Gas migration pathways, controlling mechanisms and changes in sediment acoustic properties observed in a controlled sub-seabed CO2 release experiment

Carbon capture and storage (CCS) is a key technology to potentially mitigate global warming by reducing carbon dioxide (CO2) emissions from industrial facilities and power generation that escape into the atmosphere. To broaden the usage of geological storage as a viable climate mitigation option, it is vital to understand CO2 behaviour after its injection within a storage reservoir, including its potential migration through overlying sediments, as well as biogeochemical and ecological impacts in the event of leakage. The impacts of a CO2 release were investigated by a controlled release experiment that injected CO2 at a known flux into shallow, under-consolidated marine sediments for 37 days. Repeated high-resolution 2D seismic reflection surveying, both pre-release and syn-release, allows the detection of CO2-related anomalies, including: seismic chimneys; enhanced reflectors within the subsurface; and bubbles within the water column. In addition, reflection coefficient and seismic attenuation values calculated for each repeat survey, allow the impact of CO2 flux on sediment acoustic properties to be comparatively monitored throughout the gas release. CO2 migration is interpreted as being predominantly controlled by sediment stratigraphy in the early stages of the experiment. However, either the increasing flow rate, or the total injected volume become the dominant factors determining CO2 migration later in the experiment

Marine baseline and monitoring strategies for Carbon Dioxide Capture and Storage (CCS)

The QICS controlled release experiment demonstrates that leaks of carbon dioxide (CO2) gas can be detected by monitoring acoustic, geochemical and biological parameters within a given marine system. However the natural complexity and variability of marine system responses to (artificial) leakage strongly suggests that there are no absolute indicators of leakage or impact that can unequivocally and universally be used for all potential future storage sites. We suggest a multivariate, hierarchical approach to monitoring, escalating from anomaly detection to attribution, quantification and then impact assessment, as required. Given the spatial heterogeneity of many marine ecosystems it is essential that environmental monitoring programmes are supported by a temporally (tidal, seasonal and annual) and spatially resolved baseline of data from which changes can be accurately identified. In this paper we outline and discuss the options for monitoring methodologies and identify the components of an appropriate baseline survey

Detection and impacts of leakage from sub-seafloor deep geological carbon dioxide storage

Fossil fuel power generation and other industrial emissions of carbon dioxide are a threat to global climate1, yet many economies will remain reliant on these technologies for several decades2. Carbon dioxide capture and storage (CCS) in deep geological formations provides an effective option to remove these emissions from the climate system3. In many regions storage reservoirs are located offshore4, 5, over a kilometre or more below societally important shelf seas6. Therefore, concerns about the possibility of leakage7, 8 and potential environmental impacts, along with economics, have contributed to delaying development of operational CCS. Here we investigate the detectability and environmental impact of leakage from a controlled sub-seabed release of CO2. We show that the biological impact and footprint of this small leak analogue (<1 tonne CO2 d?1) is confined to a few tens of metres. Migration of CO2 through the shallow seabed is influenced by near-surface sediment structure, and by dissolution and re-precipitation of calcium carbonate naturally present in sediments. Results reported here advance the understanding of environmental sensitivity to leakage and identify appropriate monitoring strategies for full-scale carbon storage operations

Impact of subsurface fluid flow on sediment acoustic properties, implications for carbon capture and storage

Geological Carbon Capture and Storage (CCS) is a promising climate change mitigation technology, which allows the reduction of anthropogenic carbon dioxide (CO2) emissions into the atmosphere. Although CCS is considered to have a significant potential in tackling climate change, several uncertainties remain, including the efficiency and permanency of carbon sequestration, and notably risks of CO2 leakage from the storage reservoir. A better understanding of fluid flow activity within the sedimentary overburden and the identification of the best monitoring techniques are crucial for increasing societal confidence in sequestration.This thesis reports findings from two different offshore CCS projects: a controlled sub-seabed CO2 release experiment completed in Ardmucknish Bay, Oban (Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage, QICS), and a multidisciplinary research project conducted in the vicinity of Sleipner CCS site, in the Central North Sea (Sub-seabed CO2 Storage: Impact on Marine Ecosytems, ECO2).During the QICS project, a borehole was drilled from land, allowing 37 days of CO2 release in unconsolidated marine sediments. Analysis of the time-lapse high- resolution seismic reflection data reveals development of acoustic anomalies within the overburden and water column, caused by CO2 fluxing in the vicinity of the injection site. The impacts of CO2 injection on sediment acoustic properties are investigated, where changes in seismic reflectivity, seismic attenuation, acoustic impedance and P-wave seismic velocity are detected on high-resolution seismic reflection data. CO2 migration within the overburden is interpreted to be controlled by sediment stratigraphy and injection rate/total injected volume throughout the gas release, and by the sediment stratigraphic geometry post-release. Seismic quantification of the gaseous CO2 indicates that most of the injected CO2 is trapped below a stratigraphic boundary, located at 4 m depth below the seafloor, or dissolved, throughout the gas release. These observations are in agreement with seabed gas flux measurements by passive hydroacoustics and water column bubble sampling, which suggest that only 15% of the injected CO2 emerges at the seabed, towards the end of gas release.Within the scope of the ECO2 project, increased fluid flow activity is detected along, and in the vicinity of a seabed fracture, the Hugin Fracture. Although there is no evidence of anthropogenic CO2 leakage in the Central North Sea from the current dataset, biogenic and thermogenic gas leakage at the Hugin Fracture suggest a well-established hydraulic and structural connection. The origin of the Hugin Fracture is proposed to be controlled by an E-W transtensional stress regime, and differential compaction above a buried tunnel valley system

First controlled sub-seabed CO2 release experiment: qualitative and quantitative analysis of high-resolution seismic reflection data

