Evidence of shear and tensile fracturing in the western desert earthquakes, Egypt

Anisotropic flow characteristics in single-fractured tensile and shear fractures in granite subjected to various confining stresses were investigated by experiments and experimental results-based flow simulations. Anisotropic characteristics in aspects such as the permeability reduction rate, stress dependency of permeability, and flow paths in fracture and fracture aperture distribution were analysed and compared between tensile and shear fractures. The main conclusions are as follows: (1) for tensile fractures, the permeability in the X direction, k X , is slightly lower than the permeability in the Y direction, k Y , and the average permeability anisotropic coefficient, α k ( α k = k X / k Y ), is in the range 0.236–0.779. For shear fractures, k X is obviously lower than k Y , and α k is in the range 0.038–0.163; therefore, the permeability anisotropy in shear fractures is stronger than in tensile fractures. (2) Strong channelling effects exist in both shear and tensile fractures. The stress-dependence coefficient of permeability is 0.731 MPa −1 in the X direction and 0.365 MPa −1 in the Y direction for tensile fractures, and 0.034 and 0.010 MPa −1 , respectively, for shear fractures, indicating that the dependency of permeability on stress also shows anisotropy. (3) An empirical model for α k based on stress and fracture aperture variograms is proposed. Based on α k , permeability along both the X and Y directions can be well predicted, especially under relatively high confining stress.

Quartz veins at the Mufferaw gold prospect and Komis gold deposit, northern Saskatchewan, are examples of oblique-extension veins that crystallized in shear and tensile fractures, which propagated and then opened oblique to their walls during a single continuous deformation event. The Mufferaw gold prospect consists of an interlinked mesh of conjugate shear fractures, tensile fractures, and veins. Oblique-extension veins crystallized in conjugate shear fractures, which opened during dilation of intersecting tensile fractures under the same far-field stress system. At the Komis gold mine, tensile and dextral shear fractures cut across granodiorite dikes. The tensile fractures formed perpendicular to the dike margins during stress transfer from the less-competent country rocks to the more-competent dikes. During extension of the dikes oblique to the bulk extension direction in the country rocks, the tensile fractures were reactivated as sinistral shear fractures. The reactivated tensile fractures and dextral shear fractures opened oblique to their walls, parallel to the bulk extension direction, and were sealed by the deposition of oblique-extension veins during the same deformation event.

Geothermal extraction usually requires reservoir fracturing to realize economic exploitation and fluid flow in hot-dry rock fractures has become a research hotspot. In the present study, gas flow through shear and tensile granite fractures was investigated to comparatively study effect of fracture mesoscopic structure evolution on fracture hydro-mechanical characteristics under confining stress ranging from 1 to 40 MPa and inlet pressure ranging from 0.005 to 3.6 MPa. Four major findings are reported herein. First, the conductivity of shear fractures is much higher than that of tensile fractures and the ratio of the former to the latter can be up to around 265 under the stress of 40 MPa. The second finding was that the Z2 values of tensile fractures are higher than shear fractures, indicating that the fracture surface of tensile fractures is rougher and has more mesoscopic asperities. When the two surfaces get to contact, tip contact and stress concentration happen, resulting in higher deformation in tensile fractures. The cluster coefficient characterizes fracture contact cluster property. Higher cluster coefficient indicates lower fracture deformation and lower viscous loss. Shear fractures have higher cluster coefficient than that of tensile fractures so shear fractures are weakly deformable. However, high flow velocity and Reynolds number easily occur in shear fractures, so transitional and turbulent flow dominates in shear fractures while laminar and transitional flow in tensile fractures. Third, a mechanical model relates the average imbedded depth to loading stress and lacunarity, a hydraulic aperture model based on void space and lacunarity and a friction factor model based on Reynolds number and lacunarity were built, respectively, based on the experimental data. Then a nonlinear flow model was proposed. The fourth finding was that parametric analysis on the cluster coefficient shows that the higher the cluster coefficient, the lower of fracture deformation is and the higher of hydraulic aperture is. When the cluster coefficient is high, less viscous loss occurs; however, the fluid velocity may be high, so the friction factor will be high since more inertial loss happens.

