath the craters we suggest that the craters are fault-bounded collapse structures. The fault pattern controlled the craters occurrences, size and geometry. We propose that this Triassic fault system acted as a suite of methane migration conduits and was the prerequisite step for further seafloor deformations triggered by rapid gas hydrate dissociation some 15–12 ka BP. Similar processes leading to methane releases and fault bounded subsidence (crater-formation) may take place in areas where contemporary ice masses are retreating across faulted bedrocks with underlying shallow carbon reservoirs

Repeatability of high-resolution 3D seismic data

High-resolution 4D (HR4D) seismic data have the potential for improving the current state-of-the-art in detecting shallow (≤500−1000  m below seafloor) subsurface changes on a very fine scale (approximately 3–6 m). Time-lapse seismic investigations commonly use conventional broadband seismic data, considered low to moderate resolution in our context. We have developed the first comprehensive time -lapse analysis of high-resolution seismic data by assessing the repeatability of P-cable 3D seismic data (approximately 30–350 Hz) with short offsets and a high density of receivers. P-cable 3D seismic data sets have for decades been used to investigate shallow fluid flow and gas-hydrate systems. We analyze P-cable high-resolution 4D (HR4D) seismic data from three different geologic settings in the Arctic Circle. The first two are test sites with no evidence of shallow subsurface fluid flow, and the third is an active seepage site. Using these sites, we evaluate the reliability of the P-cable 3D seismic technology as a time-lapse tool and establish a 4D acquisition and processing workflow. Weather, waves, tide, and acquisition-parameters such as residual shot noise are factors affecting seismic repeatability. We achieve reasonable quantitative repeatability measures in stratified marine sediments at two test locations. However, repeatability is limited in areas that have poor penetration of seismic energy through the seafloor, such as glacial moraines or rough surface topography. The 4D anomalies in the active seepage site are spatially restricted to areas of focused fluid flow and might likely indicate changes in fluid flow. This approach can thus be applied to detect migration of fluids in active leakage structures, such as gas chimneys

Evolution of contourite drifts in regions of slope failures at eastern Fram Strait

Geotechnical characteristics of contouritic deposition often lead to preconditioning slope instabilities and failures along glaciated and formerly glaciated continental margins. However, internal depositional geometry is also an important factor in triggering instabilities. This work highlights the importance of the tectonic and oceanographic evolution of the Northwestern (NW) Svalbard margin in determining the buildup and the internal structure of contourite drifts and the subsequent type of slope instability. The analysis of seismic reflection data reveals that the presence of two contourite drifts on the flank of an active spreading ridge in the Fram Strait—NW Svalbard margin—in an area of extensive slope instability had a major impact on the evolution of slope failure. The presence of a slope sheeted drift (or plastered drift) led to the development of rotational/translational mass movement at water depth  2500 ms the presence of sediment waves facilitated the formation of planes of shear that led to internal deformation of the lower slope through a process of slump/creep. The well-documented high seismicity of the area might have provided the necessary energy to trigger the slope instability