Carbon Capture And Storage Sites Research Articles

Technology Focus: Well Integrity (January 2024)

The context of last year’s Well Integrity Technology Focus was the repurposing of existing oil and gas wells for carbon storage, flow-wet material conformity, and associated integrity risks. Typically, oil and gas producing wells are abandoned toward the end of their life cycles with reservoir pressures depleted compared with the virgin pressures. Carbon capture and storage (CCS) projects, however, present different operating conditions. First, the pressurized containment sites have a higher life expectancy, say, between 50 and 100 years. Second, the containment pressure is possibly higher than the virgin pressure. The overpressure limit is dependent on several factors, especially the caprock sealing capacity. It goes without saying that CCS storage in wells requires long-term integrity (i.e., for the entire project life cycle). It is well-known that barriers such as cement and casings deteriorate over time. Deterioration of barriers may be accelerated by chemical reaction with carbon dioxide or thermal cycling. Hence, these storage sites could present unique life-cycle integrity risks perhaps never experienced by the oil and gas industry. To further explain the scenario, a possible breach of the confining strata resulting in pressure communication to the overburden means CO2 finding its way through the deteriorated well barriers of any active or shut-in well, resulting in CO2 ingress to the upper formations; groundwater contamination; or, in a worst-case scenario, appearance at the surface. Therefore, one of the important subsurface aspects of a CCS site is the presence of two seals (i.e., primary and secondary), with sealing capacity of both caprocks characterized robustly. Lack of double barriers or improper characterization of these confining strata could present a scenario wherein CO2 flows into overburden (i.e., above the caprock). The wells’ outer annuli then could become a potential path of least resistance for CO2 to migrate further up. Such situations can defy the primary objective of CCS projects and can require mitigation with proper attention to detail following the “as low as reasonably practicable,” or ALARP, principle. The scenario discussed is unique and complex and requires continuous managing and monitoring of CCS storage sites. Furthermore, the role of artificial intelligence and automation is seen as imperative during the operational stage of the project. In addition to the primary paper selections synopsized here, three SPE papers focusing on artificial intelligence and machine learning as tools for well integrity management are recommended as alternates for further reading. Recommended additional reading at OnePetro: www.onepetro.org. SPE 212479 Improve Well Integrity Using an Annular Barrier AI Tool by Eirik Time, Equinor, et al. SPE 211537 Transforming Production-Phase WIMS to a Game Changer Smart Well Integrity Monitoring System With Real-Time Automated Engineering Analytics by Mohammed Jasem, ADNOC, et al. SPE 216600 Paradigm Shift in Well Integrity Management Through the Use of Autonomous Wireless Well Intervention Robots by Julio Q.M. Guedes, Ouronova, et al.

Long range acoustic detection of gas bubbles in a shallow water coastal environment

As the world economy moves to net zero carbon dioxide (CO2) emissions, carbon capture and storage (CCS) is recognised a mitigation mechanism in the suite of technology options. Liquified CO2 injection into offshore subsea reservoirs will be one of the CCS strategies used. Despite being improbable, CO2 leaks could occur and impact the effectiveness of this CO2 mitigation option and undermine confidence in its use as part of the suite of tools to combat climate change. To satisfy public safety expectations and environmental regulatory requirements, long-term measurement, monitoring, and verification (MM&V) plans that are affordable will be needed as part of any commercial CCS operation. Active acoustic methods are a well-suited technique as they can detect gas bubbles in the water column at extremely low flow rates. A bespoke detection system, the Echosounder Detection of Gas Events (EDGE) was developed with the goals of long-duration monitoring and detection of low flow gas bubble releases at long ranges; the literature suggests flow rates of <2 L min−1 will be readily detectable using active acoustics, well below the upper limits of a release of 1 % of stored CO2 over a 1000 year timeframe proposed by IPCC (2005). The EDGE system was housed on a seafloor lander with a battery powered scientific echosounder connected to a narrow-beam horizontally orientated transducer mounted on a turret that rotates through 360 deg. The effectiveness of the EDGE system was tested in a range of weather conditions during two six-month deployments in ∼ 20 m depth at a proposed CCS storage site, CarbonNet, situated off the coast of Victoria, Australia. Separate experiments with controlled gas releases were conducted at a local site at Hobart, Australia to establish the detection capability at range for low flow rates. These experiments found very small gas-releases (< 2 L min−1) could be detected at ranges of up to 378 m. The ability of long-term deployment of the EDGE at the proposed CCS site and the local experiments establishing range-detection ability have demonstrated that active acoustic systems can meet the dual objectives of sustained and wide-area monitoring as a key part of an aquatic based MM&V framework.

