Heterogeneous subgreenschist deformation in an exhumed sediment‐poor mélange

Many described subduction complexes (or mélanges) exhumed from seismogenic depths comprise thick, turbidite‐dominated sequences with deformed zones containing clasts or boudins of more competent sandstone and/or basalt. In contrast, many active subduction zones have a relatively small thickness of sedimentary inputs (<2 km), turbidite sequences are commonly accreted rather than subducted, and the role of pelagic sediments and basalt (lavas and hyaloclastites) in the deforming zone near the plate interface at <20 km depth is poorly understood. Field investigation of Neoproterozoic oceanic sequences accreted in the Gwna Complex, Anglesey, UK, reveals repeated lenticular slices of variably sampled ocean plate stratigraphy (OPS) bounded by thin mélange‐bearing shear zones. Mélange matrix material is derived from adjacent OPS lithologies and is either dominantly illitic, likely derived from altered siliciclastic sediment, or chloritic, likely derived from altered volcanics. In the illitic mélange, mutually cross‐cutting phyllosilicate foliation and variably deformed chlorite‐quartz‐calcite veins suggest ductile creep was cyclically punctuated by transient, localized fluid pulses. Chlorite thermometry indicates the veins formed at 260 ± 10°C. In the chloritic mélange, recrystallized through‐going calcite veins are deformed to shear strains of 4–5 within a foliated chlorite matrix, suggesting calcite veins in subducting volcanics may localize deformation in the seismogenic zone. Shear stress‐strain rate curves constructed using existing empirical relationships in a simplified shear zone geometry predict that slip velocities varied depending on pore fluid pressure; models predict slow slip velocities preferentially by frictional sliding in chlorite, at pore fluid pressures greater than hydrostatic but less than lithostatic

Enabling large-scale hydrogen storage in porous media – the scientific challenges

Expectations for energy storage are high but large-scale underground hydrogen storage in porous media (UHSP) remains largely untested. This article identifies and discusses the scientific challenges of hydrogen storage in porous media for safe and efficient large-scale energy storage to enable a global hydrogen economy. To facilitate hydrogen supply on the scales required for a zero-carbon future, it must be stored in porous geological formations, such as saline aquifers and depleted hydrocarbon reservoirs. Large-scale UHSP offers the much-needed capacity to balance inter-seasonal discrepancies between demand and supply, decouple energy generation from demand and decarbonise heating and transport, supporting decarbonisation of the entire energy system. Despite the vast opportunity provided by UHSP, the maturity is considered low and as such UHSP is associated with several uncertainties and challenges. Here, the safety and economic impacts triggered by poorly understood key processes are identified, such as the formation of corrosive hydrogen sulfide gas, hydrogen loss due to the activity of microbes or permeability changes due to geochemical interactions impacting on the predictability of hydrogen flow through porous media. The wide range of scientific challenges facing UHSP are outlined to improve procedures and workflows for the hydrogen storage cycle, from site selection to storage site operation. Multidisciplinary research, including reservoir engineering, chemistry, geology and microbiology, more complex than required for CH4 or CO2 storage is required in order to implement the safe, efficient and much needed large-scale commercial deployment of UHSP.This work was stimulated by the GEO*8 Workshop on “Hydrogen Storage in Porous Media”, November 2019 at the GFZ in Potsdam (Germany). NH, AH, ET, KE, MW and SH are funded by the Engineering and Physical Sciences Research Council (EPSRC) funded research project “HyStorPor” (grant number EP/S027815/1). JA is funded by the Spanish MICINN (Juan de la Cierva fellowship-IJC2018-036074-I). JM is co-funded by EU INTERREG V project RES-TMO (Ref: 4726 / 6.3). COH acknowledges funding by the Federal Ministry of Education and Research (BMBF, Germany) in the context of project H2_ReacT (03G0870C).Peer reviewe

Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks

Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO2-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO2 reservoir-caprock system exposed to CO2 over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼105 years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2.Funding was provided by NERC to the CRIUS consortium (NE/F004699/1), Shell Global Solutions, for GR as part of the Center for Nanoscale Controls on Geologic CO₂ (NCGC), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-AC02-05CH11231, and DECC, which provided a CCS Innovation grant for completion of this work

Rotary shear experiments on glass bead aggregates: Stick-slip statistics and parallels with natural seismicity

