NP7.2

Seismicity associated with fluid injection into the subsurface is one of the most important issues worldwide. Fluid injection near a fault could lead to seismic sliding of the fault and as a consequence to significant seismic events. In the presented research, we study the single fault behavior under the action of a single well injection near the fault. Various cases of fluid injection and friction properties of the fault are considered. To describe the friction on the fault we use two-parameter rate-and-state law. The fault has zones characterized by velocity-weakening and velocity-strengthening friction behavior. We analyze how injection rate and volume and parameters of the friction law influence the fault sliding dynamics and seismicity level. As the result, we get conditions that are favorable for the occurrence of noticeable seismicity and dependence of seismicity parameters on injection parameters.

How to cite: Riga, V. and Turuntaev, S.: Seismic sliding of the single fault under fluid injection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9747, https://doi.org/10.5194/egusphere-egu22-9747, 2022.

This work aims to study the propagation of fractures during hydraulic fracturing operations to determine the conditions that lead to the most extensive network of secondary fractures along with the main fractures at the microlevel. The occurrence and propagation of 3D fractures were studied, considering the granular composition and complex structure of elastic-plastic rock samples. The simulation allowed us to calculate fracture networks for various loading conditions. We propose a method that will enable us to calculate the pressure of hydraulic fracturing fluid injection into the formation from the obtained numerical conditions of loading and regional stress in the reservoir rock. Based on the results of numerical modeling and recalculation of the obtained loading conditions into the pressure of the injected fluid, geomechanical engineers will choose the necessary conditions that will provide the stress-strain states that lead to the most significant degree of fracturing of the formation.

How to cite: Nachev, V. and Turuntaev, S.: Investigation of 3D Fracture Propagation in Complex Reservoirs Rocks at Microscale, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11759, https://doi.org/10.5194/egusphere-egu22-11759, 2022.

Complex structure of a skeleton of saturated porous medium can have a great influence on the processes of heat and mass transfer. 

There are various approaches to the description of  two-phase flow: direct numerical calculation of fluid flow in the pore space, multicontinuous models with the laws of mass exchange between continua, single-continuum models of non-equilibrium flow. In the family of isothermal non-equilibrium filtration models, the relative phase permeabilities and capillary pressure are functions not of current saturation but of their change history.

In this work we generalize the relaxation model of capillary nonequilibrium to the non-isothermal case. We introduce two internal thermodynamic parameters (capillary and thermal nonequilibrium) which depend on change history of saturation and temperature. In the model relative phase permeabilities and capillary pressure are functions of saturation, temperature, and current values of these internal parameters. Based on the analysis of the dissipation inequality, thermodynamically consistent kinetic equations for the evolution of these parameters are proposed. The parameters of the single-continuum model are clarified with double-porosity model of porous media with special structure. Structure of the penetration front of fluid hot (or cold) compared to the skeleton was investigated.

This work was supported by the Russian Foundation for Basic Research: grant N19-01-00592. 

How to cite: Izvekov, O., Konyukhov, A., and Cheprasov, I.: Modeling of capillary and thermal nonequilibrium flows in porous media, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8272, https://doi.org/10.5194/egusphere-egu22-8272, 2022.

It has been proposed that rock fracturing can be obtained by the powerful and fast pressure discharge at some boundary of the rock (fracture, borehole). The fracturing related with rapid pore pressure discharge looks as a “fracturing wave”.  Laboratory experiments allow to evaluate the main characteristics of the phenomena and to estimate conditions for the fracturing. Experimental data on the fracturing process were obtained with the help of transparent pressure-drop setup (plexiglass tube with length 455 mm, inner diameter 60 mm, wall thickness 5 mm) using the weak-cohesive porous samples made from the sand wetted by glycerol. The tube ends were closed by brass covers equipped by pressure transducers. Additionally, nipples for the air pumping and decompression were mounted at one of the covers. The pressure was increased by air pumping in up to 0.35 MPa, then the pressure was released through solenoid valve with flow section 15 mm. Decompression rate was controlled by the diaphragms with different diameters: 2.8, 3.0, 3.4, 4.0, 4.8 mm. To observe the fracturing, high-speed camera with frame rate 1200 frames per second was used. The dependencies of maximum depth of the fracture formations and mean distance between the fractures on decompression rate were obtained. It was found that the number of fractures and the last fracture depth grow with the pressure drop rate, while the inter-fracture distance decreases.

How to cite: Turuntaev, S. and Zenchenko, E.: Experimental study of saturated porous medium fracturing by abrupt pressure drop, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5669, https://doi.org/10.5194/egusphere-egu22-5669, 2022.