A Comprehensive Review on Carbon Dioxide Sequestration Methods

Capturing and storing CO2 (CCS) was once regarded as a significant, urgent, and necessary option for reducing the emissions of CO2 from coal and oil and gas industries and mitigating the serious impacts of CO2 on the atmosphere and the environment. This recognition came about as a result of extensive research conducted in the past. The CCS cycle comes to a close with the last phase of CO2 storage, which is accomplished primarily by the adsorption of CO2 in the ocean and injection of CO2 subsurface reservoir formation, in addition to the formation of limestone via the process of CO2 reactivity with reservoir formation minerals through injectivities. CCS is the last stage in the carbon capture and storage (CCS) cycle and is accomplished chiefly via oceanic and subterranean geological sequestration, as well as mineral carbonation. The injection of supercritical CO2 into geological formations disrupts the sub-surface’s existing physical and chemical conditions; changes can occur in the pore fluid pressure, temperature state, chemical reactivity, and stress distribution of the reservoir rock. This paper aims at advancing our current knowledge in CO2 injection and storage systems, particularly CO2 storage methods and the challenges encountered during the implementation of each method and analyses on how key uncertainties in CCS can be reduced. CCS sites are essentially unified systems; yet, given the scientific context, these storage systems are typically split during scientific investigations based on the physics and spatial scales involved. Separating the physics by using the chosen system as a boundary condition is a strategy that works effectively for a wide variety of physical applications. Unfortunately, the separation technique does not accurately capture the behaviour of the larger important system in the case of water and gas flow in porous media. This is due to the complexity of geological subsurface systems, which prevents the approach from being able to effectively capture the behaviour of the larger relevant system. This consequently gives rise to different CCS technology with different applications, costs and social and environmental impacts. The findings of this study can help improve the ability to select a suitable CCS application method and can further improve the efficiency of greenhouse gas emissions and their environmental impact, promoting the process sustainability and helping to tackle some of the most important issues that human being is currently accounting global climate change. Though this technology has already had large-scale development for the last decade, some issues and uncertainties are identified. Special attention was focused on the basic findings achieved in CO2 storage operational projects to date. The study has demonstrated that though a number of CCS technologies have been researched and implemented to date, choosing a suitable and acceptable CCS technology is still daunting in terms of its technological application, cost effectiveness and socio-environmental acceptance.

Evaluation of passive acoustic methods for ambient noise baseline and gas flow rate quantification at a proposed nearshore carbon capture and storage site in Australia

Measurement, monitoring, and verification (MMV) is an integral component of carbon capture and storage (CCS) projects. Within an operational MMV equipment, hydrophone-based passive acoustic techniques are used to establish ambient noise baseline and flow rate quantification at short range, specifically to facilitate “detect-attribute-quantify” sequence of an MMV program. However, nearshore environments are acoustically complex with different soundscape components that can disproportionately dominate ambient noise levels, potentially masking acoustic signatures of bubbles used to quantify seabed gas seeps. Therefore, a robust baseline describing ambient noise variability across the range of frequencies associated with acoustic emissions of gas seeps is required, from which changes can be detected and monitored. In this context, hydrophone measurements from a proposed nearshore CCS site in Australia are analyzed to establish a temporally resolved baseline, identifying key drivers causing overall ambient noise variability. These results are compared with acoustic bubble spectrum features and flow rate estimates from a controlled in situgas release experiment to understand the likelihood of detecting bubbles and quantifying flow rate at the proposed CCS site. Despite the complexities of nearshore environment, the evaluation highlights that passive acoustic methods can provide a practical solution to complement quantification component of operational MMV programs.