Compression‐induced tensile and shear fractures were reported to be the two fundamental fracture types in rock fracturing tests. This study investigates such tensile and shear fracturing process in marble specimens containing two different flaw configurations. Observations first reveal that the development of a tensile fracture is distinct from shear fracture with respect to their nucleation, propagation, and eventual formation in macroscale. Second, transgranular cracks and grain‐scale spallings become increasingly abundant in shear fractures as loading increases, which is almost not observed in tensile fractures. Third, one or some dominant extensional microcracks are commonly observed in the center of tensile fractures, while such development of microcracks is almost absent in shear fractures. Microcracks are generally of a length comparable to grain size and distribute uniformly within the damage zone of the shear fracture. Fourth, the width of densely damaged zone in the shear fracture is nearly 10 times of that in the tensile fracture. Quantitative measurement on microcrack density suggests that (1) microcrack density in tensile and shear fractures display distinct characteristics with increasing loading, (2) transgranular crack density in the shear fracture decreases logarithmically with the distance away from the shear fracture center, and (3) whatever the fracture type, the anisotropy can only be observed for transgranular cracks with a large density, which partially explains why microcrack anisotropy usually tends to be unobvious until approaching peak stress in specimens undergoing brittle failure. Microcracking characteristics observed in this work likely shed light to some phenomena and conclusions generalized in seismological studies.

PurposeHydrofracturing technology has been widely used in tight oil and gas reservoir exploitation, and the fracture network formed by fracturing is crucial to determining the resources recovery rate. Due to the complexity of fracture network induced by the random morphology and type of fluid-driven fractures, controlling and optimising its mechanisms is challenging. This paper aims to study the types of multiscale mode I/II fractures, the fluid-driven propagation of multiscale tensile and shear fractures need to be studied.Design/methodology/approachA dual bilinear cohesive zone model (CZM) based on energy evolution was introduced to detect the initiation and propagation of fluid-driven tensile and shear fractures. The model overcomes the limitations of classical linear fracture mechanics, such as the stress singularity at the fracture tip, and considers the important role of fracture surface behaviour in the shear activation. The bilinear cohesive criterion based on the energy evolution criterion can reflect the formation mechanism of complex fracture networks objectively and accurately. Considering the hydro-mechanical (HM) coupling and leak-off effects, the combined finite element-discrete element-finite volume approach was introduced and implemented successfully, and the results showed that the models considering HM coupling and leak-off effects could form a more complex fracture network. The multiscale (laboratory- and engineering-scale) Mode I/II fractures can be simulated in hydrofracturing process.FindingsBased on the proposed method, the accuracy and applicability of the algorithm were verified by comparing the analytical solution of KGD and PKN models. The effects of different in situ stresses and flow rates on the dynamic propagation of hydraulic fractures at laboratory and engineering scales were investigated. when the ratio of in situ stress is small, the fracture propagation direction is not affected, and the fracture morphology is a cross-type fracture. When the ratio of in situ stress is relatively large, the propagation direction of the fracture is affected by the maximum in situ stress, and it is more inclined to propagate along the direction of the maximum in situ stress, forming double wing-type fractures. Hydrofracturing tensile and shear fractures were identified, and the distribution and number of each type were obtained. There are fewer hydraulic shear fractures than tensile fractures, and shear fractures appear in the initial stage of fracture propagation and then propagate and distribute around the perforation.Originality/valueThe proposed dual bilinear CZM is effective for simulating the types of Mode I/II fractures and seizing the fluid-driven propagation of multiscale tensile and shear fractures. Practical fracturing process involves the multi-type and multiscale fluid-driven fracture propagation. This study introduces general fluid-driven fracture propagation, which can be extended to the fracture propagation analysis of potential fluid fracturing, such as other liquids or supercritical gases.

