Pressure and stress perturbations associated with volcanic activity and geothermal production can modify the porosity and permeability of volcanic rock, influencing hydrothermal convection, the distribution of pore fluids and pressures, and the ease of magma outgassing. However, porosity and permeability data for volcanic rock as a function of pressure and stress are rare. We focus here on three porous tuffs from the Krafla geothermal system (Iceland). Triaxial deformation experiments showed that, despite their very similar porosities, the mechanical behavior of the three tuffs differs. Tuffs with a greater abundance of phyllosilicates and zeolites require lower stresses for inelastic behavior. Under hydrostatic conditions, porosity and permeability decrease as a function of increasing effective pressure, with larger decreases measured at pressures above that required for cataclastic pore collapse. During differential loading in the ductile regime, permeability evolution depends on initial microstructure, particularly the initial void space tortuosity. Cataclastic pore collapse can disrupt the low-tortuosity porosity structure of high-permeability tuffs, reducing permeability, but does not particularly influence the already tortuous porosity structure of low-permeability tuffs, for which permeability can even increase. Increases in permeability during compaction, not observed for other porous rocks, are interpreted as a result of a decrease in void space tortuosity as microcracks surrounding collapsed pores connect adjacent pores. Our data underscore the importance of initial microstructure on permeability evolution in volcanic rock. Our data can be used to better understand and model fluid flow at geothermal reservoirs and volcanoes, important to optimize geothermal exploitation and understand and mitigate volcanic hazards.

How to cite: Heap, M., Bayramov, K., Meyer, G., Violay, M., Reuschlé, T., Baud, P., Gilg, A., Harnett, C., Kushnir, A., Lazari, F., and Mortensen, A.: Compaction and permeability evolution of tuffs from the Krafla geothermal system (Iceland), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2171, https://doi.org/10.5194/egusphere-egu25-2171, 2025.

Effective stress is known to be a key factor affecting permeability measurements under geological conditions. As effective stress increases, the permeability of rock containing micro-fractures will decrease significantly. Based on laboratory measurement data, several scholars have come up with empirical equations to describe permeability changes with effective stress and found that there generally exists an exponential or power-law relationships. In this study, the experimental sample is a tight sandstone formation containing microfractures from Kuche Depression in Tarim Basin, China, where gas is produced from deep reservoirs of over 6000 m. Permeability was measured using the conventional pulse-decay method using an in-house true triaxial stress cell with maximum confining pressure of 120 MPa, pore pressure of 100 MPa and axial pressure of 250 MPa. The tight sandstone contains micro-fracture and an ambient porosity of 5%. Under the condition of high pore pressure (up to 80 MPa), the Knudsen number K_n＜0.01, and the gas slippage effect appears to have little impact on the permeability, characteristic in the Darcy flow state. As the confining pressure increases, the gas permeability decreases significantly, whereas as the pore pressure increases, the gas permeability increases. It has been shown that as the effective stress increases, the gas permeability decreases, and ln(K/K₀) shows an exponential relationship with (δ - δ₀) (subscript 0 represents the initial state). As the effective stress decreases, ln(K/K₀) shows a logarithmic relationship with (δ - δ₀). Under the condition of equal effective stress, ln(K/K₀) shows a linear relationship with pore pressure. In addition, we have also noticed a strong anisotropy in the permeability when differential axial stress was applied during the permeability measurement, reflecting a preferential distribution of microfractures in the tight sandstone measured.

How to cite: Yu, B., Liu, K., and Yu, L.: Experimental investigation of the relationship between permeability and effective stress for low-permeability sandstone with micro-fractures under high pressure, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7924, https://doi.org/10.5194/egusphere-egu25-7924, 2025.

We constructed a thermo-viscoelasticity equation based on Lord-Shulman (LS) thermoelasticity with the Kelvin-Voigt (KV) model for viscoelasticity. The plane-wave analysis predicts two compressional waves and a shear wave. These two compressional waves are the fast-P and slow-P diffusion/wave (the T-wave), which have similar characteristics to the fast- and slow-P waves of poroelasticity, respectively. To overcome the nonphysical phenomenon of high-frequency P-waves in the thermo-viscoelastic (KV model), we established the thermo-viscoelasticity equation by combining LS thermoelasticity and the Zener and Cole-Cole model of viscoelasticity. Plane-wave analysis predicts two inflection points on the dispersion and attenuation curves; these are mainly affected by thermal diffusion and viscoelasticity. The dispersion curves of both types of P waves have two-level limit velocities of high frequency, and their attenuation curves also feature two attenuation peaks. Selecting appropriate parameters can cause the two-level limit velocities of high frequency and attenuation peaks to move or overlap. Finally, we consider the experiment data of P-wave velocity varying with frequency of two kinds of sandstone. Indeed, a Cole-Cole fractional model is needed to obtain a good match. These results are helpful for studying the physics of thermo-viscoelasticity and for testing experimental data and numerical algorithms for wave propagation.

