Water infiltration into crustal rocks, particularly through fractures, significantly impacts seismic wave propagation and the characterization of fracture systems. Our study (Wu et al, 2023a) investigates the acousto-mechanical behavior of fractured granite experiencing gradual water infiltration over 12 days. We reveal an order of magnitude difference in wave amplitudes when compared to intact granite, with a correlation between wave amplitudes and the movement of the wetting front. The laboratory experiments show that fracture stiffness decreases exponentially as the wetting front advances, indicating moisture-induced matrix expansion (Wu et al, 2023b) around the fracture leads to increased asperity mismatch and reduced stiffness. By back-calculating the fracture stiffness and capturing the effects of water infiltration on seismic attenuation through a numerical model, this research illuminates how elastic waves propagate across fractures undergoing moisture-induced expansion, a crucial aspect of fracture characterization and understanding of the near-surface environment's response to hydrological changes. Our research sheds light on an important question in fracture characterization: how elastic waves propagate across a fracture undergoing moisture-induced expansion.

Publications related to this research:

Wu, R., Selvadurai, P. A., Li, Y., Leith, K., Lei, Q., & Loew, S. (2023a). Laboratory acousto-mechanical study into moisture-induced reduction of fracture stiffness in granite. Geophysical Research Letters, 50, e2023GL105725. https://doi.org/10.1029/2023GL105725

Wu, R., Selvadurai, P. A., Li, Y., Sun, Y., Leith, K., & Loew, S. (2023b). Laboratory acousto-mechanical study into moisture-induced changes of elastic properties in intact granite. International Journal of Rock Mechanics and Mining Sciences, 170, 105511. https://doi.org/10.1016/j.ijrmms.2023.105511

How to cite: Wu, R., Selvadurai, P., Li, Y., Leith, K., Lei, Q., and Loew, S.: Time-lapse Acousto-Mechanical Response to Moisture-Induced Reduction of Fracture Stiffness in Granite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7192, https://doi.org/10.5194/egusphere-egu24-7192, 2024.

In geothermal systems the thermo-physical properties of the rocks change as they interact with fluids passing through the volcanic system and during discrete events such as earthquakes and magma intrusion. To characterize a geothermal system and the flow of fluid through a sub-volcanic complex, targeted rock physical tests are needed for the rocks of the area conducted at natural P-T conditions. Here, we present rock property characterization of the main geological units of the active Nevados de Chillan Geothermal System, located in the Southern Volcanic Zone (SVZ), an area with some of the largest geothermal potential in the Andes. 

Six representative blocks of the geothermal host reservoir and overlying strata were collected from the volcanic basement. The main geomechanical units of this system are (from oldest to youngest): 1) andesites, tuffs and breccia of the Cura-Mallin Formation (Miocene country rocks); 2) granodiorites and diorites of the Santa Gertrudis Bullileo Batholith (15.7 Ma and 5.9 Ma, respectively); and 3) hornfels from the contact between the granitoids and country rocks. Cylindrical core samples (26 mm diameter x 65 mm length) from each block were used to quantify density, porosity, and ultrasonic wave velocities at different confining pressures. All tests were carried out at the Rock Deformation Laboratory, University of Manchester. Polished thin sections were prepared from blocks of the same orientation as the cored directions and analyzed using petrographic.

Granodiorite has the lowest porosity at between <1 to 2% and the fastest P- and S-wave velocities (5.5 to 5.9 km/s and 3.1 to 3.5 km/s, respectively). The diorite has a higher porosity of between 4 to 6% which coincides with lower ultrasonic velocities (3.5 to 5.0 km/s and 3.5 to 5.0 km/s, respectively). This can be explained by the higher presence of macro, micro-fractures, and alteration minerals in the diorite. Hornfels has possess similar porosities (>2%) to the granodiorite and 4.8 to 5.7 km/s P-wave velocities and 2.7 to 3.3 km/s S-wave velocities. The andesitic lavas have porosities ranging 3-7%, while the tuffs and breccias have porosities of 12-30%. Elastic waves velocities in the andesitic lavas are around 2 km/s faster than the pyroclastic rocks.

Tests with cycles of increasing and decreasing hydrostatic pressure (up to approximately 150 MPa) show that granodiorite and diorite exhibit sharp increases in P-wave velocity (Vp). This is attributable to the stiffening of the rock from the progressive closure of pre-existing cracks. Above 40 MPa, the rate of increase in Vp with pressure reduces markedly, implying that the remaining porosity is less compliant. This is consistent with the maximum burial depth of the rocks suggesting that those cracks formed because of bringing the rocks to the surface.