Rock fracture surfaces are normally rough, with upper and lower surfaces presenting obvious structural properties (i.e., matching or mismatching). In this paper, rock fracture was divided into shear and tensile fractures based on its mechanical origin. A test simulation system for grouting in rough fracture (tensile and shear fractures) with continuous pressure field monitoring was proposed. A series of grouting simulation tests were employed to evaluate the effect of fracture structural property, fracture surface roughness, and aperture on the characteristics of grout flow in the rough fracture. The results show that grouting pressure exhibited a trend of rapid increase followed by a slow increase with time during the grouting in the tensile and shear fractures. Grouting pressure increases with time in a smooth relationship for tensile fracture and a fluctuating upward relationship for shear fracture. The slurry pressure distribution along the penetration length exhibited a nonlinear attenuation trend. The attenuation degree of the slurry pressure was positively related to the fracture roughness and the roughness difference in the upper–lower fracture surface and negative with the fracture aperture. The potential filtration effect during the grouting process is an important factor affecting the grouting behavior in the rough fracture.

ABSTRACT: We simulated the tensile fracturing process, capturing both microseismic events (acoustic emissions) and the dynamics of fracture propagation through high-speed photography. To record the millisecond-level fracture propagation, a synchronization circuit was employed, designed to align the 10 MHz acoustic emission recordings with high-speed images taken at a rate of 200,000 frames per second. This setup enabled us to precisely record fracture propagation within a 1.5-microsecond period. Our findings show that the relocated acoustic emission events align closely with the locations of macroscopic tensile fractures. We observed a microsecond-level time delay between the fracture propagation process and the clustering of acoustic emissions. This delay indicates that the clustering of acoustic emissions is related to the fracture nucleation process, while the high-speed photography records the coalescence and growth of fractures, reflecting the development of macroscopic cracks. Considering the temporal and spatial alignment between the acoustic emissions and the propagation of tensile fractures, it is evident that microseismic monitoring primarily captures the early stages of crack formation. Our research indicates that current microseismic monitoring results lack interpretation of the relationship between the nucleation stage and macroscopic fractures, as well as the relationship between microseismic clouds and fracture networks. 1. INTRODUCTION Microseismic monitoring represents a field technique in observing hydraulic fracturing cracks(Baig and Urbancic, 2010; Cipolla et al., 2012). While it is understood that hydraulic fracturing generates a mix of tensile and shear fractures(Busetti et al., 2014; Fischer and Guest, 2011; Wu et al., 2019), tensile fractures play a more significant role due to their greater contribution to permeability enhancement. This fact highlights the need to better understand tensile fractures. However, most established theories on microseismic inversion are based on traditional seismology(Eisner et al., 2011; Vavryčuk et al., 2008), which predominantly concentrates on shear fractures or slips. Such a concentration restricts the application of microseismic data in interpreting hydraulic fractures. Key challenges include determining whether microseismic events originate from hydraulic tensile fractures(Maxwell, 2011; Warpinski et al., 2013), understanding the relationship between the spatial extent of microseismic clouds and hydraulic fracture zones(Mayerhofer et al., 2010; McKean et al., 2019), and correlating microseismic event magnitudes with the geometric parameters of fractures(Wu et al., 2019).

Tensile and shear fracturing in predominantly compressive stress fields—a review

This paper describes a numerical investigation of hydraulic fracturing in oil sands during cold water injection. Previous studies have shown that hydraulic fracturing in unconsolidated or weakly consolidated sandstone reservoirs is highly influenced by the low shear strength of these materials and is quite different from competent rocks. As such, existing classical hydraulic fracture models are incapable of predicting the fracturing process of weak sandstone reservoirs. This paper presents a numerical tool to simulate hydraulic fracturing in oil sands and weak sandstone reservoirs. A smeared fracture approach is adopted in the simulation of tensile and shear fracturing in oil sands. The model incorporates various phenomena expected in hydraulic fracturing, including poroelasticity and plasticity, matrix flow, shear and tensile fracturing and concomitant permeability enhancement, saturation-dependent permeability, stress dependent stiffness and gradual degradation of oil sands due to dilatant shear deformation and strain localization. The results of the hydraulic fracturing simulation indicate that poroelasticity as well as shear fracturing can result in breakdown and propagation pressures larger than the maximum in-situ stress. Applying such pressures in fracturing operations can compromise the caprock integrity. It is found that at injection pressures below the vertical stress, saturation-dependent relative permeability and the development of shear fractures in the reservoir highly influence the injection response.