How to cite: wang, Z., Fu, L., and jose, C.: The behaviour of wave propagation in linear thermo-viscoelastic media, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2734, https://doi.org/10.5194/egusphere-egu25-2734, 2025.

Porous reservoir rocks like sandstones have gained utmost importance in the last decade as a potential sink for CO₂. Most of the targeted reservoirs are depleted oil and gas fields, which have caprocks to ensure the containment of the injected CO₂. Injecting CO₂ into porous reservoirs increases the pore pressure, reducing the effective horizontal and vertical stresses. Depending on the pre-injection stress condition and permeability of the reservoir, careful monitoring should be in place to define the upper limit of CO₂ injection pressure to prevent any permanent damage to the reservoir, which can lead to leakage or induced seismicity. Lab-scale experiments provide key insights into the deformation behaviour of reservoir rocks under different stress conditions, which can be upscaled to understand reservoir-scale processes. To simulate the stress perturbation caused by CO₂ injection operations, we have subjected porous reservoir rocks (core plugs) collected from different depths of offshore North Sea under realistic reservoir stress and saturation conditions, with liquid CO₂ flow-through leading to failure. The P and S wave velocities along the core plugs were recorded every 15 s to assess the change in wave properties during deformation, fluid displacement and pore pressure build-up. It was observed that during each loading cycle, wave velocities are highest at the elastic-plastic transition zone, which can be attributed to the compression of pores and closure of microcracks perpendicular to the loading direction. The wave velocities and amplitudes decrease sharply after the onset of plastic deformation, which can be attributed to the formation of microcracks in the coreplug due to increasing load. During displacement of brine with CO₂, velocities and amplitudes drop sharply. These indicators are used to develop a traffic light scenario for CCS operations to maintain safe stress conditions in the reservoir. The consistent correlation between the wave properties and mechanical response of the reservoir rocks reveals that constant monitoring of wave velocities during CO₂ injection can act as a cheaper and more efficient tool for monitoring stress state and plume movement in the reservoir, facilitating safer CO₂ storage operations.

How to cite: Chandra, D. and Barnhoorn, A.: Applicability of sonic velocities as a monitoring tool for subsurface CO2 plume migration and associated stress change, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1802, https://doi.org/10.5194/egusphere-egu25-1802, 2025.

Feldspars are a common framework grain in sandstone reservoirs targeted for carbon capture and storage (CCS). They are mechanically weak under reservoir conditions and are very likely to react with CO₂ injected into saline aquifers or depleted hydrocarbon reservoirs.  Reactions could dissolve feldspar and precipitate new minerals to an extent that fundamentally changes reservoir properties and potentially mineralises injected CO₂. The current general consensus is that these features are unlikely to impact fluid migration during the injection lifespan of any CCS project. However, the response of feldspars to saturation with aggressive CO₂-enriched fluids under stressed reservoir conditions is poorly understood.

In this contribution, the magnitude of any “feldspar effect” is re-evaluated using sandstone samples obtained from the Lower Cretaceous Captain Sandstone in the Central North Sea, which is the target reservoir for CO₂ injection in the Acorn Project (UK). 

Firstly, using petrography, SEM analysis and Pb isotopic compositions of detrital feldspars, sediment provenance and subsequent diagenesis are shown to be significant drivers on feldspar composition and texture prior to injection. This is important because it is already understood that different feldspars react with CO₂-rich fluids at different rates: thus any feldspar effect could significantly vary within a reservoir with mixed provenance and burial history on a sub-basin scale. Secondly, we conducted a suite of novel reaction experiments conducted using a triaxial ‘Nimonic’ deformation rig to investigate chemical dissolution in sandstone core plugs saturated with both CO₂-enriched fluids and water under subsurface conditions. Experiments were run at CCS reservoir pressures (70MPa confining pressure, 50MPa pore fluid pressure) and a range of temperatures (80°C – 550°C) to accelerate reaction rates and promote geological reactions in a short timescale. Microstructural and elemental analysis of post-mortem experimental samples showed enhanced fracturing and dissolution of certain feldspars along with precipitation of secondary minerals, whereas other feldspars were apparently unaffected. Experiments performed above 400°C showed replacement and dissolution of K-feldspar grains with Ca-rich plagioclase and K-bearing clays.