Carbon Capture and Storage is a promising climate change mitigation technology which allows the reduction of carbon dioxide emissions into the atmosphere. To assure a safe and permanent CO2 storage, it is vital to adapt efficient monitoring technologies allowing to better understand the fate of the injected CO2 within the subsurface, including its impact on sediment acoustic properties and migration into the overlying layers. The first-controlled sub-seabed CO2 release experiment, Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS), was completed in Ardmucknish Bay, Oban, where 4.2 tonnes of CO2 were injected into unconsolidated shallow marine sediments over 37 days. High-resolution seismic reflection data acquired syn-release reveal many CO2-related acoustic anomalies including seismic chimneys and enhanced reflectivity within the overburden, and bubbles within the water column. CO2 migration is interpreted to be controlled by sediment stratigraphy in the early stages of the experiment, whereas CO2 injection rate/ total injected volume overrode the stratigraphic control towards the end of gas release. Post-release seismic reflection data reveal that injected CO2 was mostly trapped below an erosional unconformity, Horizon 2, where a dip of 3.5° was found to significantly control the up-dip migration of the gaseous CO2 after the cessation of injection. The in situ CO2 content above Horizon 2 is also determined using the syn-release seismic reflection data combined with the Anderson and Hampton geaocoustics model, confirming that most of the injected CO2 was trapped below Horizon 2, or dissolved, during the QICS experiment

First controlled sub-seabed CO2 release experiment: Insights into gas migration pathways and impacts on sediment physical properties

Carbon Capture and Storage (CCS) is a key technology to potentially mitigate global warming by reducing the amount of carbon dioxide (CO2) from industrial facilities and power generation that escapes into the atmosphere. In order to broaden the usage of geological storage as a safe and reliable climate change mitigation option, it is vital to understand CO2 behaviour after its injection within a storage reservoir, including its migration through overlying sediments, as well as its biogeochemical and ecological impacts in the event of leakage at the seafloor. To address these issues, the first controlled CO2 release experiment, entitled 'Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage (QICS)', took place in Ardmucknish Bay, Oban, in May-July 2012. This experiment involved the injection of CO2 of known flux under shallow unconsolidated marine sediments over 36 days and repeated monitoring using geophysical and geochemical techniques. High resolution seismic reflection data (chirp and boomer), covering both pre-release and release stages, allows the detection of various CO2-related anomalies including seismic chimneys, enhanced reflectors within the sediment overburden and bubbles into the overlying water column. CO2 migration pattern is predominantly controlled by the stratigraphy in the early stages of the experiment. However, the increasing flow rate becomes the dominant factor determining CO2 migration, towards the end of the experiment. In addition, analysis of reflection coefficients and seismic attenuation indicates the effect of CO2 on sediment physical properties

Seismicity and accretion processes along the Mid-Atlantic Ridge south of the Azores using data from the MARCHE autonomous hydrophone array

The seismicity of the South Atlantic Ocean has been recorded by the MARCHE network of 4 autonomous underwater hydrophones (AUH) moored within the SOFAR channel on the flanks of the Mid-Atlantic Ridge (MAR). The instruments were deployed south of the Azores Plateau between 32° and 39°N from July 2005 to August 2008. The low attenuation properties of the SOFAR channel for earthquake T-wave propagation result in a detection threshold reduction from a magnitude completeness level (Mc) of ~4.3 for MAR events recorded by the land-based seismic networks to Mc=2.1 using this hydrophone array. A spatio-temporal analysis has been performed among the 5600 events recorded inside the MARCHE array. Most events are distributed along the ridge between lat. 39°N on the Azores Platform and the Rainbow (36°N) segment. In the hydrophone catalogue, acoustic magnitude (Source Level, SL) is used as a measure of earthquake size. The source level above which the data set is complete is SLc=205 dB. We look for seismic swarms using the cluster software of the SEISAN package. The criterion used are a minimum SL of 210 to detect a possible mainshock, and a radius of 30 km and a time window of 40 days after this mainshock (Cevatoglu, 2010, Goslin et al., 2012). 7 swarms with more than 15 events are identified using this approach between 32°et 39°N of latitude. The maximum number of earthquake in a swarm is 57 events. This result differs from the study of Simao et al. (2010) as we processed a further year of data and selected sequences with fewer events. Looking at the distribution of the SL as a function of time after the mainshock, we discuss the possible mechanism of these earthquakes : tectonic events with a "mainshock-aftershock" distribution fitting a modified Omori law or volcanic events showing more constant SL values. We also present the geophysical setting of these 7 swarms, using gravity, bathymetry, and available local geological data. This study illustrates the potential of hydrophone data to monitor segment-scale ridges processes in the vicinity of the Lucky Strike seafloor observatory (lat. 37°20'N), the Azores node of the EMSO (European Multidiciplinary Subsea Observatory) system

Monitoring techniques using Autonomous Underwater Vehicles for potential seepage of CO2 from sub-seafloor storage sites

Although a number of Carbon Capture and Storage sub-seafloor storage sites are now either in operation or planned for CO2, little is known about the effect of potential seepage on marine ecosystems. Here we describe a comprehensive field campaign to the North Sea (RRS James Cook Cruise 77) that used Autosub 6000 to test methods for detection of seepage, including formation fluids, natural gas and CO2, as it passes through the sedimentary overburden and into the water column, and develop monitoring strategies suitable for all offshore carbon capture and storage projects. In this paper we describe the Hugin Fracture, a 2 km long discontinuity imaged on the seabed, and associated fluid flow activity, revealed by geophysical observations including high reflectivity acoustic anomalies within the overburden. Further results in favour of active fluid flow along this fault will be presented, using a combination of multidisciplinary datasets comprising video photography, Eh sensor and sediment samples. <br/