Most carbonate reservoirs behave as dual porosity-permeability systems in which the rock matrix and both natural and created hydraulic fractures contribute to the hydrocarbon transport in a very complex manner. Understanding the behavior of the permeability of the matrix frame, natural fractures, and created hydraulic fractures, as a function of reservoir depletion, is vital to designing optimum stimulation treatments and to maximize the carbonate formation's exploitation. Core samples were selected from a carbonate reservoir and a testing procedure was applied to determine the stress dependant permeability as a function of various combinations of effective stresses. A tensile natural fracture was simulated by splitting a whole core by failing it under tension using a Brazilian test procedure. The stress dependant permeability was evaluated under varied effective stresses simulating a reservoir depletion scenario. A shear fractured core was selected from a given carbonate formation and a stress dependant permeability was established. The tensile fractured core was then propped with a low concentration of small mesh proppants and the permeability of the simulated propped fracture was determined. Using a new reservoir simulator the testing results and selective functions were used to predict the production performance of a carbonate reservoir under the effect of the stress dependant permeability. The experimental results indicate that the tensile fractures are much less conductive than shear fractures and the shear fractures are less conductive than propped fractures. The concept of effective stress within the rock matrix is totally different than that of natural fractures; therefore, the effective stress function for both matrix and natural fractures should be separately evaluated to obtain representative functions for any simulation study. The tensile fractures lose conductivity at very early stages of reservoir depletion. Recommendations to manage these tensile fractures for optimum hydrocarbon recovery are suggested. Practical outputs of this work are: 1) Understand how natural fractures are controlled to efficiently contribute to well productivity, 2) Quantify the effective stress concept in the matrix and fracture systems, 3) Provide stress-dependant correlations for simulation studies.

The second member of the Dengying formation (DYF M2) in the Sichuan Basin holds substantial potential for exploration and development, with fractures serving as a pivotal factor for the efficient extraction of natural gas resources within this formation. Nonetheless, there exists a notable gap in systematic research concerning the fracture formation mechanisms within the reservoir rocks of this specific geological layer, and an adequate assessment of fractures at the core scale remains elusive. To address this, our study conducted triaxial compression tests and fracture toughness experiments on the reservoir rocks, focusing on the analysis of shear and tensile fracture characteristics and mechanisms across porosity-dominated, vuggy, and fracture-dominated core types. The findings reveal that, in porosity-dominated cores, shear fractures exhibit oblique, smooth, and continuous traits, often characterized by large angles, whereas tensile fractures initiate at the contact surface and midsection, with vertical extension observed in central fractures. Conversely, in vuggy cores, shear fractures tend to deflect, connect, or bypass dissolved pores, resulting in complex trajectories for tensile fractures influenced by these pores. In the case of fracture-dominated cores, the intersection patterns between shear fractures and natural fractures display a diverse range, with tensile fractures initiating near natural fractures and expanding under their influence. Further analysis reveals that in porosity-dominated cores, the formation of shear fractures is influenced by both the internal structure and triaxial stress, with tensile fractures initiating in areas of stress concentration. In vuggy cores, the presence of dissolved pores plays a dominant role in the formation of both shear and tensile fractures, altering the mechanical properties and stress distribution, which subsequently impacts fracture development. Lastly, in fracture-dominated cores, the formation of both types of fractures is controlled by natural fractures, which modify the stress field and influence the fracture trajectories. These insights provide a comprehensive understanding of fracture formation mechanisms in DYF M2, guiding future exploration and development efforts in the Sichuan Basin.

Response characteristics of acoustic emission signal and judgment criteria for different fracture modes

An analytical model for fracture initiation in elasto-plastic soft formations based on stress path analysis

Effects of the shoulder diameter and weld pitch on the tensile shear load in friction-stir welding of AA6111/AA5023 aluminum alloys

Chapter 22 - Failure of Snow Slopes