• Energy Research

Abstract Carbon storage is required to keep rising global temperatures below 2°C, meanwhile, storage reservoirs monitoring is required for assurance of early detection of potential leakages. Projects such as QICS and STEMM-CCS have used small in-situ experiments to develop detection techniques, tools, and strategies. Given the expense of experiments it is crucial to develop accurate simulation models that replicate observed behaviours and can be extrapolated to many different scenarios. However, anomalies occur between modelled and experimental data, and a key question has been how can the models be improved? This has been approached through the development of a complex modelling system to include the effects of coastal hydrodynamics on very localised experiments, with a new multi-phase leakage model – PLUME, integrated into a high-resolution hydrodynamic model, and linked to a carbonate system for CO2 analysis. The resolution of the nested domains range from 2.5 km at the boundaries to approximately 0.5 - 1.0 m at the release sites. The efficacy of the PLUME model is demonstrated with application to the STEMM-CCS and QICS experimental sites in 120 and 9-12 m water depths respectively. Results show that the newly developed model can predict observed pCO2 and pH changes within acceptable errors. Local effects are shown to be affected greatly by both the resolution and the water currents, with momentary spikes in pCO2 and reductions in pH caused by tidal oscillation. The spatial impacts of the releases are shown to move with the tide, covering a far greater area over a tidal cycle.

Abstract 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 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.

Abstract 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.

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.

Abstract The dynamics and plume development of injected CO2 dispersion and dissolution through sediments into water column, at the STEMM-CCS field experiment conducted in Goldeneye, are simulated and predicted by a newly developed two-phase flow model based on Navier-Stokes-Darcy equations. In the experiment, CO2 gas was released into shallow marine sediment 3.0 m below the seafloor at 120 m water depth in the North Sea. The pre-experimental survey data of porosity, grain size distributions, and brine concentration are used to reconstruct the model sediments. The gas CO2 is then injected into the sediments at a rate of 5.7 kg/day to 143 kg/day. The model is validated by diagnostic simulations to compare with field observation data of CO2 eruption time, changes in pH in sediments, and the gas leakage rates. Then the dynamics of the CO2 plume development in the sediments are investigated by model simulations, including the leakage pathways, the fluids interactions among CO2/brine/sediments, and CO2 dissolution, in order to comprehend the mechanisms of CO2 leakage through sediments. It is shown from model simulations that the CO2 plume develops horizontally in the sediments at a rate of 0.375 m/day, CO2 dissolution in the sediments is at an overall average rate of 0.03 g/sec with some peaks of 0.45 g/sec, 0.15 g/sec, and 0.3 g/sec, respectively, following the increase in injection rates, when some fresh brine provided. These, therefore, lead to a ratio of 0.90~0.93 of CO2 leakage rate to injection rate.