Finally, in terms of microstructural observations, the granodiorites, diorites and hornfels have large intragranular and intergranular fractures with very high aspect ratio, which are commonly oriented and therefore impart anisotropy. In contrast, the andesitic, tuffs and breccias porosity is higher than the crystalline rocks and is mainly composed of intergranular pores with low aspect ratios and relatively isotropic.

How to cite: Mura, V., Arancibia, G., Browning, J., Healy, D., Mecklenburgh, J., and Morata, D.: Characterizing Rock Physical Properties in the Nevados de Chillán Geothermal System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11658, https://doi.org/10.5194/egusphere-egu24-11658, 2024.

Abstract: The northern part of the the Junggar Basin is the Siberian plate, and the western part is the Kazakhstan plate, which is an important part of the Central Asian orogenic belt. This study discusses how fluorescence parameters inside the mudstone caprocks of the Mobei Bugle and Mosuowan Bugle respond to the leakage of palaeo-oil zones. It is based on X-ray diffraction analysis, TOC testing, physical property testing, rock pyrolysis experiments, and image observation. First, using quantitative fluorescence technology, it was established that the appropriate particle size range for the study area mudstone is between 100 and 140 mesh by examining the QGF E intensity and sample loss rate of the control group. Then, the influence of retained primary hydrocarbons inside the mudstone on the test results of quantitative fluorescence technology is speculated to be relatively weak based on an analysis of the correlation between TOC value and total hydrocarbon value, TOC value and fluorescence parameters. The QGF index of the 5265-5302m reservoir in PD1 well ranges from 3.9 to 87.2, with an average of 16.18; the QGF index of the 7034-7195m reservoir in MS1 well ranges from 2.2 to 57.5, with an average of 11.6. The reservoirs of the PD1 and MS1 wells contain palaeo-oil zones, based on the QGF index classification criteria. Both image observation and micro resistivity imaging logging analysis demonstrate that the caprock's physical properties in the PD1 well are inferior to those in the MS1 well. The pore types of the PD1 well caprock are filled dissolution pores and residual intergranular pores, whereas the pore types of the MS1 well caprock are dissolution pores and microcracks. There is a discrepancy of approximately 9 times between the development density of fractures in the MS1 well caprock (0.989 pieces/m) and the PD1 well caprock (0.114 pieces/m). The palaeo-oil zone can be effectively sealed due to the poor physical properties of the PD1 well caprock. The oil testing conclusion of the reservoir is oil-water layer, with a daily oil production of 8.52t. Therefore, as the depth decreases, the QGF E intensity value decreases, and the QGF λmax and TSF R₁ values reflect higher hydrocarbon maturity. The MS1 well caprock has good physical properties and cannot effectively seal the palaeo-oil zone. The oil testing conclusion of the reservoir is water layer, with a daily oil production of 4.4t. As the depth decreases, the QGF E intensity value increases, and the QGF λmax and TSF R₁ values reflect lower hydrocarbon maturity. Comprehensive analysis suggests that the fluorescence parameters inside the caprock exhibit characteristics related to the degree and form of palaeo-oil zone leakage.

Keywords: Quantitative fluorescence technology, Fluorescence parameters, Mudstone caprock, Palaeo-oil zones

How to cite: Liu, K., Qu, J., Zha, M., and Ding, X.: Response of the caprock's fluorescence parameters to the leakage of palaeo-oil zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8600, https://doi.org/10.5194/egusphere-egu24-8600, 2024.

Linking the pore structure distributions and velocity-pressure relationship is important for understanding geological processes and in situ stress conditions. During the pressure loading, the pressure-velocity relationship is controlled by variation of pore-aspect-ratio spectrum distribution (pore-aspect-ratio vs. porosity) with pressure: on the one hand, the “flat crack” (aspect ratio) will be squeezed and closed, causing the large velocity variation in the low-pressure regimes; on the other hand, the shape and volume of the “round pore” (aspect ratio) are less sensitive to pressure, controlling the smaller velocity change at high pressures. The above pore-structure variation process and its effect on the velocity-pressure relationship allow us to invert and analyze the distribution characteristics from the laboratory velocity-pressure data.  

The power-law distribution has been universally applied to characterize the length and aperture distribution in natural fractures; however, it is still not well understood whether the pore-aspect-ratio spectrum distribution also follows the power law and how this power-law distribution relates to other power-law distributions of fractures. In this study, we examine the universality of the power-law relationship for the pore aspect ratio spectrum distribution and show that this distribution is the natural outcome of the power-law behavior of the length and aperture distribution in the rock fracture network.

We first collected laboratory ultrasonic velocity-pressure data for 52 rock samples with various porosities and lithologies. We then used a power law assumption to link the pore aspect ratio and porosity. The power-law exponents (hereafter referred to as the E factor) of the power-law distributions were successfully obtained by inverting the compiled velocity-pressure datasets. The inversion results showed a universal relationship between the E factor and porosity. We also demonstrated that the power-law distribution of the pore-aspect-ratio spectrum and the universal E factor-porosity trend can be explained using the length and aperture distribution characteristics of the microcrack system of rocks. This universal E factor-porosity trend provides a link between porosity and sensitivity of seismic velocities to pressure. Hence, our work not only emphasizes the relevance of natural fracture fractal distributions for a broad range of lithologies but also provides the parameterized method for relating the velocity-pressure relationship of any rock to porosity variation.

How to cite: Wang, H.-M., Tang, X.-M., and Doan, M.-L.: The power-law distribution of the rock pore structure: inversion and analysis of laboratory velocity-pressure data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4456, https://doi.org/10.5194/egusphere-egu24-4456, 2024.

Understanding the evolution of rock physical properties with changing scale is a critical challenge when characterising spatial subsurface heterogeneities. One of the possible approaches can be using elastic wave velocities at various scales, from laboratory to field, by solely tuning the sensing wavelength to the studied media. Theoretically, in the case of a dry, isotropic, and homogeneous porous medium, at all scales of investigation, the elastic properties are not dependent on the scales of analysis (non-dispersive medium). However, as already pointed out in the literature, carbonate rocks have very heterogeneous pore networks at different scales, which may lead to different Representative Elementary Volumes (REV) with changing scales. In our work, we assume that the elastic wavelength is equal to the upper bound of the REV.

In this study, we investigated marine carbonate rocks of Middle Jurassic age outcropping in the western part of France (Charentes, near Angouleme city) in four different quarries. A total of three REVs were investigated, always in dry conditions: i) the centimeter scale, acquiring P and S wave velocities (Vp, Vs) on 60 cylindrical samples of one inch-diameter using a central frequency of 500 kHz (wavelength ~ 1 cm); ii) the decimeter scale, acquiring more than 1500 measurements of Vp and Vs on outcropping carbonates with a frequency of 40 kHz (wavelength ~ 10 cm); and iii) the decameter scale acquiring seismic wave velocity measurements along a vertical profile (geophones connected to a vertical outcrop wall), where the 6 kg sledgehammer source was situated on the plateau, delivering a frequency of 100 Hz (wavelength ~ 10 m). In parallel, a thorough geological description was done at all the investigated scales, combining i) microscope-driven microstructure analysis of samples under the microscope, ii) sedimentary facies description of outcrops and iii) fracture orientation analysis on photogrammetric models of quarries.

The elastic wave velocity results were interpreted considering facies and diagenetic processes of sedimentary rock fabric. At the centimeter scale (i), for a given sedimentary facies, we show a clear control of diagenesis (cementation and dissolution) on the elastic properties, in agreement with the well-documented literature. At the decimeter scale (ii), horizontal and vertical Vp-Vs data were used to construct 2D acoustic property maps (1 square meter). Two extreme behaviors can be pointed out. On the one hand, velocity data are homogeneous and anisotropic in zones showing evidence of primary stratification and lithostatic compaction. On the other hand, data are heterogeneous and isotropic in zones exhibiting significant early diagenesis heterogeneities (“hardgrounds”). These results are thus linked to facies and diagenesis heterogeneities. Finally, at the decameter scale, seismic velocities clearly show an azimuthal anisotropy, mainly controlled by the occurrence of outcropping open joints from tectonic origin. Our study tends to highlight the crucial need to always characterise sedimentary facies, diagenesis evolution and structural overprint in carbonate reservoir rocks if one wants to interpret and correctly understand the multi-scale elastic properties of carbonates.

How to cite: Bailly, C., Léger, E., Andrieu, S., Regnet, J.-B., Bergogne, M., Monvoisin, G., Saint-Bezar, B., Mas, P., Zeyen, H., and Brigaud, B.: Behavior of elastic properties in carbonates: scale does matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12400, https://doi.org/10.5194/egusphere-egu24-12400, 2024.

How to cite: Bernardo, N., Martins-Gomes, V., Bordes, C., and Brito, D.: Exploring Seismoelectric Interface Responses at Poroelastic/Elastic Boundaries: numerical and experimental approaches, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14953, https://doi.org/10.5194/egusphere-egu24-14953, 2024.

The detection of metallic particles in the subsurface has been an important field in applied geophysics for several decades, e.g. in the exploration of ore deposits or in monitoring measurements at bioremediation sites. Due to the comparably high conductivity and high polarizability of metallic particles, geophysical methods using the electrical conductivity and especially induced polarization are highly suitable methods for applications of this kind.

To correctly interpret data measured in the field, a deep understanding of the underlying conduction and polarization mechanisms is necessary. Both, laboratory measurements with controlled experimental conditions and theoretical models describing the basic physical processes can help to achieve this kind of understanding. However, theoretical models usually are not suitable to be directly compared to laboratory measurements, because they usually consider idealized situations and exhibit a large number of free parameters that can not be controlled in measurements.

To overcome this gap, we compare experimental data of a cm-sized metallic sphere embedded in water-saturated sand with a corresponding theoretical model. To explain the remaining differences between measured and modeled data, we discuss how different processes, e.g. the geometry of the measurement setup or dissolved metal ions in the fluid, might affect the measurement and adapt our model accordingly. By doing so, we are able to reproduce the experimental results with the predictions by our theoretical model.

How to cite: Kreith, D., Breede, K., Zhang, Z., Weller, A., and Bücker, M.: Modeling of spectral induced-polarization measurements on cm-sized metallic spheres in sand-water-mixtures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9712, https://doi.org/10.5194/egusphere-egu24-9712, 2024.

This study utilized an improved sand column experimental setup to investigate the effects of microplastic on Yangtze River bank sediments. The experiments conducted included Darcy experiments, electrokinetic experiments, and wettability experiments, together for the same sample. By varying the particle size of the microplastics in the samples, we were able to observe different response of the electrical properties as well as hydrogeophysical parameters in the samples.

Results show that increasing the mass fraction of mixed microplastics generally resulted in a significant decrease in sample electrical conductivity associated with an increase in permeability. These were expected to be due to the weak conductivity and strong hydrophobic properties of plastic particles, as well as the small adhesive forces between particles, which increased the pore space of the sediments and ultimately increased permeability. However, an anomalous increase trend was observed when decreasing the particle size of the mixed microplastics. Under this condition, increasing the concentration of same size plastic particles enhanced the electrical conductivity of the sediment sample. This anomaly phenomenon was reflected in both permeability and wettability, resulting in a decrease in sample permeability and a significant increase in sample hydrophobicity. Our observations using optical microscopy revealed two types of microplastic distribution in the sediments: one case was that microplastic particles were distributed within sediment pores and they did not touch each other, the other was that they were adsorbed onto sediment particle surfaces. We hypothesized that changes in the existence form of microplastics altered the double-layer structure of sediments, ultimately changing their hydrogeophysical parameters. This work has significance and relevance for electromagnetic-based characterization of microplastic-filled porous materials; for example, estimation of microplastic abundance in sediments.

How to cite: Li, S., Xu, M., Peng, Y., and Hu, X.: Influence of microplastic occurrence on complex conductivity of river sediments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9014, https://doi.org/10.5194/egusphere-egu24-9014, 2024.

Even though stresses in the crust are triaxial (𝜎1>𝜎2>𝜎3) the overwhelming majority of rock deformation experiments are conducted under axisymmetric (or conventional triaxial) loading (𝜎1>𝜎2=𝜎3). This configuration disregards the effect of 𝜎2 on the physical and deformation properties of rocks, thus complicating and degrading the extrapolation of results to natural crustal conditions. A True Triaxial loading configuration is necessary to overcome this simplification, however, these improvements in addressing real crustal conditions come at a cost, which is the challenging boundary conditions that arise from having six loading rams rather than just two. Two main loading boundary effects can severely impact the stress distribution and failure mechanism of samples deformed in a True Triaxial Apparatus (TTA): 1) the end friction effect caused by the stiffness contrast between the rock sample and the metal loading platens, and 2) the unstressed sample edges resulting from the requirement that loading platens must necessarily be slightly smaller than the rock specimen. Managing and reducing these boundary effects is fundamental for obtaining accurate and representative data from true triaxial experiments, and for the further development of these apparatuses.

A novel TTA developed in the UCL Rock & Ice Physics Laboratory was designed to subject cubic or cuboid rock samples to truly triaxial stresses through the independent control of six loading rams. The apparatus is equipped with a confining and pore pressure system that allows for the deformation of saturated samples whilst simultaneously measuring permeability along the three axes. A suite of Finite Element Method (FEM) models was implemented to evaluate the parameters that minimize loading boundary effects in UCL’s TTA for a 50 mm edge-length cubic sample of sandstone. Our results indicate that using aluminum loading platens (𝐸𝑠𝑎𝑚𝑝𝑙𝑒/𝐸𝑝𝑙𝑎𝑡𝑒𝑛=0.47) reduces the end friction effect by a factor of two compared to using steel platens (𝐸𝑠𝑎𝑚𝑝𝑙𝑒/𝐸𝑝𝑙𝑎𝑡𝑒𝑛=0.17). In addition, we find that elevated confining pressure significantly reduces the stress concentration produced by unstressed edges. Specifically, a confining pressure of 10 MPa eliminates tensile stresses at the sample corners. These results are currently being implemented into the experimental protocol and execution in UCL’s TTA in order to ensure that we obtain reliable true triaxial data. However, these observations are generic and could therefore contribute to improved development and operation of true triaxial loading systems generally.

How to cite: Stanton-Yonge, A., Mitchell, T., Meredith, P., Browning, J., and Healy, D.: Reducing boundary effects during True Triaxial loading of rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9568, https://doi.org/10.5194/egusphere-egu24-9568, 2024.

Subglacial geology remains largely unknowns in Antarctica. Direct geological samples are limited to ice free regions along the coast, high mountain ranges or isolated nunataks, while the origin of geological material transported by glaciers themselves is often ambiguous. 3D singular and joint inversions of airborne gravity and magnetic data recovers subsurface density and susceptibility distribution. The relationship between both inverted petrophysical quantities provide crucial insight for subglacial geology and rock provinces interpretations. Validation of indirect derived subglacial geology models are critical but very challenging in Antarctica due to the sparsity of rock samples.

We present 324 new density and susceptibility measurements on rock samples from Northern Victory Land, East Antarctica. 251 samples have been measured at the National Polar Sample Archive (NAPA) from the Federal Institute for Geosciences and Natural Resources (BGR) in Berlin-Spandau, Germany and additional 73 samples were measured at the BGR in Hannover, Germany. We use the petrophysical measurements to validate our recent regional scale 3D joint inversion model of the Wilkes Subglacial Basin and the Transantarctic Mountains. Furthermore, we validate inversion results on a local scale of singular magnetic inversion based on high resolution airborne magnetic data with a flight line spacing of 500m in the Mesa Range.

We demonstrate that we can provide reliable discrimination between Ferrar Dolerites, Kirkpatrick Basalt and Granite Harbour intrusion rocks based on our local inversion model and that the recovered susceptibilities agree with those measured at rock samples from the study area. Furthermore, we show that regional scale inversion model of gravity and susceptibility distribution agrees for samples of the dominant crustal rock types. However, densities of small-scale dense intrusion bodies like Ferrar Dolerites are underestimated by the regional scale inversion, while the susceptibility range is correctly recovered.

Constraining subglacial geology with joint inversion of airborne potential field data is heavily depended on the resolution of the airborne survey, flight line coverage, the inversion scale, and the scale of the target feature. Regional scale inversion is adequate for large scale geological heterogeneities, which underestimate petrophysical quantities for small scale structures, while local scale inversions are able to resolve such structures but are more computational demanding and in the case of Antarctica lack ultra-high resolution airborne gravity data with a line spacing below 1000 – 500m.

How to cite: Lowe, M., Jordan, T., Moorkamp, M., Ebbing, J., Koglin, N., Ruppel, A., Green, C., Liebsch, J., Ginga, M., and Larter, R.: Verification of susceptibility and density relationship from 3D joint inversion of airborne magnetic and gravity data in northern Victoria Land, East Antarctica, with petrophysical measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-589, https://doi.org/10.5194/egusphere-egu24-589, 2024.

As the source material of hydrocarbon and a significant matrix constituent in organic-rich shale, organic matter influences not only the oil/gas generation and accumulation but also the mechanical behavior of shale formations. Previous researches have found that organic matter exhibits different mechanical properties from inorganic minerals, and proved that geochemical features could significantly affect the elastic behavior of organic matter in shale. Here, this work systematically investigates the influence of organic type and/or thermal maturation on mechanical behavior of organic matter. For elastic behavior, in conjunction with vitrinite reflection test, scanning electron microscope (SEM) observation, and micro-Raman analysis, nanoindentation is performed to measure the modulus of different macerals. The results indicate that with the same thermal maturity, inertinite has the highest Young’s modulus, while the modulus of bitumen is the lowest. In addition, with the increase of thermal maturity, the Young’s moduli of all kinds of maceral tend to increase, while the intensity ratio of D peak to G peak measured by micro-Raman analysis shows a decreasing trend, which indicates a higher degree of graphitization. For fracture behavior, maceral identification, focused xenon ion beam fabrication, and in situ fracture test are combined to analyze the deforming and fracturing process of different organic types. Micro cantilever beams are manufactured by using a xenon plasma focused ion beam-SEM (Xe PFIB-SEM), and then loaded in an environmental SEM (ESEM). Organic matter particle is set at the fixed end of cantilever beam, and the load is applied at the free end. Thus, the interaction between micro crack and organic matter can be observed and the corresponding mechanical data can be recorded. Test results indicate that the micro cantilever beams dominated by inertinite and vitrinite show the features of brittle failure, while those dominated by bitumen show the features of ductile failure. These microscale findings can support the upscaling model for precise prediction of mechanical properties at the macro scale, and assist with the understanding and interpretation of macroscopic elastic and fracture behavior in shale reservoirs.

How to cite: Zhao, J., Zhang, P., Zhang, W., and Zhang, D.: Influence of Geochemical Features on Elastic and Fracture Behavior of Organic Matter, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13701, https://doi.org/10.5194/egusphere-egu24-13701, 2024.

Reservoir rock properties play an important role in the overall efficiency of petrothermal systems. Potentially interesting target formations for such systems include igneous and metamorphic rocks. For such lithologies with typically low porosity values, pre-existing fractures and foliation emerge as possible controlling factors of their hydraulic and mechanical behavior, respectively. Understanding the hydromechanical behavior of the reservoir rock is vital for the planning, execution and monitoring of hydraulic stimulation and exploitation, and continued safe operation. We inferred the elastic behavior of Freiberger gneiss from millimetre to tens of meters scale from laboratory and field measurements. Laboratory measurements were performed on samples prepared from blocks collected in the Reiche Zeche mine in Freiberg, Germany, and on cores retrieved from boreholes, drilled to perform hydraulic stimulation experiments as part of the STIMTEC (STIMulation TEChnologies) project. Controlled-source, P- and S-wave ultrasonic measurements were performed on samples and cores at room pressure and temperature conditions, covering a wide range of angles between wave-propagation direction and foliation, to characterize the degree of mechanical anisotropy of the gneiss. Magnitude and anisotropy of P-wave velocities determined from the laboratory measurements on the cores are in broad agreement with velocities calculated from in-situ sonic log measurements and active ultrasonic transmission experiments  (Boese et al., 2022) representing travel path lengths from meter to decameters in the rock mass, with a mean fracture distance of decimeters. At elevated pressures, ultrasonic measurements on cylindrical samples suggest the dominance of foliation over microcracks in determining the elastic behavior. The lack of a substantial reduction in velocities, deduced from in-situ active and passive microseismic analyses (Boese et al., 2022), except in highly deformed volumes, constrains the stiffness of the in-situ fractures.

Reference
Boese, C. M., Kwiatek, G., Fischer, T., Plenkers, K., Starke, J., Blümle, F., Janssen, C., and Dresen, G.: Seismic monitoring of the STIMTEC hydraulic stimulation experiment in anisotropic metamorphic gneiss, Solid Earth, 13, 323–346, https://doi.org/10.5194/se-13-323-2022, 2022.

How to cite: Korkolis, E., Kozlov, E., Adero, B., and Renner, J.: Mechanical characterization of Freiberger Gneiss (Reiche Zeche, Germany) from laboratory to field scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15168, https://doi.org/10.5194/egusphere-egu24-15168, 2024.

Prolonged hydrocarbon production often leads to subsidence and seismicity in offshore and onshore hydrocarbon fields. In the Netherlands, tens of centimetres of subsidence has occurred above the Groningen Gas Field, with widespread induced seismicity during the 60+ years of its lifetime. These phenomena are driven by reservoir compaction at depth, resulting from gas extraction. Modelling the reversible, elastic component of compaction is straightforward. However, permanent deformation can also occur, the rate and effects of which are very poorly constrained. Furthermore, from smaller fields in the vicinity, it has already become clear that compaction may continue even now that production has stopped in 2023. To be able to confidently forecast the long-term surface impact of fluid production, for fields such as Groningen, and many other fields around the world, models are required that include the physical mechanisms responsible for reservoir compaction. These mechanisms are still poorly known and quantified at true reservoir conditions. Combining microstructural observations, obtained from field material and experimental work, and novel experimental mechanical data, obtained at simulated stress changes relevant to the reservoir, enabled us to identify the main grain-scale deformation mechanisms operating in the reservoir sandstone of the Groningen Gas Field. A key role is played by the thin intragranular clay layers present between the quartz grains making up the load-bearing framework. Compaction of and slip along these thin clay films has accommodated the permanent deformation accumulated during the production stage. After production is halted, experiments suggest that slow, time-dependent grain breakage will start to play a role as well. Microphysical models describing rate-insensitive compaction were implemented in Discrete Element models to assess sandstone compaction behaviour at the cm-dm scale. These numerical models can be used to evaluate reservoir compaction in different locations on the field due to pressure equilibration or repressurisation, with rate-sensitive mechanisms, such as stress corrosion cracking, to be added at a later stage, as their descriptions are still be developed. Eventually such small-scale numerical models should form the basis to upscale the sandstone behaviour to the reservoir scale.

How to cite: Hangx, S., Pijnenburg, R., Shinohara, T., Jefferd, M., Mehranpour, M. H., and Spiers, C.: Reservoir compaction during and after fluid production: A case study of the Groningen Gas Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12452, https://doi.org/10.5194/egusphere-egu24-12452, 2024.

Salt is playing a principal role in the current energy transition. From diapirs to salt walls, salt structures are being proposed for a growing number of non-fossil energy purposes and carbon-neutral projects (i.e., geothermics, hydrogen storage in salt caverns or CCS). However, diapiric structures are not uniform and exhibit significant compositional and structural heterogeneities that prevent an easy characterization and exploration, thus increasing the risk of exploitation. Compositional heterogeneities within salt structures arise by the occurrence of different rocks inherited from the original Layered Evaporite Sequence (LES) and transported during salt flow or created through various diagenetic processes during diapirism (e.g., caprock formation).

One of the most common heterogeneities found in salt bodies is the presence of heterometric rock masses known as stringers. The limestones of the upper Muschelkalk (M3) facies (Middle Triassic) are the primary non-evaporitic lithological unit within the Triassic LES of the South-Central Pyrenees. These limestones are exposed throughout the South-Pyrenean fold-and-thrust belt embedded in mudrocks and gypsums of the Upper Triassic Keuper facies, forming the actual caprock exposures in the region. The Estopanyà salt wall is located in the westernmost part of the South-Central Pyrenean Zone, within the Serres Marginals thrust sheet. This area shows a well-preserved 58 km² caprock exposure formed by Middle and Upper Triassic rocks that surround two adjacent salt-embedded basins known as the Estopanyà and Boix synclines. According to the stratigraphic record of these synclines, from the Upper Cretaceous to the Oligocene, the evolution of the area resulted in salt withdrawal leading to salt inflation and the occurrence of several E-W-oriented salt walls that were exposed during the deformation onset in the Lower Eocene (early Ypresian times).

The M3 stringers in the Estopanyà salt wall are embedded in a caprock matrix formed by the dissolution of halite on the surface, albeit its presence has been proposed at deeper levels based on gravimetric models. The stringers have been identified and mapped, forming decameter-thick and kilometre-long structures showing significant folding and fracturing as well as a well-preserved stratigraphic sequence. The basal part of these stringers is formed by a layered to tabular millimetre-to-centimetre-thick limestones whilst the upper part consists of tabular to massive centimetre to meter-thick limestones. The current contribution presents the petrophysical (i.e., mineral density, connected porosity, permeability and P-wave velocity) and petrothermal (thermal conductivity and specific heat capacity) properties of 40 samples collected throughout the eastern Estopanyà salt wall, covering the basal and upper succession of various stringers. The sample analysis enables us to discuss the factors controlling the studied rock properties and the viability of carbonate stringers as geothermal reservoirs and CCS, which is a novel study that can be replied in similar salt structures worldwide.

How to cite: Ramirez-Perez, P., Cofrade, G., Cruset, D., Cantarero, I., Martín-Martín, J. D., Ferrer, O., Sizun, J.-P., and Travé, A.: Outcrop analogue study of carbonate caprock stringers for CCS and geothermal reservoir: the petrophysical and petrothermal properties of the upper Muschelkalk in the Estopanyà salt wall (South-Central Pyrenees), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1531, https://doi.org/10.5194/egusphere-egu24-1531, 2024.

Carbon capture and storage (CCS) stands as a key technology for mitigating CO₂ emissions, with depleted oil and gas fields being excellent candidates for geological storage. However, injection of relatively cold, high-pressure CO₂ into higher temperature, low-pressure hydrocarbon reservoirs can induce cooling and potential freezing due to the temperature difference between the injected fluid and the reservoir, as well as Joule-Thomson cooling caused by the rapid expansion of the fluid upon entering the reservoir. This may impact wellbore integrity, and near-wellbore stability and injectivity, posing challenges for safe and cost-effective storage. To be able to accurately predict the impact of cooling on storage operations, it is important to quantify the impact of temperature cycling on the mechanical and transport properties of the rock formations in the near-wellbore area.

To address this, we performed thermal cycling experiments under realistic in-situ pressure-temperature conditions on sandstone analogous to typical hydrocarbon reservoir material. We used a novel apparatus comprising a hydrostatic pressure vessel placed inside a climate chamber providing a temperature range of -70°C to +180°C. Bleurswiller sandstone (Vosges, France; 24% porosity) was subjected to temperature changes from 100 °C to +40, +5, or -20°C at constant pore fluid pressure (5 MPa; 0.85 M NaCl brine) and confining pressure (10 or 25 MPa, i.e. similar to reservoirs of up to ~3 km depth). The effect of the rate of temperature change, brine saturation and the number of cycles on the volumetric behaviour of the sandstone were systematically investigated. Thermally treated samples were subsequently subjected to permeability measurements and conventional triaxial compression to evaluate the impact of confined temperature cycling on the transport and mechanical properties.

In all our thermal cycling experiments, we observed permanent volume change (compaction) with each cycle, though the amount of compaction decreased with subsequent cycles. Furthermore, our results showed that confined temperature cycling did not significantly alter the mechanical properties (strength, elastic properties) of Bleurswiller sandstone. This is in contrast to the strength reduction observed in other porous sandstones after unconfined thermal cycling. However, our thermally treated samples did exhibit a significant reduction in permeability by several orders of magnitude (κ = 10^-15 to 10^-16 m² post-treatment) compared to untreated reference samples (initial κ = ~10^-14 m²). Overall, permeability roughly decreased with increasing brine content (i.e. from dry to fully brine saturated), increasing number of thermal cycles, and increased temperature amplitude (i.e. more cooling). Temperature change rate did not affect the permanent volumetric strain or permeability reduction in samples that were only cooled. In experiments achieving sub-zero temperatures, including pore fluid freezing, slower temperature changes resulted in less permeability reduction.

How to cite: Amiri, S., Pijnenburg, R., and Hangx, S.: Effects of Temperature Cycling on the Mechanical and Transport Properties of Porous Sandstone: Implications for CO2 Storage in Depleted Hydrocarbon Reservoirs , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12566, https://doi.org/10.5194/egusphere-egu24-12566, 2024.

The study of caprock assumes paramount significance, particularly in elucidating alterations in the sealing conditions of petroleum reservoirs. However, it is imperative that corresponding simulation experiments transcend a singular focus on CO₂ and caprock, extending to a comprehensive study of the reservoir-caprock system. Experimental protocols were implemented, entailing the sequential flow of CO₂-rich fluid first through the reservoir and subsequently through the caprock. The chosen samples and conditions were drawn from potential CO₂ utilization and storage blocks in the Subei Basin, China, featuring sandstone reservoirs and mudstone caprocks. The experimental paradigm simulated the interaction when CO₂ migrating along the sandstone and then reaching the mudstone caprock, spanning a duration of 37 days.

Results underscore that the introduction of CO₂-rich fluid predominantly instigates the dissolution of sandstone reservoirs, with notable dissolutions observed in feldspar and clay minerals, while secondary mineral precipitation remains negligible. Upon the fluids traversal through the mudstone caprock, initial dissolution occurs in carbonate minerals, accompanied by continuous precipitation of secondary clay minerals. These secondary minerals not only occupy calcite dissolution pores but also precipitate on the surfaces of rock particles, asserting dominance in the water-rock reaction within the mudstone.

Supported by geological observations and numerical simulations, these experiments illuminate the material adjustment processes ensuing from the introduction of CO₂-rich fluid into the reservoir-caprock system. The infusion of CO₂-rich fluid induces mineral dissolution, augmenting pore space within the reservoir, with ion products subsequently transported to the mudstone caprock. Under conditions characterized by a slower flow rate and a more extensive water-rock reaction surface area in the mudstone caprock, the water-rock interaction accelerates the dissolution and reprecipitation of calcite and other minerals. The reprecipitated minerals effectively occupy caprock pores and fractures, thereby enhancing the caprock's sealing capacity.

How to cite: Zhou, B., Lun, Z., and Wang, B.: An Experimental Study of CO2 Flooding the Reservoir-caprock System: Implication for the Stability of Caprock during CO2 Intrusion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4095, https://doi.org/10.5194/egusphere-egu24-4095, 2024.