The fundamental processes of fluid flow and fracturing within porous media play a crucial role in various applications within the fields of energy geomechanics and groundwater hydrology. Design parameters include the composition and properties of the fluid, the hydraulic and mechanical properties of the porous media, and the injection methods. These parameters are selected based on the specific objectives of each application, such as inducing fractures or uniformly replacing pore fluids through infiltration, the depth of the operation, and whether the application is conducted in soil or rock.

Many of these operations take place in weakly-cemented and poorly-consolidated sands, which constitute the host rock for a significant portion of active aquifers and oil and gas reservoirs. These materials exhibit higher porosity and permeability compared to stronger rocks, resulting in a different response under fluid injection conditions. Fluid experiments are conducted on artificially cemented porous media to reveal the underlying mechanisms. The synthetic rock specimens used in the experiments are generated via microbially induced carbonate precipitation (MICP), a method that leads to the formation of calcium carbonate around silica particles. The resulting bio-treated specimens have prescribed properties (i.e., permeability, porosity, strength). The experimental behavior is then analyzed, encompassing factors such as infiltration/fracturing response regimes and fracturing patterns. Critical concepts from advanced geomechanics and groundwater hydrology are utilized to interpret the findings, including the brittleness index (BI), the cavity expansion theory, and the sand erosion problem.

How to cite: Konstantinou, C. and Papanastasiou, P.: Understanding the mechanisms of fluid flow and fracturing in poorly consolidated porous media, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8165, https://doi.org/10.5194/egusphere-egu24-8165, 2024.

Numerous applications in hydrogeology, geo-environmental and geo-energy engineering require the injection of a fluid into the formation. The interaction between the soil or rock matrix and the selected fluids is pivotal for each application, and relevant parameters in the design must be adjusted accordingly. Such designs must also prioritize safety, operational effectiveness, and the minimization of potential environmental impacts and infrastructure failure risks. Given that these weak materials have different hydraulic and mechanical properties compared to competent rocks, their infiltration and fracturing response remains largely unknown.

This study focuses on modelling the results of a series of experimental tests that were carried out on artificially cemented porous media which were generated via microbially induced carbonate precipitation (MICP), a process that results in the cementation of silica particles. The cores were subjected to an anisotropic triaxial stress state, and a prescribed fluid injection along a central small-diameter axial perforation. Depending on the flow rate, degree of cementation and stress intensity and anisotropy, the fluid caused radial fractures of various configurations in the samples.

The modelling has been undertaken with code DRAC, based on the FEM with multi-phase multi-physics features and zero-thickness interface elements (also known as cohesive elements). These elements are inserted in between standard continuum elements to represent the effect of existing or potential fractures, not only in the mechanical behavior but also flow or diffusion-wise. From the mechanical viewpoint, these elements are equipped with constitutive laws incorporating fracture mechanics principles including fracturing parameters Gf.

For the numerical simulation of the lab tests, the sample geometry includes the representation of individual grains generated randomly by Voronoi Tesselation, separated from each other by interface elements. The anisotropic confinement stresses are applied on the outer boundary while the injection is applied on a central circle (experimental perforation). The fracture parameters are adjusted to the cementation level, and the fluid flow along inter-grain interfaces follows the cubic law. The paper includes a sample of the results obtained, exhibiting realistic fracture values and fracture patterns as compared to experiments. The conclusion is that the approach used seems to be a valid approach to model the fracture of weakly consolidated sandstone samples subjected to fluid injection.

The developed micro-scale model can be used in applications where fluid flow in porous media serves as the underlying mechanism. Such applications include the generation of hydraulic barriers, fluid injection for groundwater contamination remediation, water decontamination, leaching-induced flow to groundwater, artificial ground freezing for groundwater containment, CO₂ sequestration, managed aquifer recharge (MAR), and subsurface transportation of various fluids.

How to cite: Papanastasiou, P., Konstantinou, C., Garolera, D. G., and Carol, I.: Numerical Modeling of Fluid injection in unconsolidated formations using the FEM with zero-thickness interfaces at the micro-scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18250, https://doi.org/10.5194/egusphere-egu24-18250, 2024.

In this study, we present a new apparatus for creating earthquake-like instabilities in the laboratory and testing earthquake control theories (Stefanou, 2019a; Stefanou & Tzortzopoulos, 2022; Gutiérrez-Oribio et al., 2022). This experimental setup incorporates an analogue fault surrounded by an elastic medium. The frictional properties of the analogue fault are imposed by 3D printing. The elastic medium was chosen such as to have a very low Young's modulus and allow reasonable sampling rates for testing the earthquake control theories, while enabling the upscaling of the results based on appropriate scaling laws.