Rock samples tested in uniaxial compression tests are often observed to have spallation failure at the lateral surface of the sample caused by buckling of layers developing parallel to the direction of loading. These layers are caused by extensive crack growth. Yet, the experiments show that extensive crack growth in real 3D situations requires the presence of intermediate compressive principal stress (biaxial loading). We check the hypothesis that the intermediate principal stress is generated by the direct contact with metal loading platens, where the “frictional” boundary conditions are developed at the sample ends, creating non-uniform stress distributions near the sample ends. By the use of the finite element method (FEM), we analyse the stress distributions at the immediate lateral surface of the cylindrical sample in uniaxial compression in the polar coordinate system. It is found that near the ends the sample is actually biaxially loaded: the circumferential stress could be induced to play the role of the intermediate principal stress. The sizes of the zones and the maximum magnitude of the circumferential stress depend upon the friction coefficient and the Poisson’s ratio of the rock. The biaxial load ratio is defined by the ratio between the circumferential compressive stress and the axial stress. Comparing the biaxial load ratio determined in numerical models with the critical biaxial load ratio inducing extensive crack growth, the spallation of rock samples in uniaxial compression tests can be interpreted from a new perspective.

How to cite: Wang, H., Dyskin, A., Pasternak, E., and Dight, P.: Local biaxial loading induced by end friction in uniaxial compression, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9482, https://doi.org/10.5194/egusphere-egu22-9482, 2022.

Asymmetric friction refers to different friction forces that resist sliding in opposing directions. Asymmetric friction can for instance be induced by an anisotropic block with an anisotropy axis inclined to the direction of sliding progressing in a constraint environment, (Bafekrpour et al., 2015). This study examines a Burridge-Knopoff-type model of multiple blocks connected through springs. Each block is connected to an external oscillating surface through a leaf spring; some blocks slide with asymmetric friction, while others experience conventional symmetric friction. Asymmetric friction favours slip in the direction of low friction. This increases spring forces to counteract the slip within the assembly, creating regions of tension and compression. When this model represents geological faults, regions of tension and compression can produce fractures oriented normal (in the tensile phase) or parallel (in the compressive phase) to the sliding surface.

Velocity spectra produced by the model excited by an external oscillating surface reveal that the presence of asymmetric friction creates spectra with a frequency falloff of 1/f², where f is the frequency. This is in contrast with the case of only symmetric friction blocks where oscillations result in velocity spectra with a frequency falloff of 1/f.

Recent triaxial compression of rock samples have shown slip events over shear fracture produce velocity spectra with frequency falloff that approximates 1/f²(Beeler et al., 2020). Using the results found from the model, a hypothesis on the mechanism that produces the 1/f²falloff is proposed: the shear fracture in the compression test is produced by formation of vertical micro-cracks within the rock samples. This effectively creates an anisotropic material with axes of symmetry inclined to the shear fracture, which explains the 1/f²spectra.

Acknowledgement. EP and AVD acknowledge support from the Australian Research Council through project DP210102224.

References

BAFEKRPOUR, E., DYSKIN, A., PASTERNAK, E., MOLOTNIKOV, A. & ESTRIN, Y. 2015. Internally architectured materials with directionally asymmetric friction. Scientific reports, 5, 10732-10732.

BEELER, N. M., MCLASKEY, G. C., LOCKNER, D. & KILGORE, B. 2020. Near‐Fault Velocity Spectra From Laboratory Failures and Their Relation to Natural Ground Motion. Journal of geophysical research. Solid earth, 125, n/a.

How to cite: Wong, R. X., Pasternak, E., and Dyskin, A.: The abnormal 1/f2 spectral falloff caused by asymmetric friction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1586, https://doi.org/10.5194/egusphere-egu22-1586, 2022.

Growth of real fractures is characterised by interruptions and local overlapping caused by rock heterogeneities. The interruptions and loci of overlapping can work as bridges distributed all over the fracture that connect opposite surfaces of the fracture [1]. These bridges constrain the crack opening and thus mitigate the dependence of the Mode I stress intensity factor of the fracture length (radius). In the case of hydraulic fractures, constraining the opening also affects the fluid (e.g. fracturing fluid) flow through the fracture.

The effect of bridges can be characterised by a characteristic length – the constriction length. When the fracture size is small compared to the constriction length, the fracture behaves similarly to a conventional Model I crack of the same configuration. Alternatively, when the fracture size is much greater than the constriction length both the  average fracture opening and the Mode I stress intensity factor become constant. This restricts the ability of the fracture to growth in unstable manner or dynamically.

