The ConsenCUS Project: Carbon Neutral clusters by Electricity-based Innovations in Capture, Utilisation and Storage

The project presents technological innovations in the three main components of CCUS: (i) carbon capture based on alkali absorption, coupled to a novel electrodialysis cell (100 kg CO2/h), (ii) conversion of CO2 to formate and formic acid (5 kg CO2/h) and (iii) safe cyclic loading of CO2 into salt formations and aquifers for temporary storage. Points (i) and (ii) include installing a mobile CO2 capture and utilisation demonstration plant at industrial partners: major cement, magnesia and oil refining sites. We will focus here on the methods and early results regarding the temporary storage solutions. Numerical simulations investigate the storage capacity and recovery efficiency to estimate the possible injection/production rates with a focus on the role of geochemical interactions between the injected CO2, the formation water, and the reservoir rock at the Stenlille site in Denmark. Following a comprehensive review of the thermo-mechanical behaviour of rock salt in the context of gas storage, a first series of experimental work also studied the deformation mechanics of rock salt relevant to salt cavern operation parameters

Microfracturing and microporosity in shales

Shales are ubiquitous rocks in sedimentary basins, where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories (waste waters, CO2, nuclear fuels). Moreover, when they contain organic matter, they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial, a process referred to as primary migration. And when the hydrocarbons cannot escape, these rocks can be exploited as oil or shale gas reservoirs. While the presence of fractures at the outcrop scale has been described, the existence of fractures at smaller scales, their link with microporosity, the mechanisms that created them, their persistence over geological times, and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data. Here, we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability. One question is whether such microfractures, when observed in outcrops or in drilled core samples extracted from boreholes, are related to decompaction and do not exist at depth. Another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation. The mechanisms of microfracture development are described. Some have an internal origin (fracturing by maturation of organic matter, dehydration of clays) while others are caused by external factors (tectonic loading). Importantly, the amount of microfracturing in shales is shown to depend strongly on the content in 1) organic matter, and 2) strong minerals. The nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale. Their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso- to macro-scales. Such percolating microfracture networks could control both the long-term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks. Finally, possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

Impact of second phase content on rock salt rheological behavior under cyclic mechanical conditions

Safe Underground Gas Storage (UGS) can be achieved in artificial, salt caverns to meet fluctuations in energy demand by providing adequate knowledge on rock salt when subjected to similar cyclic conditions. In this study, we performed cyclic mechanical tests on five rock salt samples with different types and amounts of second-phase mineral content. A confining pressure of 25 MPa was applied, whilst the axial stress was cycled between 4.5 and 7.5 MPa, at 0.5 kN/s loading rate, during 48 h (7200 cycles). The results demonstrate that high second-phase content such as anhydrite layering operates as a strength weakening agent by accommodating larger brittle deformation in comparison to samples with a lower content in secondary minerals. This rheological behavior is further exacerbated by the cycling mechanical conditions and recorded by a marked step on Young’s modulus and Poisson’s ratio value evolution. The microstructure analysis reveals how halite grains accommodate most of the deformation induced by the cyclic mechanical loading conditions through brittle deformation with microfracturing network development. Other structures from different deformation mechanisms are also discussed. Two types of new porosity are observed: (i) pores around isolated crystals of second-phase minerals as a result of grain rotation under cyclic mechanical deformation, and (ii) microcracks in areas with high concentration of secondary minerals (such as anhydrite, polyhalite, carnallite, or kieserite). This porosity change has strong implications for both the mechanical behavior of the material and its potential permeability

Fluid-driven cyclic reorganization in shallow basaltic fault zones

Faults represent a critical heterogeneity in basaltic sequences, yet few studies have focused on their architectural and hydromechanical evolution. We present a detailed, multi-scale characterization of passively exhumed fault zones from the layered basalts of the Faroe Islands, which reveals cyclic stages of fault evolution. Outcrop-scale structures and fault rock distribution within the fault zones were mapped in the field and in 3-D virtual outcrop models, with detailed characterization of fault rock microstructure obtained from optical and scanning electron microscopy. The fault zones record deformation localization from decameter-wide Riedel shear zones into meter-wide fault cores that contain multiple cataclastic shear bands and low-strain lenses organized around a central slip zone. Shear bands and the slip zone consist of (ultra-) cataclasites with a zeolite-smectite assemblage replacing the original plagioclase-pyroxene host rock composition. Low-strain lenses are breccias of weakly altered host rock or reworked fault rocks. Slip zone-proximal zones show significant late-stage dilatation in the form of hydrothermal breccias or tabular veins with up to decimeter apertures. We interpret these structures as evolving from alternating shear-compaction and dilation through hydrofracture. The fault core preserves slip zone reworking, which is interpreted to indicate repeated shear zone locking and migration. The alternating deformation styles of shear-compaction and dilatation suggest episodic changes in deformation mechanisms driven by transient overpressure and release. The fault zone mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing. We suggest that the model presented for fault evolution should apply widely to shallow, basalt-hosted fault zones

Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province

Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip-related deformation on upper-crustal faults in basaltic rocks is closely associated with fluid-rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt-derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault-rock petrology is determined by optical petrography and automated phase identification based on micrometer-scale chemical maps from scanning electron microscope X-ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X-ray fluorescence. The fault rocks show intense fluid-mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along-fault percolation of carbonated meteoric water, increasing fluid-rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure-solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid-mediated alteration by CO2 and water, while other species are redistributed within the fault zones

Yield envelope assessment as a preliminary screening tool to determine carbon capture and storage viability in depleted southern north-sea hydrocarbon reservoirs

This paper describes a laboratory study of the hydro-mechanical properties of samples from the Sherwood Sandstone Group (SSG), an onshore analogue of the finer grained, lower porosity portions that make up the Bunter Sandstone Formation (BSF). The study provides a yield envelope for this sandstone, and demonstrates that it is a competent sandstone at relevant reservoir depths. A theoretical yield envelope has been calculated based on the anticipated in situ stress induced by depletion and reinjection, showing that only the high porosity (35%), large grain diameter (290 µm) end-member of the BSF is likely to result in deformation of the reservoir rock. Stress analysis of four fields within the Southern North Sea suggest that depletion of 10 MPa will not result in permanent deformation of the reservoirs assuming similar porosity and grainsize characteristics to the SSG tested. Furthermore, re-inflation is unlikely to result in permanent deformation should the injection pressure not exceed the initial pre-production reservoir pore pressure

A strength inversion origin for non-volcanic tremor

Non-volcanic tremor is a particularly enigmatic form of seismic activity. In its most studied subduction zone setting, tremor typically occurs within the plate interface at or near the shallow and deep edges of the interseismically locked zone. Detailed seismic observations have shown that tremor is composed of repeating small low-frequency earthquakes, often accompanied by very-low-frequency earthquakes, all involving shear failure and slip. However, low-frequency earthquakes and very-low-frequency earthquakes within each cluster show nearly constant source durations for all observed magnitudes, which implies characteristic tremor sub-event sources of near-constant size. Here we integrate geological observations and geomechanical lab measurements on heterogeneous rock assemblages representative of the shallow tremor region offshore the Middle America Trench with numerical simulations to demonstrate that these tremor events are consistent with the seismic failure of relatively weaker blocks within a stronger matrix. In these subducting rocks, hydrothermalism has led to a strength-inversion from a weak matrix with relatively stronger blocks to a stronger matrix with embedded relatively weaker blocks. Tremor naturally occurs as the now-weaker blocks fail seismically while their surrounding matrix has not yet reached a state of general seismic failure