The apparatus allows the application of a slow, uniform deformation of the elastic medium through a system of pantographs installed at the lateral boundaries of the device. Then, when the critical state is reached, an earthquake-like instability occurs. The slip front and its propagation are measured using digital image correlation. Preliminary results show qualitative similarities with natural seismic slip, as expected.

The next step is to adjust the effective stress over the analogue fault to achieve a controlled slow slip rate of prescribed amplitude, according to our earthquake control theories and compare the results with those from existing, but simpler, experiments we have performed in the past (Gutiérrez-Oribio et al., 2023).

How to cite: Aoude, A., Stefanou, I., Semblat, J.-F., and Rubino, V.: Design of a new laboratory earthquake experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11981, https://doi.org/10.5194/egusphere-egu24-11981, 2024.

Permeability, a crucial factor in assessing porous reservoir rock quality, can be influenced by localised deformation such as fractures, shear bands, or compaction bands. Considering the arbitrary occurrence of these bands and the need to investigate the permeability difference between the overall specimen and within the localised zone, existing literature lacks a quantitative measure for such complexity. This study proposes a numerical concept that focuses on the permeability-stress connection across multiple levels from macroscopic to grain-scale bridging via the localised band, referred to as mesoscale. This framework integrates breakage mechanics into a dual-scale continuum model, tracking the permeability evolution of Bentheim sandstone under triaxial compression. The key feature lies in the link from stress to grain size distribution, due to breakage mechanics, that governs the constitutive responses, including macro, inside and outside of the localisation zone, produced from the dual-scale model following traction equilibrium condition. The successful outcome should capture the permeability inside the localisation band that is significantly lower compared to that of the outer bulk. The achievement provides insight into how important a correct mechanism of small details can influence the overall material behaviour while opening possible studies on many other material quantities while respecting the fundamental mechanics at multiple scales.

How to cite: Nguyen, N., Nguyen, G., Bui, H., and Bennett, T.: Permeability evolution of localised failure sandstone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7154, https://doi.org/10.5194/egusphere-egu24-7154, 2024.

A field experiment in the Western Canadian Sedimentary Basin, Alberta, Canada entails creation of a horizontal subsurface lens (fluid-driven fracture) through fluid injection from a vertical well. The initial lens creation is followed by cycling of smaller injection and flowback stages. After completion of these smaller injection/flowback cycles, the lens is (presumably) expanded through a larger scale injection followed by another series of smaller injection/flowback cycles. Tiltmeter data indicates subhorizontal lens orientation and nearly circular shape. Combining the lens radius inferred from tiltmeter inversion with measurements of downhole pressure and downhole caliper width (opening) measurements indicate the compliance is far below the linear elastic fracture prediction based on assumption that the parting between the fracture surfaces is complete and no intact rock bridges constrain opening of the lens. Additionally, the compliance is shown to increase with increasing maximum width attained by the lens. This history-dependence suggests intact rock bridges are progressively broken as larger injected volumes are used to attain larger widths. In this view, as the bridges break, the lens compliance increases. By considering a model wherein bridges are accounted for by springs with a prescribed ultimate breaking length, the observed crack compliance and dependence of the compliance on the width history are matched. Hence, the detailed field data combined with a suitable bridge constriction model comprises compelling evidence for rock bridges constricting fluid-driven crack apertures at field scale.

Acknowledgements. AB, AD and EP acknowledge support from the Australian Research Council through project DP190103260.

How to cite: Bunger, A., Dyskin, A., Pasternak, E., Hill, B., Lau, H., and Kasperczyk, D.: Field Evidence for Rock Bridges Constricting the Opening of Fluid-Driven Cracks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6821, https://doi.org/10.5194/egusphere-egu24-6821, 2024.

Fracture growth in rocks is not continuous; the fracture traces exhibit numerous interruptions
and overlapping. This is an indication of the presence of bridges distributed (in 3D) all over
the fracture. While the bridges themselves make little obstruction for the fluid flow in the
hydraulic fracture, they constrict the fracture opening and thus narrow the channel transmitting
the fracturing fluid. The effect of distributed bridges on the fracture opening can be modelled
by introducing a Winkler layer in the fracture with an effective stiffness combining the average
bridge stiffness and the number of bridges per unit area of the fracture.
When the fracturing fluid is injected and the leakoff is not negligible, the fracture size is
controlled by the fracturing fluid left in the fracture after leakoff rather than by the stress
concentration/singularity at the fracture contour. The flow of the fluid and filling of the fracture
depends upon the fracture geometry and is also controlled by the bridges. However, the bridges
weakly influence the time dependence of the fracture dimensions: the KGD and PKN fractures
grow proportionally to the square root of time: the effect is only in coefficients. Only in disc-
like cracks the constriction reduces the rate of crack growth and weakens the time dependence
of the fracture radius.
Acknowledgement. The authors acknowledge support from the Australian Research Council
through project DP190103260.