Evaluation of passive acoustic methods for ambient noise baseline and gas flow rate quantification at a proposed nearshore carbon capture and storage site in Australia

Measurement, monitoring, and verification (MMV) is an integral component of carbon capture and storage (CCS) projects, providing valuable feedback for management practices and assuring integrity. Within an operational MMV equipment, hydrophone-based passive acoustic techniques are used to establish ambient noise baseline and flow rate quantification at short range, specifically to facilitate ‘detect-attribute-quantify’ sequence of an MMV program. But nearshore environments are acoustically complex with overlapping components of biophony, geophony, anthropophony, and deployment noise resulting from multiple instruments used for MMV. These soundscape components can disproportionately dominate ambient noise levels, potentially masking acoustic signatures of bubbles used to quantify seabed gas seeps. Therefore, a robust baseline describing ambient noise variability across the range of frequencies associated with acoustic emissions of gas seeps is required, from which changes can be detected and monitored. In this context, multi-year hydrophone measurements (20 Hz to 24 kHz) from a proposed nearshore CCS site in Australia are analysed to establish a temporally resolved baseline, identifying key drivers causing overall ambient noise variability. These results are compared with acoustic bubble spectrum features and flow rate estimates from a controlled in situ gas release experiment. The experiment was performed using a custom-made bubble generator at 1, 10, and 25 m range from the hydrophone to understand the likelihood of detecting bubbles and quantifying flow rate at the proposed CCS site. Despite the complexities of nearshore environment, the evaluation highlights that low-cost hydrophone-based passive acoustic methods can provide a practical solution to complement quantification component of operational MMV programs.

Integrating risk assessment methods for carbon storage: A case study for the quest carbon capture and storage facility

This paper documents how tools developed by the National Risk Assessment Partnership (NRAP) complement the Bowtie risk assessment approach through the demonstration of a risk-based Area of Review (AoR) determination for the Quest Carbon Capture and Storage (CCS) facility near Edmonton, Alberta with the NRAP Open-Source Integrated Assessment Model (NRAP Open-IAM). The Bowtie risk assessment for the Quest CCS site was developed by Shell Canada Ltd., who operates the asset on behalf of the Athabasca Oil Sand Project — a joint venture, owned by Canadian Natural Resources Ltd, Chevron Canada and Shell Canada Ltd. The Quest Bowtie treats site risk comprehensively – considering a large set of technical and project risks (e.g., operational risks, above ground infrastructure, public perception, etc.). NRAP tools, methods, and analyses are focused on quantitative assessment of subsurface technical risks. NRAP-Open-IAM was used to evaluate potential groundwater impacts at the Quest site, i.e., a sufficient change above background groundwater quality levels to be measurable, should CO2 and/or brine leak from the reservoir via a faulty and/or compromised wellbore. Our results show there is a low potential for groundwater impact and support a further reduction (from ∼460 to 100 km2) to the established AoR for the Quest site. Integrating both approaches early in the site characterization period could significantly benefit commercial-scale projects by drastically reducing the region over which leakage risks (e.g., legacy wells) need to be evaluated, i.e., the AoR. This is especially true for brownfield sites, which could contain hundreds to thousands of legacy wells. Further, basing monitoring network design on those regions where impacts are most likely to occur has the potential to facilitate more efficient deployment of monitoring technologies and improve the likelihood of detecting a leak should one occur.

Variability in acoustic backscatter and fish school abundance at a shallow water CCS site

Carbon capture and storage (CCS) is one approach used to reduce carbon dioxide (CO2) emissions to the atmosphere. Subsea CO2 storage sites must have low risk of leakage and have minimal impact on the marine environment. Single beam echosounders (SBES) are a good technology for detecting CO2 leakage in the water column for application in a marine Measurement, Monitoring and Verification (MM&V) framework. Characterisation of active acoustic data is needed to determine whether levels of water column backscatter enable CO2 gas leaks to be detected, and to understand acoustic false positives caused by naturally occurring acoustic signals (e.g., fish schools). We used acoustic data collected by SBES fitted to two mobile and two fixed sensor platforms over four years to characterise the site of the proposed CarbonNet CCS Project subsurface CO2 storage reservoir. We observed diel and seasonal variability in water column backscatter and fish school abundance. Water column backscatter was highest at night and in Austral summer and autumn periods but was at a level that would allow small CO2 gas leaks to be detected. Fish abundance was highest during the day and in Austral summer and autumn. To reduce the likelihood of acoustic false positives, surveys designed to detect CO2 gas leaks using active acoustic technologies could prioritise data collection at night and during the months where fish school abundance is lowest. Understanding these sources of natural variability can guide survey strategies and response actions.

Integrating underground gas storage experience for CCS optimisation

Carbon capture and storage (CCS) sites require diligent assessments to account for potential leakage over time. Leakage pathways include both natural and engineered (e.g. wells) invoking the requirement for a structured technical analysis as part of a risk inventory. This paper focuses on lessons learnt from underground gas storage (UGS) sites to guide CCS technical workflows to handle natural geological risk elements. UGS facilities have been operating for over a 100 years. Their injection and withdrawal histories, by their very nature, test the complex changes that occur to the reservoir and cap-rock over time and provide valuable insight to demonstrate the feasibility of CCS and its potential for long-term storage. Geomechanical assessment of the reservoir and cap-rock requires an inventory of core material, log and seismic data with the aim of selecting the appropriate data to model the ability of that formation to contain a column of CO2. As supercritical CO2 is injected into the formation and pore pressure builds, the geomechanics of the reservoir needs diligent review to assess the potential to trigger failure on the cap-rock and pre-existing faults and fractures. UGS injection/withdrawal histories assist in the reservoir and overburden static and dynamic modelling along with designing a measurement, monitoring and verification process for CCS sites. Technical workflows, from seismic and static modelling to geomechanics and dynamic simulation and back to seismic for containment monitoring, are discussed that have been refined by UGS experience to benefit the selection of potential CCS sites.

Silixa Chief Expects CCS Boom To Create “Huge Opportunity” for Fiber Optics

_ Fiber optics represents one of the biggest technological success stories in the recent history of the oil and gas industry. A single fiber-optic cable installed outside the casing, inside the tubing, can deliver a wealth of data over a well’s life cycle. It can sense minute shifts in acoustics, temperature, and has recently been developed to detect changes in near-wellbore strain—all prized clues as to what is helping or hurting productivity. But less than a decade ago, the ultrathin glass cables were considered exotic, and only a handful of first adopters dared to deploy them downhole. The sensing technology has, of course, gone on to become a mainstay of subsurface diagnostic programs for operators working across the planet. Now, with their skills sharpened in the upstream industry, many fiber-optics developers are gearing up for what appears to be a coming boom in carbon capture and storage (CCS) capacity. Among them is UK-based Silixa. Founded in 2007, Silixa’s innovations have improved on the spatial resolution of fiber measurements and enabled real-time monitoring of fracture growth and production. Last year, the company installed the world’s first subsea distributed acoustic sensing (DAS) system for BP to enable 4D seismic acquisition in the Gulf of Mexico. Glynn Williams, CEO of Silixa, shares in this Q&amp;A how all this experience points to fiber having a major role to play in the rise of the CCS sector—especially when it comes to safety and regulatory matters. His background features more than 30 years spent in the upstream industry’s data acquisition arena. Williams also holds a degree in mining engineering from the University of Wales and is a member of SPE. Where do you see the carbon sequestration sector headed—is an inflection point coming? This now consumes a lot of my time. We see CCS as a huge opportunity because we’ve been engaged with the sector for a long period of time. For the type of solutions that we deliver, we can extract perhaps $1 of value for each ton per year of CO2 that is stored. When you look at the projections by the IEA, which suggest we may be storing over 7.5 gigatons of CO2 by 2050, that relates to $7.6 billion of value. That’s if we were to be successful in securing all of that—there will be other players alongside us. But to this point, we’ve provided services to 17 CCS sites going back as early as 2012. So, we’ve got a track record with our environmental business unit that serves CCS, and we’re seeing in the current period a twofold increase in pipeline opportunities. We feel that particular business unit will account for 40% of our overall pipeline in 2023.

Dawsonite as a Temporary but Effective Sink for Geological Carbon Storage

The possibility of using dawsonite mineral trapping as a carbon capture and storage (CCS) strategy intrigues many. In this study, we used a dawsonite-rich (∼10%) CO 2 gas reservoir in the Hailar basin in northern China as a natural analogue of a CO 2 storage site, along with numerical modeling, to demonstrate that a large amount of dawsonite can be generated in sandstone formations, provided sufficient Na-rich feldspar and CO 2 gas are available. While precipitated dawsonite can be preserved only in a hydrodynamically-closed system in the long term under high CO 2 fugacity and log((Na + )/(H + )) activities in solution, short-term trapping of CO 2 in dawsonite (on the order of 10 kyr) is possible and lowers CO 2 pressure, which mitigates the risk of CO 2 leakage to the ground surface or overlying drinking water aquifers. The re-dissolution of dawsonite after a few thousand years facilitates progressive dissipation of the gas phase CO 2 over time. Consideration of reservoirs or saline aquifers with minerals or formation water that can provide a high abundance of dissolved sodium, significantly increases the number of potential CCS sites globally. Furthermore, alternating water-and-gas injection regimens could enhance the precipitation of dawsonite in Na-rich aquifers. Future editions of the Carbon Storage Atlas should consider aquifer geochemistry in the site selection for secure long-term carbon storage in addition to the volumetric considerations for short-term operation.

Ranking and Evaluation of CO2 Storage Sites Using an Advanced Workflow

The world is in urgent need of carbon capture and storage (CCS) sites/facilities to achieve ambitious net carbon dioxide (CO2) emission reduction goals. After CO2 capture and transport, storage is the third step of the CO2 journey. Accessing and utilizing regional seismic information is a significant part of any workflow attempting to identify and characterize proposed subsurface CO2 storage sites. In this paper, we have developed and implemented a workflow over a proof-of-concept (PoC) area to assess CCS storage capacity and containment at a candidate saline aquifer site. Injectivity and monitoring/monitorability are assessed as part of an extended workflow. The integrated PoC CCS site assessment workflow allows validation of the various workflow elements and technologies, with the view to creating an efficient and scalable tool for regional site identification and characterization. The current project has been established using a PGS regional multi-client broadband seismic dataset in the North Sea which comprises an extensive cross-border regional dataset in the UK and Norway. The broadband nature of the seismic data allows significant and efficient site assessment, by providing detailed 3D characterization of the subsurface, away from areas with well control, and the parameterization of more accurate/reliable attributes for key storage attributes such as net-to-gross, porosity, and thickness. Finally, the integrated workflow and data integration allowed us to perform an efficient carbon storage site risk assessment as part of an overall site ranking process.

Detecting and mapping a CO2 plume with novel autonomous pH sensors on an underwater vehicle

We report the first successful use of chemical sensors integrated on to an underwater vehicle to locate, map and estimate flux from a controlled sub-seabed CO2 release, analogous to a leak from a Carbon Capture and Storage (CCS) reservoir. This has global implications for the efficacy and cost of monitoring of offshore CCS sites and hence public and regulatory confidence as this tool for addressing climate change is considered and rolled out. A remotely operated vehicle (ROV) equipped with three different pH sensors was deployed to determine the spatial extent of the controlled release. The sensors each operated on a different principle (spectrophotometric, fluorescence, and electrochemical) and the strengths and weaknesses of each sensor are discussed. The sensor data demonstrated that evidence of the plume was limited to within 3 m of the seafloor, as predicted by previous modelling work. The data were then utilised to develop a model of the plume, to extend the spatial coverage of the data. This comparison of the three sensors and the insight into plume dynamics provided by the model would assist in the planning of future plume surveys to ensure the sensor and vehicle combination can resolve the plume of interest.

A Threshold Line for Safe Geologic CO2 Storage Based on Field Measurement of Soil CO2 Flux

Carbon capture and storage (CCS) is an established and verified technology that can implement zero emissions on a large enough scale to limit temperature rise to below 2 °C, as stipulated in the Paris Agreement. However, leakage from CCS sites must be monitored to ensure containment performance. Surface monitoring of carbon dioxide (CO2) concentrations at onshore CCS sites is one method to locate and quantify CCS site leakage. Employing soil accumulation chambers, we have established baseline data for the natural flux of CO2 as a threshold alert to detect CO2 leakage flux to ensure the safety of onshore CCS sites. Within this context, we conducted on-site CO2 measurements at three different locations (A, B, and C) on the INAS test field at the Ito campus, Kyushu University (Japan). Furthermore, we developed a specific measurement system based on the closed-chamber method to continuously measure CO2 flux from soil and to investigate the correlation between CO2 flux from the soil surface and various parameters, including environmental factors and soil sample characteristics. In addition, gas permeability and the effect of different locations on soil CO2 flux are discussed in this study. Finally, we present an equation for estimating the soil CO2 flux used in the INAS field site that includes environmental factors and soil characteristics. This equation assists in defining the threshold line for an alert condition related to CO2 leakage at onshore CCS sites.

Effects of impurities N2 and O2 on CO2 storage efficiency and costs in deep saline aquifers

Carbon capture and storage (CCS) is one of the key technologies for reducing CO2 emissions, but it has suffered from the high costs for the purification of captured CO2 streams. Storing CO2 with two major impurities (i.e., N2 and O2) in geological formations would be more economical; however, the cost savings might be offset by the reduced CO2 storage efficiency. To understand the relationships between the reduced CO2 storage efficiency and the CO2 storage costs in deep saline aquifers, the effects of impurities N2 and O2 on the CO2 storage efficiency and CO2 storage costs in deep saline aquifers are evaluated. We established a 2D model by developing a new module incorporated into TOUGH2, to simulate the migration and dissolution of CO2-N2-O2 mixtures in aquifers based on the hydrogeological conditions of the Shenhua CCS site in China. It is found that the partitioning between CO2 and the impurities during migration can reduce CO2 storage efficiency by raising impurity concentrations in gas phase. But higher pressure and temperature in reservoirs can alleviate the reduction of CO2 storage efficiency, while retaining the benefit of the impurities on the CO2 dissolution efficiency. With 10 mol% N2, the CO2 storage efficiency is decreased by at least 32%, while the CO2 storage cost in saline aquifers is increased less than 0.01 US$/t CO2.

Potential effect of CO2 seepage at high pressure on the marine organic matter

Seepage from a carbon capture and storage site (CCS) under the seabed can lead to CO2 enrichment of the surrounding seawater. This seepage may cause changes that alter the composition of the marine dissolved organic matter (DOM) pool. This study evaluated possible changes in the composition of the DOM in the event of CO2 seepage in sediments from a CCS site in the Baltic Sea. High-pressure (9 atm) and low pH (pH 7) conditions were simulated using a hyperbaric titanium tank over a period of 40 days. The DOM composition was analyzed by liquid chromatography coupled to a high-resolution mass spectrometer (LC-HRMS) with electrospray (ESI) as ionization mode in both positive and negative polarities. An effect of the pH7 conditions on the DOM compounds was detected in both ionization modes. In this study, it was possible to determine whether the changes observed on the DOM were caused by natural seawater variation or by the effect of pH 7. Results showed that the composition of organic compounds can be impacted in seepage areas where a pH 7 occurs.

Defining a biogeochemical baseline for sediments at Carbon Capture and Storage (CCS) sites: An example from the North Sea (Goldeneye)

Injection of carbon dioxide (CO2) into subseafloor reservoirs is gaining traction as a strategy for mitigating anthropogenic CO2 emissions to the atmosphere. Yet, potential leakage, migration and dissolution of externally-supplied CO2 from such reservoirs are a cause for concern. The potential impact of CO2 leakage on the biogeochemistry of sediments and overlying waters in the North Sea was studied during a controlled subsurface CO2 release experiment in 2019 at a potential carbon capture and storage site (Goldeneye). This study describes the natural (unperturbed) biogeochemistry of sediments. They are classified as muddy sand to sandy mud with low organic carbon content (∼0.6 %). Distributions of dissolved inorganic carbon (DIC) and total alkalinity (TA) in sediment porewaters are reported in addition to in situ benthic fluxes of dissolved nutrients and oxygen between the sediments and the overlying water. Oxygen fluxes into the sediment, measured using benthic chambers and eddy covariance, were 6.18 ± 0.58 and 5.73 ± 2.03 mmol m−2 d-1, respectively. Diagnostic indicators are discussed that could be used to detect CO2 enrichment of sediments due to reservoir leakage at CCS sites. These include the ratio TA and ammonium to sulfate in sediment porewaters, benthic fluxes and chloride-normalized cation distributions. These indicators currently suggest that the organic carbon at Goldeneye has an oxidation state below zero and is mainly degraded via sulfate reduction. Carbonate precipitation is apparently negligible, whereas decreases in Mg2+ and K+ point toward ongoing alteration of lithogenic sediments by reverse weathering processes.

Towards improved monitoring of offshore carbon storage: A real-world field experiment detecting a controlled sub-seafloor CO2 release

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.

Climate change mitigation effects: How do potential CO2 leaks from a sub-seabed storage site in the Norwegian Sea affect Astarte sp. bivalves?

Carbon capture and storage (CCS) is one of the most promising mitigation strategies for reducing the emissions of carbon dioxide (CO2) to the atmosphere and may substantially help to decelerate global warming. There is an increasing demand for CCS sites. Nevertheless, there is a lack of knowledge of the environmental risk associated with potential leakage of CO2 from the storage sites; and even more, what happens when the seepage stops. Can the environment return to the initial equilibrium? Potential effects on native macrofauna were studied under a scenario of a 50-day CO2 leakage, and the subsequent leak closure. To accomplish the objective, Trondheim Fjord sediments and clams were exposed to an acidified environment (pH 6.9) at 29 atm for 7 weeks followed by a 14-day recovery at normal seawater conditions (pH 8.0, 29 atm). Growth and survival of clams exposed to pressure (29 atm) and reduced pH (6.9) did not significantly differ from control clams kept at 1 atm in natural seawater. Furthermore, bioaccumulation of elements in the soft tissue of clams did not register significant variations for most of the analysed elements (Cd, Cr, Pb, and Ti), while other elements (As, Cu, Fe, Ni) had decreasing concentrations in tissues under acidified conditions in contrast to Na and Mg, which registered an uptake (Ku) of 111 and 9.92 μg g−1dw d−1, respectively. This Ku may be altered due to the stress induced by acidification; and the element concentration being released from sediments was not highly affected at that pH. Therefore, a 1 unit drop in pH at the seafloor for several weeks does not appear to pose a risk for the clams.

Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK

Petroleum-rich basins at a mature stage of exploration and production offer many opportunities for large-scale Carbon Capture and Storage (CCS) since oil and gas were demonstrably contained by low-permeability top-sealing rocks, such as shales. For CCS to work, there must be effectively no leakage from the injection site, so the nature of the top-seal is an important aspect for consideration when appraising prospective CCS opportunities. The Lower Cretaceous Rodby Shale and the Palaeocene Lista Shale have acted as seals to oil and gas accumulations (e.g., the Atlantic and Balmoral fields) and may now play a critical role in sealing the Acorn and East Mey subsurface carbon storage sites. The characteristics of these important shales have been little addressed in the hydrocarbon extraction phase, with an understandable focus on reservoir properties and their influence on resource recovery rates. Here, we assess the characteristics of the Rodby and Lista Shales using wireline logs, geomechanical tests, special core analysis (mercury intrusion) and mineralogical and petrographic techniques, with the aim of highlighting key properties that identify them as suitable top-seals. The two shales, defined using the relative gamma log values (or Vshale), have similar mean pore throat radius (approximately 18 nm), splitting tensile strength (approximately 2.5 MPa) and anisotropic values of splitting tensile strength, but they display significant differences in terms of wireline log character, porosity and mineralogy. The Lower Cretaceous Rodby Shale has a mean porosity of approximately 14 %, a mean permeability of 263 nD (2.58 × 10−19 m2), and is calcite rich and has clay minerals that are relatively rich in non-radioactive phases such as kaolinite. The Palaeocene Lista Shale has a mean porosity of approximately 16% a mean permeability of 225 nD (2.21 × 10−19 m2), and is calcite free, but contains abundant quartz silt and is dominated by smectite. The 2% difference in porosity does not seem to equate to a significant difference in permeability. Elastic properties derived from wireline log data show that Young’s modulus, material stiffness, is very low (5 GPa) for the most shale (clay mineral)-rich Rodby intervals, with Young’s modulus increasing as shale content decreases and as cementation (e.g., calcite) increases. Our work has shown that Young’s modulus, which can be used to inform the likeliness of tensile failure, may be predictable based on routine gamma, density and compressive sonic logs in the majority of wells where the less common shear logs were not collected. The predictability of Young’s modulus from routine well log data could form a valuable element of CCS-site top-seal appraisals. This study has shown that the Rodby and Lista Shales represent good top-seals to the Acorn and East Mey CCS sites and they can hold CO2 column heights of approximately 380 m. The calcite-rich Rodby Shale may be susceptible to localised carbonate dissolution and increasing porosity and permeability but decreasing tendency to develop fracture permeability in the presence of injected CO2, as brittle calcite dissolves. In contrast, the calcite-free, locally quartz-rich, Lista Shale will be geochemically inert to injected CO2 but retain its innate tendency to develop fracture permeability (where quartz rich) in the presence of injected CO2.

Microbial diversity of two natural CO2-rich springs with contrasting hydrochemical features

Microbial communities in CO2-rich springs have adapted to environmental pressure including low pH resulting from high concentrations of CO2. Microbial community composition and hydrochemical characteristics in the CO2-rich springs are important features affected by high levels of CO2 in groundwater ecosystems, and they may be useful indicators of CO2 leakage from carbon capture and storage (CCS) sites where CO2 is injected into deep geological reservoirs for the mitigation of climate change. To determine how long-term CO2 leakage affects the groundwater ecosystems, we analyzed the bacterial and archaeal community composition and hydrochemical features of two different natural CO2-rich springs, Hansil (HS) and Chosukol (CS), located in south-eastern Korea which are natural analog sites for groundwater ecosystems affected by CO2 leakage from geological CO2 storage sites. Analyses of hydrochemical characteristics and 16S rRNA gene amplicon sequencing of bacteria and archaea revealed a marked distinction between the two springs. The pH, the concentrations of bicarbonate ion (HCO3-), major cations including Ca2+ and Mg2+, and specific trace elements including Fe and Mn were higher in HS as compared to CS while no distinction in pCO2 between the two springs was observed. Microbial community composition was distinct between the two springs at the genus level. Specifically, the relative abundance of Gallionella, an iron-oxidizer, was significantly higher in HS whereas that of Albidiferax known as dissimilatory iron-reducing bacteria was higher in CS than HS, reflecting the difference of iron concentration in the two springs. In our study, microbial community composition appeared to be more strongly affected by the concentration of iron than that of CO2. Our results enhance the understanding of hydrochemical features and microbial communities in natural CO2-rich springs and suggest that microbial communities may be useful indicators of CO2 leakage from CCS sites.