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

Ion-species in pore fluids with opposite effects on limestone fracturing

At ten percent of sedimentary rocks, limestones are common geo-energy reservoirs. Being highly soluble, limestones are prone to fluid-assisted deformation and their mechanical behaviour likely susceptible to fluid chemistry. In this study, we saturated limestone samples with 0.4 M MgSO4 or 0.4 M Na2SO4 CaCO3-saturated solutions (naturally present in many reservoirs) or a reference CaCO3-saturated solution for 1, 50 or 200 days prior to mechanical testing. Triaxial deformation tests were then performed at 7, 30, and 70 MPa of confining pressure room temperature, under drained conditions. Our results show that exposure to one different cation impacts the strength of this rock (up to of dry rock strength) and its failure dynamics, associated with different microstructural damage distribution. A 200 day exposure to MgSO4 promotes strengthening whilst similar exposure to Na2SO4 leads to weakening. We posit that these strength changes may be related to changes in surface charges on the mineral surfaces. More data on fluid–rock interaction will be key to fully understand fracture propagation in natural carbonate formations

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

Hydraulic fracturing: a review of theory and field experience

This report summarises the current state-of-the-art knowledge of the hydraulic fracturing process used by the shale gas/oil industry using open peer-reviewed literature and from government commissioned research reports. This report has been written to make statements on our knowledge of the following questions: • How do hydrofractures form? • How far do hydrofractures extend during stimulation? • What dictates where hydrofractures propagate? • How do hydrofractures interact with the existing fracture network? • Can the size and distribution of hydrofractures be controlled? Gaps in our knowledge have been highlighted, with the largest of these resulting from differences between North American and European shale rocks

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

Laboratory micro-seismic signature of shear faulting and fault slip in shale

This article reports the results of a triaxial deformation experiment conducted on a transversely isotropic shale specimen. This specimen was instrumented with ultrasonic transducers to monitor the evolution of the micro-seismic activity induced by shear faulting (triaxial failure) and subsequent fault slip at two different rates. The strain data demonstrate the anisotropy of the mechanical (quasi-static) compliance of the shale; the P-wave velocity data demonstrate the anisotropy of the elastic (dynamic) compliance of the shale. The spatio-temporal evolution of the micro-seismic activity suggests the development of two distinct but overlapping shear faults, a feature similar to relay ramps observed in large-scale structural geology. The shear faulting of the shale specimen appears quasi-aseismic, at least in the 0.5 MHz range of sensitivity of the ultrasonic transducers used in the experiment. Concomitantly, the rate of micro-seismic activity is strongly correlated with the imposed slip rate and the evolution of the axial stress. The moment tensor inversion of the focal mechanism of the high quality micro-seismic events recorded suggests a transition from a non-shear dominated to a shear dominated micro-seismic activity when the rock evolves from initial failure to larger and faster slip along the fault. The frictional behaviour of the shear faults highlights the possible interactions between small asperities and slow slip of a velocity-strengthening fault, which could be considered as a realistic experimental analogue of natural observations of non-volcanic tremors and (very) low-frequency earthquakes triggered by slow slip events