The outcome of our re-evaluation is that the impact of feldspars in CCS reservoirs has likely been overlooked, but until further experimental work is carried out to constrain how quickly feldspar interactions will impact fluid flow within the reservoir, uncertainties will remain with regard to their impact on CO₂ injectivity and storage capacity.

How to cite: Farrell, N., Yang, L., Flowerdew, M., Badenszki, E., Mark, C., Ardo, B., Taylor, K., Waters, J., Hughes, L., and Paul, L.: Experimental and microstructural analysis of feldspar solubility in CCS reservoirs, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11208, https://doi.org/10.5194/egusphere-egu25-11208, 2025.

Though the energy transition aims to phase out fossil fuels while continuing to exploit the subsurface for other storage solutions (e.g. geological CO₂ storage, temporary hydrogen storage), natural gas, as a low-carbon energy carrier, will continue to play a role in our energy mix for the foreseeable future. In general, human activities in the subsurface change the physical and chemical environment, which in turn can lead to surface subsidence and induced seismicity. These phenomena may continue even after activities have stopped, as observed for natural gas extraction from the giant Groningen Gas Field in the Netherlands. They are largely caused by deformation in the reservoir rock, driven by fluid pressure changes. However, in-situ strain measurements from the Groningen Gas Field demonstrate that the clay-rich over- and underburden formations of the reservoir are also affected by these fluid pressure changes, displaying slow compaction. To make accurate predictions of reservoir deformation and to allow reliable assessment of the associated surface subsidence and induced seismicity, a detailed understanding of the deformation processes controlling deformation in these clay-rich formations is needed. Understanding which processes caused deformation in past (hydrocarbon) operations will help in understanding what may happen now that we plan to store other fluids in the subsurface.

We performed rock mechanical experiments at in-situ conditions on the Opalinus Claystone (Switzerland), an analogue to the Groningen over- and underburden claystones, to assess the grain-scale mechanisms responsible for deformation. We designed an innovative and comprehensive multi-step experimental procedure that provides new, coherent data on the time-dependent deformation of clay-rich rocks. The experiments were performed in a triaxial compression apparatus, applying systematic steps of constant stress while controlling the pore fluid pressure in the sample. These steps were either stepped up or down in differential stress during an experiment. At each differential stress we systematically analyzed the instantaneous and time-dependent deformation.

We observed general compaction of the samples upon increasing stress, and time-dependent expansion of the sample when stepping down in stress. Our results demonstrate that deformation in clay-rich rocks is strongly affected by the fluid-transport properties of the rock. We infer that sorption of fluids to the clay-rich matrix plays an important role in the deformation of clay-rich rocks, along with frictional slip controlling grain rearrangement. However, matters are complicated by slow diffusion of pore fluid pressure, which leads to an additional time-dependent component. Overall, our results demonstrate that over half of the observed deformation is permanent, even at low differential stresses. A detailed understanding of the time-dependent deformation of clay-rich rocks is crucial for accurate predictions of the impact of human activities in the subsurface, as sorption of fluid to the clay material may also be important during CO₂ and hydrogen storage.

How to cite: Sep, M., Hangx, S., and de Bresser, H.: Time-dependent deformation of clay-rich rocks enveloping reservoirs exploited for geo-energy purposes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11807, https://doi.org/10.5194/egusphere-egu25-11807, 2025.