The effect of constriction is even more pronounced in the case when the fracture gets open in the displacement-controlled mode. In this case the dependence of the stress intensity factor of disc-like fracture on the fracture radius is no longer monotonic. The stage of decrease of the stress intensity factor with the fracture radius leads to the emergence of stable fracture growth when increase in the displacement is required to maintain fracture propagation. It is important that without taking the constriction into account the corresponding stage of stable fracture propagation can be taken for the effect of rock heterogeneity.

The theory developed is essential for predicting and monitoring growth of Mode I fractures, in particular hydraulic fractures.

• He, J., Pasternak, E. and A.V. Dyskin, 2020. Bridges outside fracture process zone: Their existence and effect. Engineering Fracture Mechanics, 225, 106453.

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

How to cite: Pasternak, E. and Dyskin, A.: Growth of fractures with constricted opening, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4550, https://doi.org/10.5194/egusphere-egu22-4550, 2022.

Stress field in geomaterials is not uniform; in particular due to the presence of spatial fluctuations. These are induced under external loading due to multiple randomly located heterogeneities and small-scale defects/fractures or by fluid transport. Stress fluctuations can be a form of residual stresses caused for instance by phase transformation (e.g. magma solidification), or as a result of strong seismic events. Subsequently, the stress field can be represented as a superposition of slowly changing large scale stress and small scale self-equilibrating spatial stress fluctuations (random stress field with zero average).

At the first glance, the field of spatial stress fluctuation taking alone is not supposed to cause any large scale development of fractures considered as linear objects (fracture opening linearly depending upon the applied load) since the average stress is zero. However, often the fractures are not independent of the stress field but produced in the zone where the stress fluctuations exhibit high tensile stress. The fracture initial size is of the order of the characteristic size of the zone of high tensile stress, that is the correlation length of the random field of stress fluctuations. The fracture will then be able to propagate further. The simplest model of such a fracture is a disc-like crack opened in the centre by a pair of concentrated forces with magnitude equal to the total force of the tensile stress in that zone [1].

Model [1] predicts the extensive fracture growth to be stable that is further increase in its size would require increase in the concentrated forces that is increase in the amplitude of stress fluctuations. In the case when the stress fluctuations represent residual stress, it is not possible as the residual stress can only decrease [2]. Yet, self-equilibrating residual stresses can cause macroscopic failure. This requires a new paradigm of crack/fracture growth in self-equilibrating field of stress fluctuations. To this end we accept that (1) the fracture opening is bilinear such that the local compressive stress just closes the fracture and hence cannot equilibrate the corresponding local tensile stress; (2) the fracture growth is not planar as passing through zones of compression is not possible leading to local overlapping.

The simplest model that accommodates the above features of fracture growth is a fracture with distributed bridges (fracture with constraint opening represented as a crack with Winkler layer [4]). We show that such a fracture will exhibit unstable growth forming a mechanism of both breakage due to residual stress and large (e.g. geological) scale fracture formation.

• Dyskin, A.V. 1999. On the role of stress fluctuations in brittle fracture. J. Fracture, 100, 29-53.
• Dyskin V. and E. Pasternak, 2019. Residual strain mechanism of aftershocks and exponents of modified Omori’s law. J. Geophys. Research: Solid Earth, 10.1029/2018JB016148.
• He, J., E. Pasternak and A.V. Dyskin, 2020. Bridges outside fracture process zone: Their existence and effect. Engineering Fracture Mechanics, 225, 106453.

Acknowledgement. The authors acknowledge support from the Australian Research Council through project DP210102224.

How to cite: Dyskin, A. and Pasternak, E.: Fractures in geomaterials driven by spatial stress fluctuations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4482, https://doi.org/10.5194/egusphere-egu22-4482, 2022.

A meter-level direct shear test system and a true triaxial test system were designed by placing the traditional test apparatus into sealed cabins subjected to high water pressure. The influences of three-dimensional seepage water pressure on the shear and compression deformation of rock mass in Xiluodu Hydropower Station were studied. The test results showed that the changes in water pressure caused obvious shear deformation of the interlayer dislocation zone and tensile deformation and reduction in the triaxial compression strength of the fractured rock mass. The effect of water pressure on shear displacement and tensile displacement had a hysteresis effect. This was consistent with deformation data collected through field monitoring. The deformation mechanism in the reservoir valley was the coupling of the stress and seepage fields caused by reservoir impoundment. The effective stress was reduced, the mechanical parameters were weakened, and the change of the initial stress field led to the slightly overall shear slip and tensile deformation of the bank slope.

How to cite: Ban, Y., Xie, Q., Fu, X., and Wu, A.: A Hydro-Mechanical Coupling Test System for Simulating Rock Masses in High Dam Reservoir Operations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2006, https://doi.org/10.5194/egusphere-egu22-2006, 2022.