Work packages

Efficient and accurate detection of CO₂ leakage from CCS sites is critically dependent on having detailed and reliable knowledge of the natural environmental and ecological baselines around the storage site, so that deviations from “normal” conditions can be quickly detected and traced.

Establishing a baseline at an offshore storage site is a hugely complex and potentially costly exercise, requiring detailed observation and measurement of a wide range of parameters that encompass the physical, biological and chemical characteristics of seawater and the seafloor environment. Understanding the degree of variability in these characteristics over different spatial and temporal scales (episodic, seasonal, annual, decadal) is crucial for the effective and safe monitoring of CCS operations.

WP2 used observational, analytical and modelling techniques to map geomorphology and biological community structure; quantified the spatio-temporal variability of key chemical parameters in both water column and sediments; developed indices for biological community composition and health, and provided estimates of the existing stressors (e.g. trawling, pollution) on the marine environment. All these inputs were used to establish a sound baseline for the STEMM-CCS experimental site in the North Sea. Models were used to deliver comprehensive fields of physical and chemical properties, enabling predictions and projections of future baseline conditions against which regulators and operators can plan and future-proof environmental modelling.

By combining existing information with a suite of newly gathered data and advanced numerical modelling, STEMM-CCS has constructed a comprehensive morphological, physico-geochemical and ecological description of the seabed above the Goldeneye storage complex. This information has been synthesised to create a comprehensive report and a seabed habitat map that also includes major human pressures such as fishing. Such documents are vital for determining the areas of seabed that potentially have the greatest sensitivity to CO₂ leakage, and therefore identify those areas around which to focus environmental monitoring.

WP2 Objectives:

• Design and implement an effective environmental baseline survey at the STEMM-CCS experimental site
• Identify appropriate biological and chemical indices of impact, suitable for industrial-scale application above CCS sites
• Use modelling approaches to extend spatially and temporally limited field observation data to support a comprehensive environmental baseline

The location and likely intensity of CO₂ leakage from a CCS site depend crucially on the distribution, orientation and permeability of fluid pathways in the subsurface. Sub-seafloor geological structures known as chimneys or pipe structures can be up to 500m wide and commonly occur in sedimentary basins such as the North Sea. Thought to have formed as a result of fluid migration and hydrofracturing, they present potential leakage pathways from a storage complex to the seabed and therefore require careful investigation.

STEMM-CCS used new 3D seismic and sediment imaging techniques alongside direct sampling to investigate fluid pathways in seafloor sediments. A comprehensive geophysical investigation of the pipe structure below Scanner Pockmark in the North Sea yielded important data on its permeability characteristics. Combined with direct sampling of fossil chimney structures on land, this enabled numerical models to be developed that give quantitative predictions of the efficiency of fluid flow through such structures. Permeability created by fracture networks in subsurface sediments were assessed via borehole sampling, geophysical seafloor imaging techniques and cutting-edge 4D seismic technology. These datasets were combined to form the basis for a numerical modelling study of overall fluid migration patterns and expected volumes, which demonstrated that CO₂ can migrate much faster through such a structure than through the undisturbed host rock around it. However, apart from showing that pipe structures can transgress across different lithologies, the simulations also revealed the large variety of pipe structures, both in terms of size and formation mechanism. This clearly documents the demand for further studies of pipe structures in a range of sedimentary basins to put the findings on a more robust statistical footing.

These results will inform the design of a state-of-the-art cost-effective geophysical survey of the near-surface geology at the Goldeneye site in order to assess the potential for CO₂ escape and to identify priority locations for monitoring.

WP3 Objectives:

• Determine the efficiency of leakage pathways for CO₂ transfer
• Determine the CO₂ permeability of chimney structures
• Determine how long chimney structures remain open for CO₂ transfer
• Determine the physical properties of chimney structures

The ability to efficiently and accurately detect, trace and quantify CO₂ leakage from storage reservoirs is critical for ensuring the safe operation of CCS in the marine environment. STEMM-CCS set out to determine the most viable and effective methods of detecting any escape, focusing on both the physical expression of CO₂ as gas in seafloor sediments and the water column, and the chemical dynamics of CO₂ in the dissolved phase.