How to cite: Shapiro, S., Dyskin, A., and Pasternak, P.: Growth of hydraulic fractures with constricted opening and leakoff, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15854, https://doi.org/10.5194/egusphere-egu24-15854, 2024.

The sliding surfaces of faults and shear fractures in rocks are rarely smooth. They possess local undulations or asperities that act as distributed points of sliding disturbance. These asperities work as bridges connecting the opposite sliding surfaces and creating forces that non-linearly depend on deformation. These forces can even act in the direction of sliding. The mechanism of generating these forces is the local dilation caused by the fact that the sliding makes some asperities of one surface climb over the asperities of the opposite surface in the presence of the external compressive stress. Therefore, the dilation creates an effective friction angle which is the original (microscopic) friction angle controlling friction between the surfaces of the opposite asperities plus the inclination angle between the tangent line to the asperity surfaces at the point of contact and the direction of the movement. The inclination angle depends upon the position of the contact between the asperities making the effective friction angle non-linearly dependent upon the displacement. After passing the point of maximum dilation, the inclination angle becomes negative. Therefore, depending upon the surface friction angle, the effective friction angle can become negative. This creates the effect of local negative stiffness, where the energy is supplied by the convergence of the opposite faces of the fault/fracture (after the top of the contacting asperity is passed) in the presence of the external pressure.

The development of the zones of negative friction which are the zones of energy release can be accompanied by microseismisity (one zone - one signal) generated without involving local fracturing. Understanding this mechanism will contribute to refining of the existing monitoring methods of faults/fracture movement.

Acknowledgement. The authors acknowledge support from the Australian Research Council through project DP190103260.

How to cite: Pasternak, E. and Dyskin, A.: Asperities in sliding surfaces as frictional bridges. Negative friction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12467, https://doi.org/10.5194/egusphere-egu24-12467, 2024.

The presence of multiple asperities in the contacting fault surfaces produces sliding resistance associated with the contact between asperities of the opposite sliding surfaces. The pairs of contacting asperities essentially work as frictional bridges: the fault sliding can only proceed when all bridges are broken, that is the corresponding pairs of asperities are either broken or their resistance is overcome by one asperity climbing over its counterpart. Furthermore, each step of sliding produces new pairs of contacting asperities forming new frictional bridges independent of the previous ones. The number of new bridges is usually high. We assume it to be proportional to the sliding velocity. The condition of producing a step of sliding is the failure of all the bridges resisting to the sliding in the area associated with the sliding step. Each bridge is characterised by a different strength (either the actual asperity strength or the force needed to make one asperity to climb over its counterpart). Given the large number of bridges involved in each step, this condition can be expressed in terms of the distribution of the maximum strengths. Using the theory of order statistics, we obtained a logarithmic dependence of the resulting friction coefficient upon the sliding velocity, which is observed in the experiments. Thus, our model suggests a new interpretation of the experimentally observed rate-dependent friction. The model parameters are associated with the parameters of bridge distribution, which are related to the morphology of the sliding surfaces.

Acknowledgement. The authors acknowledge support from the Australian Research Council through project DP190103260.

How to cite: Dyskin, A. and Pasternak, E.: Rate-dependent friction associated with frictional bridges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12306, https://doi.org/10.5194/egusphere-egu24-12306, 2024.

Rock masses consist of nesting rock blocks with various scales separated by weak structural layers, and their complex hierarchical structures play a significant role in dynamic deformation and stress wave propagation. Based on the Cosserat theory, a dynamic model of pendulum-type and rotational waves in blocks rock mass with complex hierarchical structures is established to determine the influence of hierarchical structures on dynamic deformation. Then, aiming to low-frequency and low-velocity characteristics of pendulum-type waves, dispersion equations of waves are determined and solved in different hierarchical structures based on the Bloch theorem, and furthermore, the dispersion relation and velocity characteristics of waves are investigated. Finally, mechanism of low-frequency characteristics of pendulum-type waves is revealed on the basis of solid energy band theory, and the possibility of pendulum-type and rotational waves inducing rock bursts is discussed based on the research results. It is indicated that ignoring higher-order hierarchical structures of rock masses may underestimate displacement and overall deformation of rock masses, resulting in unsafe numerical results. Under the action of long wave disturbance, for the first mode pendulum-type waves (the acoustic branches), the dispersion is not significant and propagation velocity decreases, and higher-order hierarchical structures inside rock masses hinder the wave propagation. However, the dispersion of other waves (the optical branches) is significant so that they hardly exist and propagate independently. The low-frequency pendulum-type waves are dominant, which have slower attenuation and longer propagation distance than the high-order mode waves and traditional P and S-waves.

How to cite: Jiang, K. and Qi, C.: Research on pendulum-type and rotational waves in 2D discrete blocky rock masses with complex hierarchical structures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3271, https://doi.org/10.5194/egusphere-egu24-3271, 2024.