The goal of predicting earthquakes remains elusive despite decades of instrumental observations and research, and a much longer historical record. Even the practice of seismic hazard analysis is a topic of heated debate, in part due to our inability to accurately determine the rate and size distribution of earthquakes that a fault can produce. The main reason for these deficiencies is the lack of a validated, physics-based theory of earthquakes. Heuristic attempts to discover patterns in seismicity based on its phenomenology have produced ambiguous and sometimes contradicting results, due to the relatively short instrumental record of big earthquakes compared to their rate of occurrence. From a geodynamics perspective, earthquakes are bursts of energy release as the lithosphere is loaded due to the motion of tectonic plates at rates of a few centimeters per year. Similar behavior, known as crackling, is observed when shearing granular aggregates. Loosely packed particles behave collectively as a fluid, giving rise to small instabilities only. At a critical packing fraction, the size distribution of the instabilities approaches power law scaling. This suggests that the aggregate is at a phase transition and that long-range correlations are a key characteristic of its macroscopic behavior. Above the critical packing fraction, the collective behavior of the particles is similar to that of a solid. In that solid-like regime, the aggregates alternate between power law distributed event sizes and quasi-periodic stick-slip. A significant number of laboratory studies have employed granular media to explore the dynamics of critical systems in the context of seismicity and fault gouge rheology. These studies have been performed either at low normal stress (< 1 MPa) or to limited shear displacements (< 50 mm), and often under dry conditions. It is not known whether the macroscopic behavior of granular aggregates remains the same under higher normal stress and larger displacements, or in the presence of pressurized water. If not, is it possible to determine what mechanisms are responsible for the change? The rotary shear experiments presented in this thesis expand the envelope of the experimentally tested conditions up to 8 MPa normal stress and 165 mm of shear displacement, \textit{simultaneously}. This enabled us to infer the emergence of correlations under these conditions, through changes in the statistics of granular avalanches. Because the elevated stress conditions do not allow direct visual observation of the glass bead samples, a specially developed AE monitoring system was used to detect and locate the source of crackling. The findings of this thesis highlight the importance of emergent, long range correlations in sheared granular media, as a function of experimental conditions. We infer that the key parameter that determines the scaling of avalanche statistics is the packing fraction, which in turn depends on normal stress, wear rate, and particle size distribution

Permeability of Bituminous Coal to CH4 and CO2 Under Fixed Volume and Fixed Stress Boundary Conditions: Effects of Sorption

Permeability evolution in coal reservoirs during CO2-enhanced coalbed methane (ECBM) production is strongly influenced by swelling/shrinkage effects related to sorption and desorption of CO2 and CH4, respectively. Recent research has demonstrated fully coupled stress–strain–sorption–diffusion behavior in small samples of cleat-free coal matrix material exposed to a sorbing gas. However, it is unclear how such effects influence permeability evolution at the scale of a cleated coal seam and whether a simple fracture permeability model, such as the Walsh elastic asperity loading model, is appropriate. In this study, we performed steady-state permeability measurements, to CH4 and CO2, on a cylindrical sample of highly volatile bituminous coal (25 mm in diameter) with a clearly visible cleat system, under (near) fixed volume versus fixed stress conditions. To isolate the effect of sorption on permeability evolution, helium (non-sorbing gas) was used as a control fluid. All flow-through tests reported here were conducted under conditions of single-phase flow at 40°C, at applied Terzaghi effective confining pressures of 14–41 MPa. Permeability evolution versus effective stress data were obtained under both fixed volume and fixed stress boundary conditions, showing an exponential correlation. Importantly, permeability ((Formula presented.)) obtained at similar Terzaghi effective confining pressures showed (Formula presented.) > (Formula presented.) >> (Formula presented.), while (Formula presented.) -values measured in the fixed volume condition were higher than those in the fixed stress case. The results show that permeability to CH4 and CO2, under in situ conditions where free swelling of rock is not possible, is strongly influenced by the coupled effects of 1) self-stress generated by constrained swelling, 2) the change in effective stress coefficient upon sorption, 3) sorption-induced closure of transport paths independently of poroelastic effect, and 4) heterogeneous gas penetration and equilibration, dependent on diffusion. Our results also show that the Walsh permeability model offers a promising basis for relating permeability evolution to in situ stress evolution, using appropriate parameter values corrected for the effects of stress–strain–sorption

Time-resolved model for geothermal engineering in high porosity Slochteren sandstone

This work is an extension of the time-resolved poro-elasto-plastic Mohr-Coulomb model by Fokker et al to include a more realistic constitutive model. Experiments on Slochteren sandstone revealed that the inelastic deformation contributes significantly in compressive deformation almost in all stages of loading and instigated the development of a Cam-Clay-like model to reproduce the Slochteren sandstone behavior. This model was adopted for the current extension. A typical behavior of this model is that the presence of a borehole causes both elastic and inelastic deformation everywhere in the reservoir, as opposed to the conventional philosophy with plastic and elastic zones. Our solution handles the spatial and temporal evolution of pressures, permeability, elastic and plastic properties under the assumption of symmetric loading. The applicability of the approach is demonstrated through a number of cases, like fluid injection, shut-in, production, stimulation, and an injection-production sequence

Surface microstructures developed on polished quartz crystals embedded in wet quartz sand compacted under hydrothermal conditions

Intergranular pressure solution plays a key role as a deformation mechanism during diagenesis and in fault sealing and healing. Here, we present microstructural observations following experiments conducted on quartz aggregates under conditions known to favor pressure solution. We conducted two long term experiments in which a quartz crystal with polished faces of known crystallographic orientation was embedded in a matrix of randomly oriented quartz sand grains. For about two months an effective axial stress of 15 MPa was applied in one experiment, and an effective confining pressure of 28 MPa in the second. Loading occurred at 350 °C in the presence of a silica-saturated aqueous solution. In the first experiment, quartz sand grains in contact with polished quartz prism (1¯¯¯010) faces became ubiquitously truncated against these faces, without indenting or pitting them. By contrast, numerous sand-grain-shaped pits formed in polished pyramidal (176¯¯¯3) and (4¯¯¯134) crystal faces in the second experiment. In addition, four-leaved and (in some cases) three-leafed clover-shaped zones of precipitation formed on these prism faces, in a consistent orientation and pattern around individual pits. The microstructures observed in both experiments were interpreted as evidence for the operation of intergranular pressure solution. The dependence of the observed indentation/truncation microstructures on crystal face orientation can be explained by crystallographic control of stress-induced quartz dissolution kinetics, in line with previously published experimental and petrographic data, or possibly by an effect of contact orientation on the stress-induced driving force for pressure solution. This should be investigated in future experiments, providing data and microstructures which enable further mechanism-based analysis of deformation by pressure solution and the effect of crystallographic control on its kinetics in quartz-rich sands and sandstones

Microphysics of Inelastic Deformation in Reservoir Sandstones from the Seismogenic Center of the Groningen Gas Field

Physics-based assessment of the effects of hydrocarbon production from sandstone reservoirs on induced subsidence and seismicity hinges on understanding the processes governing compaction of the reservoir. Compaction strains are typically small (ε < 1%) and may be elastic (recoverable), or partly inelastic (permanent), as implied by recent experiments. To describe the inelastic contribution in the seismogenic Groningen gas field, a Cam–clay-type plasticity model was recently developed, based on the triaxial test data obtained for sandstones from the Groningen reservoir (strain rate ~ 10−5 s−1). To underpin the applicability of this model at production-driven strain rates (10−12 s−1), we develop a simplified microphysical model, based on the deformation mechanisms observed in triaxial experiments at in situ conditions and compaction strains (ε < 1%). These mechanisms include consolidation of and slip on µm-thick clay films within sandstone grain contacts, plus intragranular cracking. The mechanical behavior implied by this model agrees favourably with the experimental data and Cam–clay description of the sandstone behavior. At reservoir-relevant strains, the observed behavior is largely accounted for by consolidation of and slip on the intergranular clay films. A simple analysis shows that such clay film deformation is virtually time insensitive at current stresses in the Groningen reservoir, so that reservoir compaction by these mechanisms is also expected to be time insensitive. The Cam–clay model is accordingly anticipated to describe the main trends in compaction behavior at the decade time scales relevant to the field, although compaction strains and lateral stresses may be slightly underestimated due to other, smaller creep effects seen in experiments

Effect of pore fluid chemistry on uniaxial compaction creep of Bentheim sandstone and implications for reservoir injection operations