New sensors and detection techniques developed during the project were tested during the controlled release experiment. The most visually obvious evidence of CO₂ escape - bubbles - were monitored using passive sonar techniques alongside visual surveys to detect, quantify and trace bubble streams in the water column. Other indicators of potential CO₂ escape include displacement of natural fluids and gases from the surrounding sediments, which can be detected against expected background levels established during baseline assessment. Such seepages are likely to precede any CO₂ escape, and can therefore act as an early warning of reservoir integrity breach.

Artificial tracers have been successfully used in land-based CCS projects but until now have not been fully tested for use in marine storage systems. A range of chemically and isotopically distinct substances were added to the CO₂ in the STEMM-CCS controlled release experiment. This allowed us to assess their suitability as sensitive tracers of gas movement through the marine environment, to quantify the volume and rate of release, and to investigate how the CO₂ interacts with the sediments.

All methodologies tested during the controlled release experiment were capable of detecting the leakage of injected CO₂, but only some were capable of detecting leakage at the lowest injection rates, or at distances of more than a metre or so above the seabed CO₂ seeps. Not all techniques were capable of quantifying the flux of CO₂ that escaped across the seabed. Gas migration through the seabed sediments can be detected remotely by sub-bottom profiler imaging (Chirp), but gas escape features need to be >~5cm in diameter. While ship-based echosounders readily detect the release of gas bubbles from the seabed, bubble fluxes are best quantified using hydrophones; however, they need to be located within ~10m of the gas seeps. As concentrations of dissolved CO₂ in the water column are highly variable, anomalies due to leakage are best demonstrated using stoichiometric techniques, such as the C_seep technique developed in STEMM-CCS. Suitable variables include the ratio of CO₂/O₂ both of which can be measured by in situ sensors with high precision. Fluxes of dissolved CO₂ across the seabed are best determined by analysis of artificial tracers that can be co-injected with CO₂ into the storage reservoir. Crucially, these tracers can also be used to unambiguously confirm the source of fugitive CO₂ emissions in the marine environment.

It is important to recognise that the CO₂ leakage rates tested by STEMM-CCS were relatively low (≤ 50 L/min). CO₂ leakage from a sub-seabed CO₂ storage reservoir is most likely to occur at the injection point, through faults or from abandoned exploration wells, which can be expected to have much higher gas flow rates (from ~100 L/min at abandoned wells, to ~50,000 L/min at the injection point). In these circumstances, the footprint of the CO₂ anomaly in both the seabed sediments and the water column can be expected to be significantly larger, which would greatly increase the utility of many of the methodologies tested by STEMM-CCS for leakage detection.

WP4 Objectives:

• Detect, locate and quantify CO₂ leakage from the controlled sub-seabed CO₂ release experiment (WP1)
• Assess the utility of artificial and natural tracers of CO₂ in the marine environment
• Develop effective methodologies for quantification of CO₂ fluxes across the seabed and dispersion in the water column
• Develop coupled and nested model systems to assess CO₂ and tracer dispersion in sediments and the water column for a range of leakage scenarios

Innovation and technology development formed the core of STEMM-CCS. From major engineering challenges to improvements in the detection of CO₂ escape, the project went beyond the current state-of-the-art in crucial areas that will provide confidence in the large-scale roll-out of operationally safe and environmentally sound CO₂ storage in offshore geological reservoirs. This innovation included the design and application of new techniques and strategies, as well as the development and improvement of instruments and platforms for the detection and quantification of CO₂ escape.

WP5 addressed capability gaps in the instrumentation, sensors, observational systems and techniques required to reduce the cost and uncertainty in measurement of the environment at proposed and operational CCS sites. These measurements enable site characterisation and selection, baseline survey, leak detection, leak quantification, environmental impact assessment and mitigation/remediation decisions and efficacy assessment.