The quality of the sandstone reservoir is critically influenced by the presence of clay coatings on the surfaces of quartz grains. These coatings play an essential role in determining porosity and permeability, key parameters that govern the storage and flow potential of sandstone reservoirs used for geothermal energy, groundwater, and hydrocarbons. This study employs a multiphase-field model, a versatile tool widely used in materials science, to simulate the complex interplay of interface motion and phase transitions within geological systems. By generating a detailed three-dimensional digital representation of sandstone, the model provides precise control over quartz grain coatings and composition, enabling a thorough investigation of their impact on reservoir properties. Two central aspects are explored: (1) the effect of varying clay coating coverage on quartz grains, and (2) the influence of coating distribution on the evolution of porosity and permeability during quartz precipitation. Computational fluid dynamics (CFD) simulations further quantify the changes in permeability at different stages of grain growth, revealing intricate relationships between the distribution of the coating, the properties of the rock, and the dynamics of fluid transport. The findings show that sandstones with a higher proportion of coated grains exhibit enhanced permeability due to the cement growth limiting effects of clay coatings on quartz grains. These insights provide a deeper understanding of the mechanisms that govern sandstone reservoir quality and offer practical implications for optimizing applications in geothermal energy, water resource management, and carbon and hydrogen storage.

How to cite: Kumar, A., Prajapati, N., Schneider, D., Busch, B., Hilgers, C., and Nestler, B.: Revealing the Hidden Dynamics of Clay-Coated Quartz Grains in Sandstone with Multiphase-Field Modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3471, https://doi.org/10.5194/egusphere-egu25-3471, 2025.

This study integrates 3-D seismic reflection and petrophysical data to investigate the Lopingian bedded salt formations of the Delaware Basin, part of the Greater Permian Basin in the United States. Focusing on the Castile and Salado Formations, the analysis identifies a zone of thickened and deformed strata associated with an intra-salt fold-thrust belt in the southwestern portion of the seismic volume. Adjacent to this fold-thrust belt lies a geophysically distinct region termed the buffer zone.

Petrophysical analysis of the Castile Formation within the buffer zone reveals a unique composition, deviating from the expected cyclical anhydrite-halite members. Instead, this zone consists exclusively of anhydrite. This compositional anomaly challenges previous interpretations that halite absence results from dissolution, suggesting instead that gypsum deposition followed by conversion to anhydrite may have occurred.

The overlying Salado Formation displays significant heterogeneity and karst features, highlighting potential geohazards and complexities for underground energy storage. These findings emphasize the necessity of combining geophysical and petrophysical approaches to accurately characterize subsurface conditions, assess risks, and optimize the placement of salt caverns for energy storage applications.

How to cite: Schuba, N., Moscardelli, L., Dooley, T., Martinez-Doñate, A., and Melani, L.: Geophysical and Petrophysical Insights into Bedded Salt Formations: Implications for Underground Energy Storage in the Delaware Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14303, https://doi.org/10.5194/egusphere-egu25-14303, 2025.

Seismoelectric (SE) methods are potentially interesting for subsurface characterization by exploiting the coupling between seismic waves and electromagnetic fields in fluid-saturated porous media. While traditional SE techniques have primarily focused on body-wave-induced signals, recent research has highlighted the significant advantages of surface wave-induced SE signals, including enhanced amplitudes and increased sensitivity to near-surface heterogeneities. These characteristics make surface wave-induced SE signals particularly valuable for detailed subsurface investigations.

We conducted controlled laboratory experiments using a water-saturated sandstone sample (19.7% porosity, 310 mD permeability) and a planar acoustic source to generate surface waves at a water-sandstone interface. SE signal variations were systematically measured as a function of receiver distance from the interface, and array-based measurements were performed to analyze the velocity and characteristics of the induced SE surface waves. High signal-to-noise ratio SE surface waves were successfully measured across multiple excitation frequencies (100 kHz, 200 kHz, 300 kHz, 400 kHz, and 500 kHz), demonstrating the robustness of the phenomenon across a broad frequency range.

Our results show that SE signals were only observed in the presence of the porous medium, confirming that they originate from the fluid-porous interface. The SE signal amplitude decayed rapidly with increasing distance from the surface, which is consistent with surface wave behavior. Notably, the SE waveforms exhibited propagation velocities matching those of acoustic surface waves. They showed significantly shorter durations and different frequency content than the corresponding acoustic signals, indicating potential for enhanced spatial resolution in subsurface imaging. Ongoing work focuses on extracting the dispersion and attenuation characteristics of the measured SE surface waves across different frequencies. These findings will provide a foundation for more effective geophysical workflows, particularly in scenarios requiring detailed near-surface characterization.

How to cite: Liu, Y. and Smeulders, D.: Acoustically Induced Seismoelectric Surface Waves at a Fluid-Saturated Sandstone Interface: Multi-Frequency Experimental Observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13204, https://doi.org/10.5194/egusphere-egu25-13204, 2025.