In the transition to a sustainable energy system, natural gas may be an interim source for relatively low-carbon energy production. However, hydrocarbon production worldwide is leading to reservoir compaction and, consequently, surface subsidence and induced seismicity, hampering the potential of natural gas. Reservoir compaction may potentially be mitigated by fluid injection. Fluid injection into porous subsurface reservoirs is also required in other technologies envisioned in a sustainable energy system, such as geothermal energy production and temporary storage of renewable energy. However, fluid injection into porous reservoirs may create a chemical disequilibrium between the pore fluid and host rock, potentially activating fluid–rock interactions that can cause compaction of the reservoir. These chemically activated fluid–rock interactions are not well-understood, and, therefore, we performed uniaxial compaction experiments at 35, 75 and 100 MPa effective stress, employing samples of Bentheim sandstone saturated with supercritical phases (i.e. N2, CO2, wet-N2 and wet-CO2), distilled water and aqueous solutions (i.e. 3.7 pH HCl solution, AMP solution and AlCl3 solution), as well as low-vacuum (dry) conditions. Creep strain and acoustic emissions (AEs) accumulated with increasing stress and sample porosity. While saturation with supercritical fluids produced slightly less creep strain than dry conditions, flooding with distilled water doubled the creep strain. The acidic solutions inhibited compaction creep compared to distilled water saturation. AE activity and microstructural analysis revealed that microcracking controlled deformation, presumably via stress corrosion cracking. While the supercritical fluids may have dried crack tips, distilled water likely reduced the stress required for Si-O bond breakage. The acidic solutions inhibited microcracking through, presumably, a change in surface energy. Our results suggest that fluids devoid of water, with low water content or acidic in nature can be injected into quartz-rich porous reservoirs without increasing reservoir compaction rates

Volumetric response of crushed dunite during carbonation reaction under controlled σ-PT conditions

Naturally, olivine reacts with CO2-rich fluids, producing carbonates and silica. If in completion, this reaction will cause a large increase in the solid volume (~85%), which can generate a significant stress/force when it occurs in a confined space. This may be used to fracture the surrounding rocks in the context of the injection of industrially captured CO2 into peridotites for permanent sequestration. Contrarily, this volume-increasing reaction may also clog transport paths and thus inhibit CO2 access, leading to little or no volumetric increase at industrial time scales. Although observations from natural systems suggest that reaction-induced fracturing during peridotite carbonation can occur, the fracturing mechanism has not been experimentally reproduced under in-situ stress-temperature-chemical conditions. Here, we report 9 flow-through experiments performed on pre-compacted Åheim dunite (containing ~85% olivine) powders (grain size 36-50 µm) during carbonation reaction under controlled σ-P-T conditions. This was done using a purpose-built apparatus, consisting of a flow-through system accommodated with a uniaxial servo-controlled loading system. Before experiments, the dunite powders were compacted stepwise up to 250MPa to form a disc-shape sample with starting porosity of ~25%. The sample was covered by a thin Teflon sleeve plus Vaseline to reduce the friction against the vessel wall. The experiments were performed at a constant temperature of 150° and constant (Terzaghi) effective stress of 1, 5, 15MPa, respectively. The sample was first exposed to deionized (DI) water at a pore fluid pressure of 10MPa, and then the DI water was replaced, maintaining constant pore pressure of 10MPa, by flow-through of a certain chemical fluid, such as CO2 saturated brine (containing 1M NaCl plus 0.64M NaHCO3, pH~3), CO2 saturated water (pH~3), NaHCO3 saturated solution (pH~9) and NaHSO4 solution (pH~3). The permeability was measured for all experiments using the flow-through system by means of the steady-state method, and each experiment took 2-4 weeks. The experiments show that the samples exhibited 0-0.37% compaction strain when CO2 saturated brine, CO2 saturated water, and NaHCO3 saturated solution flow through, independently of poroelastic effects, and the sample permeability drops in the order from 10-17 to 10-20 m2. By contrast, for the NaHSO4 flow-through experiment where no carbonation reaction occurred, the sample permeability increased from 2*10-17 to 7*10-17 m2, associated with 0.05% compaction. The sample mass after the NaHSO4 flow-through experiment reduced ~5%, suggesting that magnesium and silica may be partly leached out from the sample. Microstructure observations and XRD analysis on these samples demonstrate a drastic reduction in porosity of the reaction zone where CO2 was integrated into the crystal structure of the product carbonates by means of carbonation reactions. The mechanism responsible for the observed behavior seems to be that the dissolution of olivine that occurred first at the grain contact surface leads to compaction, followed by precipitation of carbonates at porous that clogs the transport paths and thus reduces the permeability, though the detailed chemical analysis is still performing. As a result, our current findings suggest that the volume-increasing precipitation produced via the carbonation reaction under in-situ subsurface conditions will clog transport paths