STEMM-CCS developed novel sensor technology that allows - for the first time - in situ characterisation of the marine carbonate system, thus enabling detection of CO₂ leaks from sub-seabed CO₂ storage sites. These technological advances included the design, development and demonstration of technologies:

• CO₂ flux and leak measurement across the sediment-water interface, including benthic boundary layer landers for gradient flux, relaxed eddy accumulation measurements and sediment microprofilers
• Collecting baseline environmental data, including landers equipped with novel chemical sensors for characterising natural biogeochemical variability at high resolution.

In addition, novel and commercial off-the-shelf sensors were integrated with a number of platforms including landers, AUVs and ROVs, for autonomous spatial and temporal high-resolution surveys of the CO₂ release site and profiling the CO₂ plume. STEMM-CCS demonstrated that chemical sensor-based technologies can effectively detect and quantify CO₂ leakage from the seabed even in the absence of visual evidence (i.e. bubbles).

To help CCS operators assess and select an offshore, sub-seabed CO₂ storage site, STEMM-CCS developed and constructed an online decision support tool which can also assist with the identification and prioritisation of appropriate techniques that could form part of an operational site monitoring programme

WP5 Objectives:

• Develop new technologies and optimise methods to locate, detect and quantify CO₂ leakage
• Develop new technologies and techniques for measurement of biological and chemical parameters, including natural variability
• Reduce the cost of data collection by developing new technologies and techniques
• Assimilate measurement and model information to produce a decision support tool to guide mitigation and remediation action
• Develop automated low-cost seabed imaging and mapping technologies and techniques for baseline and impact assessments

STEMM-CCS invested significant effort in exporting knowledge and products to many international stakeholders through networking and presentations at international conferences such as GHGT-14, bilateral discussions with other CCS research teams in Australia and Japan, and ensuring project results are published in a timely manner in the scientific literature, including a special volume of the International Journal of Greenhouse Gas Control

WP6 Objectives:

• Capitalise on collaborative interactions with international research and development to maximise the impact of STEMM-CCS
• Maximise the exposure and dissemination of project outputs to an international audience of CCS stakeholders including industry, regulators, governments and researchers

WP7 hosted a series of events to share the project’s results and knowledge gained with a range of CCS stakeholders. These included two policy-focused events involving representatives from academia, industry and policymaking groups, as well as an open final meeting held in partnership with the International Energy Association Greenhouse Gas Group’s Offshore Geologic Storage Workshop which attracted 150 participants from all over the world.

In its four years of work, STEMM-CCS has invested in the development and training of young scientists across the spectrum of offshore CCS-relevant disciplines. More than 20 postgraduate (PhD) and postdoctoral researchers were directly involved in STEMM-CCS effort, and indeed they represent a critical component of the project’s work force without whom it would have been impossible to achieve such significant scientific advances. STEMM-CCS has encouraged development (at all career stages) beyond the boundaries of everyday research by providing opportunities and financial support to undertake placements in industry and elsewhere in the CCS research community. A series of training events has enabled project researchers to broaden their perspectives, learn from CCS experts outside the consortium, and strengthen their use of cross-platform tools and techniques to increase the impact of their research.

The project has also undertaken outreach and dissemination activities with local communities who may be affected by CCS operations in the North Sea, and provided a range of resources to those wanting to learn more about the environmental aspects of offshore CCS. The controlled release expedition in spring 2019 provided excellent outreach opportunities: as well as a steady social media feed and a daily blog from the ship, a series of live sessions from the ship enabled direct engagement with schoolchildren in the UK.

• Ensuring that accurate information about CO₂ storage is available by providing clear and accessible communications designed for specific audiences
• Facilitating interaction between project scientists and stakeholders
• Establish a comprehensive training programme for the community of postgraduate and postdoctoral researchers involved in the project, and industry and regulatory stakeholders