CO2 sequestration potential in Indian basalts

India currently produces 7 – 8% of annual global CO2 emissions, with energy production accounting for 69% of India’s emissions. As such there is growing interest in the potential for emissions reductions through Carbon Capture and Storage (CCS). Vishal et al., (2021) estimated the theoretical storage capacity of India. However, many large emission sources are located distant from sedimentary basin settings where CO2 storage may conventionally be considered. Continental flood basalts of the Deccan Traps cover an estimated 500,000 km3 in west-central India, and have been identified for potential CO2 storage. The concept of CO2 storage in basalt relies on CCS by mineralisation (CCSM) as basalt can act as a ready source of divalent cations such as Mg2+ and Ca2+. These can combine with carbonate, CO32-, from dissolved CO2, to produce carbonate minerals such as Magnesite (MgCO3). The CarbFix project in Iceland is a successful example of CCSM in basalt. CO2 is captured from the industrial emission site and piped to the CarbFix operation, where CO2 gas is dissolved in water to produce carbonated water and injected into the basalt (Matter and Kelemen, 2009). A key point to note is that a successful basalt storage scheme requires a target formation with sufficient porosity and permeability to sustain the required injection rates. We present an experimental study focused on characterising basalt of the Ambenali and Poladpur formations sampled from the Killari-1 borehole to assess the storage potential of Deccan Trap basalts. A density log for the Killari-1 borehole is shown in Figure 1, illustrating significant physical property variations throughout the site. Rubbly flow tops and vesicular layers which may be considered as potential injection intervals can be identified from the density log. Higher density layers may act as low permeability top seals. X-ray computed tomography (CT) was performed on six selected core samples (c. 120 mm in length and 54 mm in diameter) using a Geotek rotating X-ray computed tomography (RXCT) core scanner at the British Geological Survey’s (BGS) Core Scanning Facility (CSF). Sample depths are shown on Figure 1. Total and connected porosity was calculated using digital rock analysis (PerGeos by ThermoFisher). Figure 2 illustrates the porosity network for sample KIL-110 taken from a low-density zone at a depth of 231 m. The left image (blue) shows total porosity, while the right image (purple) shows total connected porosity which is 15.57%. For comparison, KIL-123, at a depth of 168 m has no porosity and is therefore unlikely to contribute to fluid flow unless open fractures or joint networks are present. Previous geochemical experiments carried out on Deccan Trap samples have indicated that major dissolution of primary carbonates and precipitation of secondary minerals such as siderite occurs during CO2 exposure. This confirms the potential for mineral trapping of CO2. Using samples from the Killari-1 borehole, a new series of experiments has been initiated to explore these geochemical processes more fully, including the reaction rates and implications for CO2 storage. An ongoing series of batch experiments using powdered basalt samples at a range of temperatures is currently underway, with regular fluid sampling and monitoring to provide valuable information on reaction rates. The experiments are conducted under 100 bar CO2 headspace and at temperatures of 50°C, 100°C and 150°C. As well as providing some relatively high temperature data points where reactions will progress more rapidly, the selected temperature range reflects the significant variation in geothermal gradient across the Deccan Volcanic Province. SEM and XRD reacted solids will provide information on changes from pre to post experiment. The experiments will be complemented by additional batch experiments conducted at similar conditions using cut, rather than powdered sample material. As well as generating more realistic data on reaction rates, this approach will enable detailed characterisation of the reacted material surfaces through a before and after comparison of specific surface sites via SEM. A subsequent suite of flow-through experiments will be conducted on core samples to investigate the impact of flowing water with a high dissolved CO2 content through the basalt. These experiments will enable the assessment of the dissolution and precipitation potential of porous basalt systems, and, therefore, the potential impacts on flow within a representative flow zone. Effluent chemistry will be monitored to assess directions and rates of reaction within the system, allowing an assessment of the storage potential of Deccan Trap basalts. The experiments will be conducted under pressure and temperature appropriate to potential storage depths. Acknowledgements The research was part of the BGS International NC programme ‘Geoscience to tackle Global Environmental Challenges’, NERC reference NE/X006255/1. Nimisha Vedanti acknowledges ECCSEL-ERIC for supporting transnational access to ECCSEL research infrastructure at the British Geological Survey. The Director, NGRI is acknowledged for permission to conduct research on NGRI Basalt samples. The extended abstract is published by permission of the Director of the British geological Survey. References Gupta, H.K., Srinivasan, R., Rao, R.U.M., Rao, G.V., Reddy, G.K. and others. 2003. Borehole investigations in the surface rupture zone of the 1993 Latur SCR earthquake, Maharashtra, India: Overview of results. Memoir of the Geological Society of India, 54, 1–22. Matter, J., Kelemen, P. 2009. Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation. Nature Geosci 2, 837–841. Vishal, V., Verma, Y., Chandra, D. and Ashok, D. 2021. A systematic capacity assessment and classification of geologic CO2 storage systems in India. International Journal of Greenhouse Gas Control, 111, 103458

Lithological constraints on borehole wall failure: a study on the Pennine Coal Measures of the United Kingdom

Stress-related borehole deformation features have been documented across the United Kingdom, most commonly as borehole breakouts and drilling induced tensile fractures (DIFs). Recent studies using borehole imaging have allowed more detailed investigation of these features and the processes that control their formation. Within the Pennsylvanian Pennine Coal Measures Group (PCM) of the United Kingdom borehole imaging has highlighted a disproportionately high number of breakouts occurring within paleosols located immediately below coal seams. To understand the processes controlling breakout formation, a 10.5 m section of core from the Melbourne 1 borehole, incorporating a typical coal seam / paleosol sequence, was analyzed using multiple techniques including: scanning electron and optical microscopy, X-Ray Fluorescence (XRF), X-ray radiography, Point Load testing, wireline petrophysics and track-based core scanning for physical properties. Strength measurements highlight that breakouts form preferentially in poorly consolidated sediments, with low tensile strength, cross-cut by listric surfaces. The formation and termination of breakouts also corresponds to zones of diagenetic iron mineral growth with a lower propensity to fail. These coincide with greater preservation of sedimentary structures and an increase in the rock’s tensile strength; this intra-unit variation in tensile strength constrains breakout length. This demonstrates that secondary diagenetic processes, including the growth of iron minerals impose, lithological controls on the formation and length of borehole breakouts within the United Kingdom PCM