scholarly journals Mutual interference of layer plane and natural fracture in the failure behavior of shale and the mechanism investigation

2020 ◽  
Author(s):  
Peng Zhao ◽  
Ling-Zhi Xie ◽  
Zhi-Chao Fan ◽  
Lei Deng ◽  
Jun Liu
Keyword(s):  
Failure Mechanism ◽  
Failure Process ◽  
Fracture Pattern ◽  
Natural Fracture ◽  
Particle Flow Code ◽  
Particle Model ◽  
Failure Behavior ◽  
Significant Implication ