EMRP1.5 | Rock fracturing and applications across disciplines, from surface processes to deep geothermal reservoirs
EDI

This session comprises fracture focused research that spans disciplines and scales but is all intimately linked to coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes and factors. We divide the session into two parts related to shallow and deep processes, respectively.

The first part is focused on progressive rock failure (PRF) and its applications to surface processes, rock physics and engineering research. Because fractures influence the hydromechanical properties of rocks such as porosity, permeability, erodibility and strength, the rock mechanics and rock physics of PRF is intimately linked to virtually all surface and critical zone processes and also extends beyond the natural world to our own built environment. Yet, the potentially central role that PRF may play in these fracture-related systems has been largely unrecognized or misconceived across surface-process, engineering, and rock physics applications.

The second part of the session is related to THMC processes in geothermal reservoirs with focus on the role of fractures and faults on the reservoir performance, its sustainable use and related risks. We invite contributions including: (i) fluid flow, permeability, fluid conductivity; (ii) heat flow, thermal conductivity and diffusivity; (iii) deformation either compression, shear, or tension; seismic or aseismic; (iv) fracture and fault (re)activation and related seismic risks; (v) coupled THM-processes in fractured and intact reservoir rocks.

Public information:

This session comprises fracture focused research that spans disciplines and scales but is all intimately linked to coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes and factors. We divide the session into two parts related to shallow and deep processes, respectively.
 
The first part is focused on progressive rock failure (PRF) and its applications to surface processes, rock physics and engineering research. Because fractures influence the hydromechanical properties of rocks such as porosity, permeability, erodibility and strength, the rock mechanics and rock physics of PRF is intimately linked to virtually all surface and critical zone processes and also extends beyond the natural world to our own built environment. Yet, the potentially central role that PRF may play in these fracture-related systems has been largely unrecognized or misconceived across surface-process, engineering, and rock physics applications. 
 
The second part of the session is related to THMC processes in geothermal reservoirs with focus on the role of fractures and faults on the reservoir performance, its sustainable use and related risks. We invite contributions including: (i)    fluid flow, permeability, fluid conductivity; (ii)    heat flow, thermal conductivity and diffusivity; (iii)    deformation either compression, shear, or tension; seismic or aseismic; (iv)   fracture and fault (re)activation and related seismic risks; (v)    coupled THM-processes in fractured and intact reservoir rocks.

Co-organized by GM3
Convener: Martha-Cary Eppes | Co-conveners: Guido Blöcher, Philip Meredith, Jean Schmittbuhl, Mauro Cacace, Lucille CarbilletECSECS, Sophie KenmareECSECS
Orals
| Tue, 25 Apr, 14:00–15:45 (CEST)
 
Room -2.21
Posters on site
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
Hall X2
Posters virtual
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
vHall TS/EMRP
Orals |
Tue, 14:00
Wed, 10:45
Wed, 10:45

This session comprises fracture focused research that spans disciplines and scales but is all intimately linked to coupled Thermal-Hydraulic-Mechanical-Chemical (THMC) processes and factors. We divide the session into two parts related to shallow and deep processes, respectively.
 
The first part is focused on progressive rock failure (PRF) and its applications to surface processes, rock physics and engineering research. Because fractures influence the hydromechanical properties of rocks such as porosity, permeability, erodibility and strength, the rock mechanics and rock physics of PRF is intimately linked to virtually all surface and critical zone processes and also extends beyond the natural world to our own built environment. Yet, the potentially central role that PRF may play in these fracture-related systems has been largely unrecognized or misconceived across surface-process, engineering, and rock physics applications. 
 
The second part of the session is related to THMC processes in geothermal reservoirs with focus on the role of fractures and faults on the reservoir performance, its sustainable use and related risks. We invite contributions including: (i)    fluid flow, permeability, fluid conductivity; (ii)    heat flow, thermal conductivity and diffusivity; (iii)    deformation either compression, shear, or tension; seismic or aseismic; (iv)   fracture and fault (re)activation and related seismic risks; (v)    coupled THM-processes in fractured and intact reservoir rocks.

Orals: Tue, 25 Apr | Room -2.21

14:00–14:10
|
EGU23-5126
|
EMRP1.5
|
ECS
|
On-site presentation
Till Mayer, Missy Eppes, and Daniel Draebing

Rockwall erosion by rockfall processes proceed at rates between 0.05 ±0.03 to 14.4 mm a-1 (Draebing et al., 2022) and are key agents of alpine landscape evolution. Previous studies suggest that frost weathering is a major contributing process to alpine rockwall erosion (Draebing and Mayer, 2021). Frost weathering occurs primarily by frost cracking driven by ice segregation, but our current process understanding is based on studies focusing on high-porosity low-strength rocks. However, rock types forming alpine rockwalls are characterized by crack-dominated porosity and high rock strength, therefore, it is unclear how past findings from low-strength rocks apply in these settings. In this study, we will perform laboratory ice segregation tests on rock samples with different saturation levels and fracture density to quantify their influence on frost cracking efficacy.

We used Wetterstein limestone rock samples in laboratory experiments and exposed rocks to realistic-rockwall freezing conditions while monitoring acoustic emissions as a proxy for cracking. To differentiate triggers of cracking, we modelled ice pressures and thermal stresses. We tested the influence of (i) saturation (low versus full initial saturation), (ii) crack density (0.4 versus 0.6 % rock porosity), and (iii) temperature range (-10 to 0°C) on the efficacy of ice segregation.

(i)  Our data showed that the efficacy of ice segregation is not controlled by initial water content in alpine rocks. These results suggest that water available at depth within alpine rock masses can rapidly travel along fractures to form ice lenses near the rock surface.

 (ii) Crack density has a direct impact on the elastic properties of rocks, which shifts the stress threshold for crack propagation. A fractured rock with high crack density is less prone to ice segregation as its lower brittleness increases the critical fracture toughness.

(iii) Our data revealed temperature patterns promoting ice segregation with highest rates of frost cracking at temperatures between -10 and -7 °C in high strength Wetterstein limestone.

We conclude that frost cracking efficacy in high alpine environments is more impacted by temperatures than by initial rock moisture, which potentially results in more rockfall at colder north- than warmer south-facing rockwalls.

 

Draebing, D., Mayer, T., Jacobs, B., and McColl, S. T.: Alpine rockwall erosion patterns follow elevation-dependent climate trajectories, Communications Earth & Environment, 3, 21, https://doi.org/10.1038/s43247-022-00348-2, 2022.

 

Draebing, D., and Mayer, T.: Topographic and geologic controls on frost cracking in Alpine rockwalls, Journal of Geophysical Research: Earth Surface, 126, e2021JF006163, https://doi.org/10.1029/2021JF006163, 2021.

How to cite: Mayer, T., Eppes, M., and Draebing, D.: Quantifying controlling factors of ice segregation in alpine rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5126, https://doi.org/10.5194/egusphere-egu23-5126, 2023.

14:10–14:20
|
EGU23-16084
|
EMRP1.5
|
ECS
|
On-site presentation
Hao Chen, Paul Antony Selvadurai, Antonio Salazar Vasquez, Patrick Bianchi, Qinghua Lei, Claudio Madonna, and Stefan Wiemer

Brittle creep in rock that results from time-dependent subcritical crack growth often plays a fundamental role in the emergence of precursory phenomena of impending catastrophic events in the upper crust. Laboratory has been used to investigate time-dependent cracking in brittle rocks, and signals of acoustic emission (AE) and X-ray tomography were employed as proxies for damage accumulation, which increase non-linearly towards failure (Heap et al., 2009; Renard et al., 2020). Despite these studies, the evolution of damage in real time and especially the potential impact of strain localization on dynamic critical transition of failure yet are understudied.

We study brittle creep in a dry Herrnholz granite (with an initial porosity of 2.2%) under triaxial stress conditions. The test procedures consisted of (i) confining the sample to Pc =10 MPa and (ii) then applying and holding a differential stress σd = 234 MPa. This stress was held constant and a standard creep response was observed exhibiting a clear trimodal behavior that culminated with the formation of a shear fracture and catastrophic failure of the sample. We used the distributed strain sensing (DSS) fiber optic technology to obtain local estimations of strain and calculate volumetric strain on the surface of the sample using an interpolation strategy. During the primary creep phase, deformation mapped with DSS was found to be sparsely distributed in the form of volumetric deformation in general uniform throughout the sample and expressed on the surface. The transient acceleration of creep (creep burst) was only identified in local strain measurements near the final faulting position and occurred during the steady-state creep phase. This clearly indicates that strain began to localize around the ultimate location of fracture, which was also confirmed by the postmortem 3D optical scanning.

Using this better understanding of progressive strain localization, we searched for indications of damage evolution and critical behavior. During the creep phases, changes in certain properties of DSS array were examined for potential precursory signatures. We analyzed the statistics of damage rate and incremental strain and detected a significant breaking of scaling during the creep phase which led to a critical interpretation of fracture. Prior to the critical point, creep bursts correlated with the nucleation and growth of the main fault, which likely indicates the onset of scaling divergence where damage began to self-organize toward failure. These results show that strain localization which drives the fracture development can be captured by DSS technology and the brittle creep processes in Herrnholz granite follow a critical point transition which can be attributed to a self-adjustment of local strains after creep burst.

 

References:

Heap, M. J., Baud, P., Meredith, P. G., Bell, A. F., & Main, I. G. (2009). Time-dependent brittle creep in Darley Dale sandstone. Journal of Geophysical Research, 114(B7), B07203. https://doi.org/10.1029/2008JB006212

Renard, F., Kandula, N., McBeck, J., & Cordonnier, B. (2020). Creep burst coincident with faulting in marble observed in 4‐D synchrotron X‐ray imaging triaxial compression experiments. Journal of Geophysical Research: Solid Earth, 125, e2020JB020354. https://doi.org/ 10.1029/2020JB020354

How to cite: Chen, H., Selvadurai, P. A., Vasquez, A. S., Bianchi, P., Lei, Q., Madonna, C., and Wiemer, S.: Illumination of Damage and Critical Transition During Time-Dependent Deformation in Herrnholz Granite Using Distributed Strain Sensing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16084, https://doi.org/10.5194/egusphere-egu23-16084, 2023.

14:20–14:30
|
EGU23-9609
|
EMRP1.5
|
On-site presentation
Elli-Maria Charalampidou, Maria-Eleni Taxopoulou, Nicolas Beaudoin, Charles Aubourg, Alexis Cartwright-Taylor, Ian Butler, Robert Atwood, and Stefan Michalik

Deformation bands, or tabular zones of localised strain, are a common manifestation of deformation in upper crustal sedimentary rocks. Any mining or energy-related engineering applications must consider the possibility of reactivating these pre-existing failure planes because doing so can cause seismicity and compartmentalise the reservoir. However, there has only been a small amount of research done on laboratory-induced deformation in rocks with natural deformation features.

On a low porosity bioclastic calcarenite from the Cotiella Basin, Spanish Pyrenees, our current experimental work aims to capture, for the first time to our knowledge, the dominant failure mechanisms during the reactivation of natural deformation bands oriented at different angles to the principal stress direction. At the I12-JEEP beamline at the synchrotron facility of Diamond Light Source, UK, we carried out triaxial compression experiments using a modified version of the Mjolnir cell used by Cartwright-Taylor et al., (2022) to examine how these highly heterogeneous rocks respond to additional mechanical deformation. During the deformation experiments, 4D (time and space) x-ray tomography images (8 m voxel size resolution) were acquired. We tested confining pressures between 10 MPa and 30 MPa.

The mechanical data demonstrate that the existence of natural deformation features within the tested samples weakens the material. For instance, solid samples of the host rock subjected to the same confining pressures had higher peak differential stresses. Additionally, our findings demonstrate that new deformation bands form as their angle, θ, to σ1 increases, while the reactivation of pre-exiting deformation bands in this low porosity carbonate only occurs for dipping angles close to 70o. The spatio-temporal relationships between the naturally occurring and laboratory-induced deformation bands and fractures were investigated using time-resolved x-ray tomography and Digital Volume Correlation (DVC). Volumetric and shear strain fields were calculated using the SPAM software (Stamati et al., 2020). The orientation of the recently formed failure planes is influenced by the orientation of the pre-existing bands, as well as their width and the presence (or absence) of porosity along their length. Additionally, pre-existing secondary deformation features found in the tested material trigger additional mechanical damage that either promotes the development or deflects the new failure planes.

References

Cartwright-Taylor et al. 2022, Nature Communications 13, 6169, https://doi.org/10.1038/s41467-022-33855-z

Stamati et al. 2020, Journal of Open Source Software, 5(51), 2286, https://doi.org/10.21105/joss.02286

How to cite: Charalampidou, E.-M., Taxopoulou, M.-E., Beaudoin, N., Aubourg, C., Cartwright-Taylor, A., Butler, I., Atwood, R., and Michalik, S.: Failure mechanisms in low-porosity carbonate rocks during the reactivation of deformation bands with various orientations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9609, https://doi.org/10.5194/egusphere-egu23-9609, 2023.

14:30–14:31
14:31–14:41
|
EGU23-3676
|
EMRP1.5
|
ECS
|
On-site presentation
Alexandra Kushnir, Michael Heap, Patrick Baud, Thierry Reuschlé, and Jean Schmittbuhl

Rock masses are often criss-crossed by generations of discontinuities, including veins, fractures, and joints. The presence of fractures and joints can increase rock mass permeability and decrease rock mass strength. However, fluid flow within rock masses can result in secondary mineral precipitation within these spaces. Secondary mineralisation can reduce permeability, with important consequences for fluid flow in systems that rely on discontinuity-dominated permeable networks. Here we investigate if variably sealed joints can be reactivated during deformation and the role joint reactivation plays on permeability. We deformed 20 mm in diameter by 40 mm long cores of un-jointed and jointed (variably sealed) bedded sandstones. Samples were cored such that their dominant structural feature (i.e., bedding or joint) was oriented parallel, perpendicular, or at approximately 30° to the sample axis. We find that the permeability of the undeformed samples is sensitive to the presence and orientation of bedding. In jointed samples, well-sealed joints can act as barriers to fluid flow, but partially filled joints neither inhibit nor promote fluid flow with respect to their joint-free counterparts. While all rocks in this study deformed in the brittle regime under triaxial deformation conditions, the location of the experimentally induced fractures depends on the extent to which joints are sealed. The mineralisation that fills well-sealed joints also permeates the surrounding sandstone matrix, locally reducing porosity and forming a cohesive bond between the joint-fill and the host-rock that increases rock strength: experimentally induced fractures do not exploit pre-existing joint surfaces in these samples. By contrast, strain is localised on the joint surface in samples containing partially sealed joints and the strength of these samples is lower than their un-jointed counterparts. The permeability of all samples increased after deformation, but permeability increase was largest in samples with pre-existing, poorly filled joints. We conclude that partially sealed joints act as planes of weakness within rock masses and that their reactivation can result in significant permeability increase. Well-sealed joints, however, may locally increase rock strength and never become reactivated during deformation: consequently, these joints may never re-contribute to the permeability of a rock mass. These observations provide insight into how fluid flow in the crust may evolve, with possible implications for how these systems weather over time.

How to cite: Kushnir, A., Heap, M., Baud, P., Reuschlé, T., and Schmittbuhl, J.: Reactivating sealed joints: rock strength reduction and permeability enhancement … sometimes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3676, https://doi.org/10.5194/egusphere-egu23-3676, 2023.

14:41–14:51
|
EGU23-7101
|
EMRP1.5
|
On-site presentation
Philip Meredith, Yang Yuan, Monica Rasmussen, Karin Hofer Apostolidis, Yochitaka Nara, Patrick Webb, Thomas Mitchell, Tao Xu, Russell Keanini, Amit Mushkin, Uri Shaanan, Maxwell Dahlquist, Alex Rinehart, and Martha Eppes

Fractures in rock are ubiquitous; from cold dry planetary bodies to the hottest, wettest climates on Earth, and from km-scale tectonic fractures deep in Earth’s crust to microcracks in surficial rocks. Yet, many of these fractures propagate progressively over geologic timescales, making their development complex and enigmatic. Therefore, to measure how fractures have developed in rocks exposed at Earth’s surface over millennia, and how this consequently changes rock physical properties, we collected ten ~25 cm diameter granitic boulders from two sites in the Eastern Sierra, California, USA. The boulders were deposited on the surface of alluvial terraces and fans during geologically instantaneous glacial and alluvial events at different times since about 148ka BP, then the depositional surfaces were subsequently abandoned. The chronosequences of geomorphic surfaces provide a natural laboratory in which rocks of consistent lithology have been exposed to similar environmental conditions for different lengths of time, allowing us to compare rock property evolution on the order of 0 to 105 years of environmental exposure; an approach that allows us to better understand and characterize mechanical weathering processes, especially long-term changes in rock fracturing. Note that fresh (time-zero) rocks in this study are represented by boulders found within active channels, and that the measured changes in rocks with longer exposure times are interpreted by comparison with the fresh rocks. Focusing only on similarly sized boulders removes any ambiguities in tectonic and exhumation history that might arise in outcrop samples, thus ensuring that rocks from each site have experienced similar stress conditions; namely those restricted to the environment.

We performed laboratory measurements on 10 granitic boulders (four from Lundy Canyon, with exposure ages of ~0 to ~148 ka; six from Shepherd Creek, with exposure ages of ~0 to ~117 ka) to quantify how rock physical properties changed as a function of environmental exposure age. We measured key parameters commonly used as proxies for crack damage, including porosity, compressional wave velocity (Vp), and shear wave velocity (Vs). We hypothesize that changes in crack damage are likely to affect rock mechanical properties, so we also measured tensile strength, uniaxial compressive strength (UCS), and Young’s modulus (E). We find that all measured parameters evolve as a function of exposure age, with systematic increases in porosity, and systematic decreases in Vp, Vs, tensile strength, UCS, and E. For example, porosity increases from 0.5 – 1.0 % in the fresh rock to 2.6 – 3.2 % in the oldest rocks. We interpret these changes as reflecting progressive subcritical crack growth that arises due to ubiquitous, but relatively low magnitude, environmental stresses continuously acting on the boulders, as opposed to differences inherited before their erosion from bedrock.

Apart from demonstrating the importance of environmentally driven cracking in rock weathering, these observations of progressive crack damage accumulation also have significant implications for the interpretation of any measurements made on rocks exposed at Earth’s surface, even if the age of exposure is relatively short compared to the age of the geologic deposit itself.

How to cite: Meredith, P., Yuan, Y., Rasmussen, M., Hofer Apostolidis, K., Nara, Y., Webb, P., Mitchell, T., Xu, T., Keanini, R., Mushkin, A., Shaanan, U., Dahlquist, M., Rinehart, A., and Eppes, M.: Evidence for increase in crack damage in rocks with duration of exposure at Earth’s surface., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7101, https://doi.org/10.5194/egusphere-egu23-7101, 2023.

14:51–14:55
14:55–15:05
|
EGU23-4110
|
EMRP1.5
|
On-site presentation
Fanbao Meng, Lu Shi, Stephen Hall, Patrick Baud, and Teng-fong Wong

Previous investigations of the compressive failure of porous rocks under true triaxial compression have focused on the brittle faulting regime. These studies have underscored the dependence of the peak stress state on the interplay of the three principal stresses. In comparison, there is a paucity of systematic investigations of ductile failure under true triaxial compression. In this study we selected Bleurswiller sandstone, which has been extensively investigated in relation to the brittle-ductile transition under conventional triaxial compression at room temperature. Experiments were conducted in Wuhan on water-saturated samples with the size of 100mm×50mm×50mm at the minimum and intermediate principal stresses ranging up to 70 MPa and 170 MPa, respectively. Previous conventional tests have shown that the initial yield points of Bleurswiller sandstone fall on a linear cap relating the differential and mean stresses. Our new data show that initial yielding under true triaxial loading at a fixed Lode angle is also characterized by a Mises effective shear stress that decreases linearly with increasing mean stress, in agreement with the prediction of an elastic-plastic pore collapse model. Subsequent yielding was manifested by various degrees of strain hardening, that would culminate in a spectrum of failure modes (high-angle shear bands, conjugate shear bands, compaction bands, distributed cataclastic flow). The 3D complexity and geometric attributes of these failure modes have been characterized by X-ray CT imaging of the failed samples.   

How to cite: Meng, F., Shi, L., Hall, S., Baud, P., and Wong, T.: Inelastic compaction and failure mode of Bleurswiller sandstone under true triaxial compression, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4110, https://doi.org/10.5194/egusphere-egu23-4110, 2023.

15:05–15:15
|
EGU23-8009
|
EMRP1.5
|
On-site presentation
Anett Blischke, Ásta R. Hjartardóttir, Egill Árni Gudnason, Thorbjörg Ágústsdóttir, Ásdís Benediktsdóttir, Arnar M. Vilhjálmsson, Unnur Þorsteinsdóttir, Gunnlaugur M. Einarsson, Auður A. Óladóttir, and Anette K. Mortensen

We present a new structural model of the Theistareykir high-temperature geothermal field, located in NE Iceland within a volcanically active rift zone. Various interdisciplinary geoscientific methods are applied and cross-analyzed. We use remote sensing data, structural and geological surface and subsurface mapping, drone surveys, borehole images, seismicity, potential field, and CO2 surface emissions data. This composite data modelling approach aims to highlight primary fault zones and fractured intervals at the surface by assigning fault types and their spatial orientation. The compiled surface and subsurface datasets were used for 3D fault projections and the delineation of fault-block compartments. Fault plane solutions from earthquakes supported fault property assignments, slip direction, and stress-field orientations. Surface fault segments and fracture intensity maps highlight areas of relay ramping, fault damage, and accommodation zones in between the NW-SE, NE-SW to N-S striking fault systems of the Theistareykir rift segment. The fault systems are intersected by WNW-ESE striking embryonic transfer zones that form boundaries between rift valley graben segments south and within the Theistareykir geothermal field area. These WNW-ESE transfer zones accommodate the differential and oblique opening of the rift zone, which overall follows the right-lateral opening direction of the region south of the Husavik-Flatey transform fault. Our structural model of the Theistareykir geothermal field area is subdivided into six structural domains that form fault block compartments, with varying degrees of faulting and fracturing, reflecting the different quality of hydraulic connectivity across the field.

How to cite: Blischke, A., Hjartardóttir, Á. R., Gudnason, E. Á., Ágústsdóttir, T., Benediktsdóttir, Á., Vilhjálmsson, A. M., Þorsteinsdóttir, U., Einarsson, G. M., Óladóttir, A. A., and Mortensen, A. K.: 3D structural mapping of the Theistareykir geothermal field, NE-Iceland: Fault and fracture connectivity within a volcanically active rift zone., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8009, https://doi.org/10.5194/egusphere-egu23-8009, 2023.

15:15–15:25
|
EGU23-11236
|
EMRP1.5
|
ECS
|
On-site presentation
Entela Kane, Anne Pluymakers, and André Niemeijer

Human intervention in subsurface geoenergy systems, such as fluid injection, can lead to induced seismicity. Particularly in geothermal systems where faults or fractures serve as fluid pathways, fault reactivation is a significant risk. Therefore, we must elucidate under which stress conditions faults become reactivated. The geometry of fault planes evolves as a function of fault displacement. Mature faults (i.e. with 10-100 m displacement) are most likely gouge-filled due to material weathering during movement. This study investigates the Lower Carboniferous system and specifically the Dinantian formation. This specific formation is particularly interesting for deep geothermal energy in the Netherlands, Belgium and Germany but can serve as a proxy for fractured carbonate deep geothermal reservoirs worldwide. The Dinantian carbonates exhibit pre-existing fractures which mainly contribute to rock permeability. However, there is little or no knowledge of the fault geometry and filling material. Subsequently, it is essential to investigate a spectrum of carbonate fault geometries and gouge material to deliver fault stability conditions. 

Here, we aim to experimentally characterise fault strength through all stages of their temporal evolution, from bare rock to highly strained fault gouges at a range of normal stresses. All experiments are performed at room temperature, using de-ionised (DI) water as pore fluid. The bare rock surfaces are gouge-free saw-cut samples, loaded into a Hoek cell embedded in a 500 kN uniaxial loading machine. These experiments are performed at a range of confining pressure from 10 to 50 MPa, corresponding to normal stresses from 60 to 90 MPa and undrained conditions. We determined the critical range of axial, shear and normal stress values per experiment at which fault reactivation was initiated. We used a rotary shear apparatus to increase fault gouge maturity through the shearing of a simulated gouge material in drained conditions. In a single experiment and sample, we changed the normal stress with a protocol of 2-4-6-8-10-8-6-4-2 MPa. For every stress interval, we performed a slide-hold-slide procedure, where the slide-hold times were 10-10-10-100-10-1000-10 sec and the velocity was 20-0-20-0-20-0-20 μm/sec respectively. After each hold time, the reactivation leads to a different peak shear stress. By characterizing the different peak stresses we can quantify the evolution of critical shear stress as a function of fault inactivity time. We used the reactivation stresses for both experimental types to calculate the intercept and angle of the reactivation envelopes in a Mohr-Coulomb context using linear regression, which corresponds to the cohesion and friction coefficient of the laboratory faults. 

Our preliminary results for the rotary shear experiment show that mature carbonate laboratory faults exhibit a cohesion of <1 MPa and friction coefficient up to 0.65 under wet conditions. Moreover, the cohesion of the fault decreases as a function of healing time and the friction coefficient increases. Future plans include the investigation of fluid chemistry on the reactivation envelope. To conclude, we aspire to give insight to the operators on how to safely design the geothermal injection and production schemes accounting for the geomechanical constraints.

How to cite: Kane, E., Pluymakers, A., and Niemeijer, A.: Reactivation envelopes of immature and mature faults of Dinantian carbonates targeted for geothermal energy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11236, https://doi.org/10.5194/egusphere-egu23-11236, 2023.

15:25–15:35
|
EGU23-8589
|
EMRP1.5
|
On-site presentation
Ioannis Stefanou, Georgios Tzortzopoulos, Philipp Braun, and Diego Gutierrez-Oribio

We perform laboratory experiments of decametric scale using a novel triplet apparatus (Fig. 1) that allows (a) to reproduce earthquake-like instabilities and (b) to prevent them by active fluid pressure adjustment. The dynamic earthquake events are prevented using high-end, robust stabilizing controllers that stabilize the system even in the absence of knowledge about its friction, elasticity and other complex phenomena that are hard to quantify in practice.

Two scenarios are investigated experimentally. In the first scenario, the system is loaded close to its instability point and then fluid is injected in order to provoke a seismic event. We observe how the controller automatically adjusts the fluid pressure in order to prevent such instabilities and immobilize the system. In the second scenario, the controller adjusts the fluid pressure automatically in order to drive the system in a new stable equilibrium of lower energy in an aseismic manner. Despite the inherent unstable behavior of the system, uncertainties related to friction, elasticity and multiphysics couplings, the earthquake-like events are avoided and controlled. We expect our methodology to inspire earthquake mitigation strategies regarding anthropogenic and/or natural seismicity.

References

[1] Stefanou, I. (2019). Controlling Anthropogenic and Natural Seismicity: Insights From Active Stabilization of the Spring‐Slider Model. Journal of Geophysical Research: Solid Earth, 124(8), 8786–8802. https://doi.org/10.1029/2019JB017847
[2] Tzortzopoulos G., Braun P., Stefanou I. (2021), Absorbent Porous Paper Reveals How Earthquakes Could be Mitigated, Geophysical Research Letters 48. https://doi.org/10.1029/2020GL090792.
[3] Stefanou, I., Tzortzopoulos, G. (2022). Preventing instabilities and inducing controlled, slow-slip in frictionally unstable systems. Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/2021JB023410
[4] Gutiérrez-Oribio D., Tzortzopoulos G., Stefanou I., Plestan F. (2022). Earthquake Control: An Emerging Application for Robust Control. Theory and Experimental Tests. http://arxiv.org/abs/2203.00296
[5] Papachristos, E., Stefanou, I. (2022), Controlling earthquake-like instabilities using artificial intelligence. http://arxiv.org/abs/2104.13180.
[6] Gutiérrez-Oribio D., Stefanou I., Plestan F. (2022). Passivity-based Control of a Frictional Underactuated Mechanical System: Application to Earthquake Prevention. https://arxiv.org/abs/2207.07181

 

Fig.1: Experiments of decametric scale using a novel triplet apparatus for preventing earthquake-like instabilities

How to cite: Stefanou, I., Tzortzopoulos, G., Braun, P., and Gutierrez-Oribio, D.: Controlling earthquake-like instabilities in the laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8589, https://doi.org/10.5194/egusphere-egu23-8589, 2023.

15:35–15:45
|
EGU23-14681
|
EMRP1.5
|
ECS
|
On-site presentation
Victoria Alegria Jimenez Martinez and Joerg Renner

The creation of fractures in boreholes by hydraulic stimulation is of importance for enhancing fluid transport in the context of geothermal reservoirs. Detecting whether a fracture is created and evaluating its capacity to transport fluid are usually performed by locating the acoustic emissions generated during the rock failure and by comparing the hydraulic properties before and after this type of hydraulic stimulation using pumping tests, respectively. However, free oscillations, exerted by rapid changes in pumping parameters, can be used as well to detect the existence of a fracture and to evaluate its permeability. The diagnostic properties of free pressure oscillations are their spectral components, i.e., frequency and damping coefficient. The hydraulic system, which includes technical equipment such as tubes and hoses, and the rock formation, which can be tight or leaky. In a tight system, the free pressure oscillations are attenuated by the viscous interaction of the fluid and the borehole wall and the local coupling of the fluid compression and deformation of the borehole due to the pressure variation. In a leaky system, the attenuation of free pressure oscillations includes fluid exchange between the borehole and the hydraulic conduits of the rock. We developed an analytical solution starting from the dispersion relation of fluid-flow waves in a tight borehole by accounting for the fluid exchange as a modified boundary condition. Deviation of spectral components of observed oscillation from the analytical solution for a tight borehole is an indication that the free pressure oscillations contain information of the hydraulic properties of the penetrated formation. The oscillations typically last for tens of seconds, which allows assessing the success of the stimulation operation on a near-real-time basis. We analyzed the characteristics of free operations recorded in boreholes during several hydraulic stimulation campaigns. We investigated the evolution of the spectral components in the course of the stimulation and with changes in mean injection pressure and obtained transmissivity values that favorably compare to the results of conventional analyses.

How to cite: Jimenez Martinez, V. A. and Renner, J.: Discerning fracturing and constraining hydraulic properties from the characteristics of free pressure oscillations in boreholes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14681, https://doi.org/10.5194/egusphere-egu23-14681, 2023.

Posters on site: Wed, 26 Apr, 10:45–12:30 | Hall X2

X2.233
|
EGU23-5158
|
EMRP1.5
Martha-Cary Eppes, Mike Heap, Patrick Baud, Thomas Bonami, Max Dahlquist, Russell Keanini, Cyril LaCroix, Monica Rasmussen, Alex Rinehart, Youness El Alaoui, and Adrien Windenberger

The growth of rock fractures enables physical and chemical rock erosion, sets the pace for infrastructure-, rockfall- and landslide-hazards, and influences rock hydrologic, and therefore chemical, processes. Although fracture growth ultimately lowers rock strength, when rocks are subject to stresses lower than that magnitude, laboratory experiments indicate that the growth of fractures can counterintuitively make rock more resilient to subsequent fracture growth through ‘stress memory’ or ‘fatigue-limit’ phenomena such as the Kaiser effect.  Thus, over geologic time scales, all other things being equal, fracturing rates may decrease, which would have important implications for understanding and interpreting a wide range of landscape evolution processes. To date, however, there have been few if any data explicitly showing that fracture-resilience feedback phenomena arise naturally in subaerially exposed rock.

Here we test for a natural stress memory in two ~25 cm diameter boulders for which we have 1-4 years of known environmental exposure history.  The granite boulders were collected from an unvegetated bar in an ephemeral channel issuing from the south flank of the San Bernardino Mountains, California. As such, we infer that natural abrasion in the channel had removed any major cracks or heterogeneities, effectively ‘resetting’ the rock to a relatively pristine state. The rocks were left on the ground in full sun exposure for 1 and 3 years respectively in humid temperate North Carolina and semi-arid temperate New Mexico, USA. Per-minute rock surface and environmental conditions and cracking (using acoustic emissions) were monitored. Prior work (Eppes et al., 2016 & 2020) indicates that thermal stresses were the primary driver of cracking in the rocks during these time periods. The boulders were then cut in half, and 20x40mm cores were collected from various locations within the rock interior, in duplicate and triplicate for locations of varying distance to the rock exterior. We measured core porosity and P-wave velocity in the cores as proxies for initial rock crack composition, as well as thermal conductivity. We then subjected sets of cores collected at different distances from the rock exterior to increasing magnitudes and number of thermal stress cycles in a temperature-monitored oven, beginning with our best approximation of those matching the maximum stresses leading to observed cracking during the 1 and 3 year observation periods. Our preliminary results reveal that initial crack characteristics vary as a function of distance from rock exterior, as might be expected due to the different magnitudes of thermal stresses experienced within these locations within the rock. Thus, we hypothesize that areas starting with the highest porosities and lowest velocities will experience less change following heating cycles than those parts of the rock with few inferred fractures. We hope that these data will help elucidate mechanisms and feedbacks of natural rock fracturing phenomena that occur over geologic time scales.

How to cite: Eppes, M.-C., Heap, M., Baud, P., Bonami, T., Dahlquist, M., Keanini, R., LaCroix, C., Rasmussen, M., Rinehart, A., El Alaoui, Y., and Windenberger, A.: Testing natural fracture growth-fracturing resilience feedbacks in rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5158, https://doi.org/10.5194/egusphere-egu23-5158, 2023.

X2.234
|
EGU23-7443
|
EMRP1.5
Jill Marshall, Alex Rinehart, Martha Cary Eppes, and Phillip Meredith

Geomorphology context: Earth surface scientists have long posited what controls bedrock to soil conversion rates, which we can now test (assuming steady state) using cosmogenic nuclides. Additionally, near-surface geophysics allows us to image the near-surface with increasing fidelity, such that weathering states from ‘fresh’ bedrock to weathered rock to soil can be inferred over hillslope scales. Current models do not always match field data, and we are yet unable to predict soil thickness. Pedologist Hans Jenny's five factors of soil formation (climate, organisms, topography, parent material, and time) complement the factors geomorphologists presume drive soil production rates (and thus thickness). Geomorphology considers soil production rates from the top-down - whereas the existing soil thickness controls the efficacy of climate and organisms in converting bedrock into disaggregated material, and climate, topography and organisms control the transport efficiency necessary to remove soil - thus keeping the boundary between rock and soil thin enough for more top-down weathering. In most settings, we have few observations of in situ physical weathering. Weathering mechanisms (e.g., thermal, ice segregation, wind-driven tree sway, plant water uptake) are cyclic over brief (seconds to minutes), diurnal, or seasonal cycles. Almost all bring water to the crack network. Unlike traditional laboratory experiment conditions, surface rock is buffered by a soil layer and is subject to disturbance agents that can remove loose fragments - thus modifying the stress state and the crack network. 

Rock physics context: Laboratory experiments to date only consider bare rock. While frost weathering has a rich history of physical experimentation, we know of no other physical experiments that directly test near-surface weathering conditions specifically. While all near-surface rock is to some degree broken by tectonics, the journey to the surface, or contraction cooling; a threshold density of cracks is necessary for cracks to intersect significantly. Because crack growth rate is a function of the crack length and eventually, degree of stress accommodation due to increasing porosity, crack growth in non-uniform over time and thus physical weathering is non-uniform even if conditions remain constant. In its simplest form, considering only mechanical sources of damage, the 'Kaiser effect' suggests that under conditions of cyclic loading, cracking happens only when the previous maximum stress is exceeded. However, in natural environments, each cycle of opening refreshes water at the crack tip, allowing chemical damage to accrue, and for fracture propagation. Most progressive rock failure experiments are run monotonically, with the fracture under a consistent loading, or with rapid, cyclic loading—neither replicating conditions experienced in the natural world necessary to estimate material property change through time.

Interdisciplinary context: Geomorphologists and soil scientists have generally ignored factors governing fracture propagation, and rock physicists, focused on index properties and detailed process understanding, have not simulated relevant field conditions. Here, we explore such as above in asking if and how the non-uniform nature of subcritical cracking may be a first order control on soil production and bedrock landscapes, and if so, what experiments exist or are needed to arrive at a new type of soil production function?

How to cite: Marshall, J., Rinehart, A., Eppes, M. C., and Meredith, P.: Should we? Can we? apply experimental rock physics knowledge to reconsidering soil production functions?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7443, https://doi.org/10.5194/egusphere-egu23-7443, 2023.

X2.235
|
EGU23-17555
|
EMRP1.5
|
ECS
Clay Wood, Chun-Yu Ke, Jacques Rivière, Derek Elsworth, Chris Marone, and Parisa Shokouhi

Understanding the poromechanical response of fractured rock is necessary for applications ranging from hydraulic stimulation of the subsurface to teleseismic impulses from earthquakes that may reactivate faults or otherwise breach reservoir seals. We describe laboratory experiments that seek to decouple the role of fracture interface stiffness and fracture infill (sediment transport) in the hydraulic and elastodyamic properties of fractured rock. Experiments are conducted on multiple samples of Westerly granite with different uniform roughness (silicon carbide grit-roughened or milled) that were loaded under triaxial stresses in a pressure vessel while permeability evolution is measured from the flow-through of deionized water. In some experiments, thin layers of synthetic wear (gouge) material are added to the fracture interface to simulate mature, sheared, fractures whose poromechanical response is dominated by clogging and unclogging of pore throats. Oscillations of pore pressure and normal stress are applied at amplitudes ranging from 0.2 to 1 MPa at 1Hz. The experiments also consider the influence of fracture aperture with effective stress perturbations applied at normal stresses ranging from 5 to 20 MPa (reducing aperture with increasing effective normal stress). Before, during, and after the dynamic stressing, an array of piezoelectric transducers (PZTs) continuously transmits and receives ultrasonic pulses across the fracture to monitor the evolution of fracture stiffness and fluid transport due to the dynamic stressing. These allow evaluation of stress-induced changes in transmitted ultrasonic wave velocity and amplitude to estimate the contact acoustic nonlinearity of the fracture interface concurrent with permeability evolution. We compare the results for samples with and without synthetic wear (gouge) material to understand the role and evolution of fracture stiffness and clogging-unclogging mechanisms in pore throats of porous and fractured media.

How to cite: Wood, C., Ke, C.-Y., Rivière, J., Elsworth, D., Marone, C., and Shokouhi, P.: Decoupling the poromechanics of particle remobilization and interface stiffness of dynamically stressed tensile fractured rock, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17555, https://doi.org/10.5194/egusphere-egu23-17555, 2023.

X2.236
|
EGU23-15452
|
EMRP1.5
|
ECS
|
Highlight
Evangelos Korkolis and Joerg Renner

Environmental effects on cracking have important implications in geoengineering applications, such as mechanized tunnel construction. The time required and the cost of excavation could be reduced by employing techniques such as special cutting fluids that lower the strength of the rock.

At first contact, the cutting tool acts as an indenter, penetrating the rock surface and causing distributed brittle damage and subsequently localized deformation. We performed indentation tests using blunt indenters, mimicking a section of a standard cutting disk, on unconfined cylindrical Anroechter sandstone specimens using a servo-hydraulic press. The loading rate was varied over two orders of magnitude. A first set of experiments was performed at room temperature and humidity; in a second, specimens had their top surface wetted with water during indentation. Preliminary results show that in the presence of water, the peak indentation force is reduced by approximately 13%. The short duration of the tests (a few minutes) and the relatively low porosity of the sample material (approximately 10%) suggest a fast-acting weakening mechanism. Currently, we are focusing our efforts on understanding the effect of wetting fluid chemistry on peak indentation pressure and exploring the interplay with loading rate.

How to cite: Korkolis, E. and Renner, J.: Experimental investigation of environmentally affected cracking during indentation testing of Anroechter sandstone, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15452, https://doi.org/10.5194/egusphere-egu23-15452, 2023.

X2.237
|
EGU23-4576
|
EMRP1.5
|
ECS
Gabriel Meyer and Marie Violay

With increasing depth, crustal rocks gradually transition in deformation type, from being brittle/cataclastic to being crystal plastic, as well as in deformation mode, from being localized (faults, shear zones) to being ductile (homogeneous flow). This transitional layer, commonly referred to as brittle-ductile transition (BDT) has recently become the focus of economic development with the advent of Superhot Rock geotherm Reservoirs (SHR). Superhot Rock geothermal projects (e.g., Japan Beyond-Brittle Project, Iceland Deep Drilling Project, and Newberry Volcano) seek to extract heat from geothermal reservoirs where water reaches a supercritical state (≥ 400 °C). These could multiply the power output of geothermal plants by a factor ten, a progress that is critical in the context of the climate crisis.

However, SHR reservoirs are generally localized at the BDT in semibrittle rocks (rocks deforming through a mixture of brittle and crystal plastic processes) which hydraulic properties are poorly understood.

Here, we report experiments conducted in TARGET, a newly designed gas-confining triaxial apparatus located at EPFL, CH. We deformed cylindrical cores of Lanhelin granite of dimension 40 x 20 mm at a confining pressure of 100 MPa and temperatures ranging from 200 to 800°C and a strain rate of 10-6 s-1. While deforming, sample permeability was recorded using the pore pressure oscillation method with an oscillation amplitude of 5 MPa and a period of 2400 s.

Lanhelin granite transitions from being localized with the formation of a sample scale fracture to being ductile between 600 and 800°C. In the localized regime, samples have an ultimate strength of around 600 to 650 MPa. In this regime, permeability initially slightly decreases upon loading from its initial value of 10-20 m2 before increasing with continued deformation. Permeability eventually plateaus upon sample failure and remains constant with further deformation. In the localized regime, permeability increase never exceed 2x10-19 m2. In the ductile regime, sample strength is halved and, past the initial decrease upon loading, permeability increases monotically by more than an order of magnitude.

We interpret these data has being the result of sample bulk controlling the sample permeability. In our localized experiments, the fracture never connected the ends of the rock core but would concentrate all of the strain after nucleation, limiting permeability improvement by micro-cracking in the bulk. In the ductile regime, since no localization occurs, bulk permeability of the rock would continuously improve with strain. These results bear important implications for the engineering of permeability in semibrittle reservoirs as well as for the understanding of hydrothermal circulation in the continental crust.

How to cite: Meyer, G. and Violay, M.: Permeability of Lanhelin granite through the brittle-ductile transition., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4576, https://doi.org/10.5194/egusphere-egu23-4576, 2023.

X2.238
|
EGU23-12406
|
EMRP1.5
|
ECS
Line Hähnel, Wolfgang Bauer, and Harald Stollhofen

Germany's geothermal potential outside the known geothermal provinces has been little investigated so far, as it does not involve deep sedimentary basins. The crystalline basement offers a yet untapped potential for producing geothermal energy with enhanced geothermal systems (EGS). In the research alliance "Geothermal Alliance Bavaria", which is funded by the Bavarian State Ministry of Science, we are investigating the potential for EGS in northern Bavaria.

A region in Franconia with a geothermal anomaly has already been delimited and is the focus of further investigations. A granite body covered by several kilometers of sediments was identified as the source of this geothermal anomaly. For a more detailed investigation of the hydraulic conditions in a fault zone-controlled granite reservoir, a surface analogue was found in a granite quarry in the Fichtelgebirge in northeastern Bavaria.

The quarry is transected by a fault zone and shows a narrow fracture network at the surface. A testfield consisting of 15 wells with depths between 15 and 25 m was set up in the quarry to analyse the influence of a fault zone-controlled fracture network on the hydraulic permeability. A photogrammetric model, surface geophysical measurements and borehole geophysics have been carried out to record the fracture network in detail. The influence of the fracture network on the hydraulic permeability is to be determined by various hydraulic tests.

Slug and pulse tests show variable, but overall low hydraulic permeabilities in the individual boreholes with values between 10-7 – 10-10 m/s. Slightly higher permeabilities assumingly correlate with more prominent fractures or fracture zones detected in image logs and several geophysical logs. Double packer tests on selected fractures/fracture zones will determine single fracture permeabilities in order to clarify which fractures and whether certain fracture properties mainly influence the hydraulic permeability. Furthermore, these double packer tests are intended to identify connectivity between individual wells through specific fractures or fracture zones.

In a further step, hydraulic packer tests will be used to determine fracture opening pressures and the stress field.

First results of hydraulic tests evaluated so far will be presented and the influence of recorded fracture properties of single fractures and fracture zones on the observed hydraulic permeability will be presented and discussed.

How to cite: Hähnel, L., Bauer, W., and Stollhofen, H.: Influence of fractures and their properties on hydraulic permeability in a fault zone-controlled fractured granite – basic scientific research for an EGS feasibility study, Northern Bavaria, Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12406, https://doi.org/10.5194/egusphere-egu23-12406, 2023.

X2.239
|
EGU23-14219
|
EMRP1.5
|
ECS
|
Highlight
Arjan Marelis, Jan-Diederik van Wees, and Fred Beekman

Keywords: geothermal, high resolution, fault stability, induced seismicity.

High resolution predictions of three-dimensional subsurface stress changes are required for the assessment of geothermal operations with respect to fault stability and the potential risk for induced seismicity. The effects of long-term cooling on reactivation and seismicity potential of faults near a geothermal doublet require quantification and management for safe and effective application of geothermal energy. This work presents a detailed analysis of the sensitivity for fault reactivation and induced seismicity based on different scenarios for reservoir characteristics and production parameters. To this end, analytical solutions are used as well as a TNO-proprietary tool known as MACRIS (Mechanical Analysis of Complex Reservoir for Induced Seismicity) (Van Wees et al., 2019) that allows for both poro- and thermo-elastic stress evaluations in structurally complex (i.e. highly faulted) reservoirs. The stress evaluations take as input the pressure and thermal field of the reservoir and over- and underburden which are obtained from the Open Porous Media (OPM) Flow reservoir simulator (Rasmussen et al., 2021). In this workflow, high resolution stress change solutions at the faults are available.

The workflow has been applied to a high resolution three-dimensional reservoir model, including over- and underburden rock, marked by a single fault. Key elements in the dynamic and mechanical behaviour of the reservoir are varied, along with different production scenarios. Simulated stress evolutions in MACRIS and alternative analytical solutions show a predominant sensitivity for fault reactivation to the thermo-elastic parameters, i.e. the Young’s modulus and thermal expansion coefficient. Furthermore, in cooling reservoirs, the intersection area of the cold-water volume in direct contact with the fault plane is shown to be the main driver for fault reactivation and subsequent seismic potential.

 

References

Rasmussen, A.F., Sandve, T.H., Bao, K., Lauser, A., Hove, J., Skaflestad, B., … and Thune, A. (2021). The Open Porous Media Flow reservoir simulator, Computers and Mathematics with Applications, 81, 159-185.

Van Wees, J.D., Pluymaekers, M., Osinga, S., Fokker, P. A., van Thienen-Visser, K., Orlic, B., Wassing, B. B. T., Hegen, D., and Candela, T. (2019). 3-D mechanical analysis of complex reservoirs: a novel mesh-free approach, Geophysical Journal International, 219 (2), 1118-1130.

 

How to cite: Marelis, A., van Wees, J.-D., and Beekman, F.: A sensitivity analysis of stress changes related to geothermal direct heat production in clastic reservoirs and potential for fault reactivation and seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14219, https://doi.org/10.5194/egusphere-egu23-14219, 2023.

X2.240
|
EGU23-14427
|
EMRP1.5
Fracturing of rocks in Bavarian geothermal reservoirs – Experimental and numerical investigation of borehole stability
(withdrawn)
Justin Mattheis, Catharina Drexl, Leon Herrmann, Ridvan Yildirim, and Kurosch Thuro
X2.241
|
EGU23-15244
|
EMRP1.5
|
ECS
Regina Fakhretdinova, Alexis Sáez, and Brice Lecampion

Deep heat mining requires activation of slip on pre-existing geological discontinuities and the creation of hydraulically conductive fracture networks. Fluid injection or diffusion of ground waters can rise the fluid pressure near pre-existing fractures and faults, which may induce frictional slip. The fracturing process depends strongly on the initial stress conditions and rupture planes orientation. It is known that vertical stress is varying linearly with depth whereas horizontal stresses are likely not to exhibit linear dependence. Nevertheless, within certain length scales, one may assume linear relations for all stress tensor components.

In the previous study [1], it was shown that for a planar rupture which is propagating due to fluid injection under a constant overpressure in the absence of stress gradient, the solution is self-similar and depends only on one dimensionless parameter which determines two limiting regimes. The first so-called "critically-stressed limit” designates that the fault is initially close to failure, whereas the “marginally pressurized limit” represents the case when the fluid pressure is “just sufficient” to activate the fault. One of the main features of the solution in the uniform stress case is that the rupture tips are propagating symmetrically.

In our work, we investigate how linear stress gradient acting initially on the fault affects the shear rupture growth, namely, how it breaks the symmetry of the rupture propagation. The problem couples quasi-static elastic equilibrium and fluid flow on the fault plane via a Coulomb shear failure criterion with a constant friction coefficient. From a scaling analysis, it is shown that the problem is governed by two dimensionless parameters, To (similar to the one found in [1]) and dimensionless time. Parameter To is the ratio between the initial distance to failure and the strength of injection [1] calculated at the injection point. To determines two propagation regimes similar to those found in [1] (critically stressed and marginally pressurized limits). Dimensionless time parameter determines symmetric and asymmetric propagation periods and encapsulates the information about stress-gradient values. At early times, the solution is similar to the homogeneous stress case and the rupture stays symmetrical. At times near the characteristic time of each regime, the non-uniform in-situ stress distribution makes the rupture to propagate asymmetrically. We investigate the transition time for each limiting regime and compare it with real field observations. Our solution can also provide a benchmark for numerical solvers.

 

REFERENCES

[1] Viesca, R., 2021 Self-similar fault slip in response to fluid injection, Journal of Fluid Mechanics, vol. 928

How to cite: Fakhretdinova, R., Sáez, A., and Lecampion, B.: The effect of in-situ linear stress gradient on the frictional shear rupture growth in 2D., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15244, https://doi.org/10.5194/egusphere-egu23-15244, 2023.

X2.242
|
EGU23-16563
|
EMRP1.5
Brice Lecampion, Alexis Sáez, Regina Fakhretdinova, and Ankit Gupta

Hydraulic stimulation of pre-existing fractures is used in deep geothermal development in order to increase reservoir permeability and achieve economical flow rates – with mixed success [1, 2]. Although the primary idea is to shear dilate these pre-existing discontinuities via injection, in a number of field tests [3], a large increase of permeability is only observed when fracture opening has been reached (sometimes denoted as hydraulic jacking). Shearing of pre-existing discontinuities can also occur during more traditional hydraulic fracturing operations in oil and gas reservoirs, either by direct fluid pressurization or via stress transfer from the main fractures. In this contribution, we discuss the development of a robust numerical solver for the modeling of the fluid-driven growth of a shear crack along frictional discontinuities, accounting for shear-induced dilatancy as well as possible transition to opening hydraulic fracturing. An elasto-plastic constitutive relation with a non-associated flow rule is used to model the frictional and cohesive behavior of the pre-existing discontinuity with or without slip-dependent frictional properties. A fully coupled hydro-mechanical solver is developed for this class of problem. It combines a boundary element discretization of the fracture(s) for the solution of the quasi-static elastic equilibrium of the rock mass with a finite element discretization of the width-averaged fluid mass conservation and momentum in the fractures. Using implicit time-stepping, the resulting non-linear system of coupled equations is solved via a Newton-Raphson procedure using the consistent tangent elasto-plastic operator obtained from the local integration of the interfacial constitutive relation via a predictor-corrector scheme. We present a number of stringent verification problems for strictly frictional as well as strictly hydraulic fracturing conditions. We then investigate the evolution of both the shear and opening front in terms of the properties of the pre-existing discontinuities (friction and dilatancy), the in-situ and injection conditions [4, 5, 6]. We highlight relevant conditions associated with deep geothermal reservoirs, and discuss the occurrence of different propagation regimes from purely frictional to hydraulic fracturing type.

References

[1] R. Jung. EGS - Goodbye or Back to the Future. In ISRM International Conference for Effective and Sustainable Hydraulic Fracturing. International Society for Rock Mechanics, 2013.

[2] M. W. McClure and R. N. Horne. An investigation of stimulation mechanisms in Enhanced Geothermal Systems. Int. J. Rock Mech. Min. Sci., 72:242–260, 2014.

[3] Y. Guglielmi, C. Nussbaum, P. Jeanne, J. Rutqvist, F. Cappa, and J. Birkholzer. Complexity of fault rupture and fluid leakage in shale: Insights from a controlled fault activation experiment. Journal of Geophysical Research: Solid Earth, 2020.

[4] K. Hayashi and H. Abe. Opening of a fault and resulting slip due to injection of fluid for the extraction of geothermal heat. Journal of Geophysical Research: Solid Earth, 87(B2):1049–1054, 1982.

[5] A. Sáez, B. Lecampion, P. Bhattacharya, and R. Viesca. Three-dimensional fluid-driven stable frictional ruptures. J. Mech. Phys. Sol., 160:104754, 2022.

[6] E. Detournay. Mechanics of hydraulic fractures. Annual Review of Fluid Mechanics, 48:311–339, 2016.

How to cite: Lecampion, B., Sáez, A., Fakhretdinova, R., and Gupta, A.: Development of a robust numerical simulator for mixed shear and opening modes fluid driven fracture propagation along pre-existing discontinuities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16563, https://doi.org/10.5194/egusphere-egu23-16563, 2023.

Posters virtual: Wed, 26 Apr, 10:45–12:30 | vHall TS/EMRP

vTE.8
|
EGU23-4804
|
EMRP1.5
|
ECS
Abolfazl Baghbani, Thomas Baumgartl, and Vilim Filipovic

Water penetration, changes in the groundwater level and moisture content changes can affect the physical and chemical properties of coal in an open pit mine. Water levels in open coal pit mines can vary throughout the year, resulting in a number of wet and dry cycles for brown coal. Wet and dry cycles occurring throughout the year can affect the mechanical strength of the stone's microstructure and macroscopic structure. Loss of strength can have severe negative impacts if such rock is integral component in landform design. Until now, no research has been conducted on the effects of wet and dry loading cycles on brown coal. This study investigates the effect of wet and dry cycles on brown coal's strength by conducting a series of unconfined compressive strength (UCS) laboratory tests. For this purpose, nine laboratory samples with dimensions of 38 x 76 cm were prepared. Samples were placed inside distilled water chambers in a temperature-controlled environment. Afterwards, the samples were subjected to unconfined compressive strength (UCS) tests following 0, and 3 cycles of wet and dry conditions. The results of the UCS test show that as the number of wetting and drying cycles increased, the UCS of the samples decreased from 2150 to 330 kPa after three cycles of wetting and drying. In addition, the results indicate that the elastic modulus of brown coal has decreased from 10500 to 1200 kPa. Also, the Poisson ratio decreased from 0.34 to 0.27. This study confirms the importance of paying attention to the wet and dry cycles in brown coal mines.

How to cite: Baghbani, A., Baumgartl, T., and Filipovic, V.: Effects of Wetting and Drying Cycles on Strength of Latrobe Valley Brown Coal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4804, https://doi.org/10.5194/egusphere-egu23-4804, 2023.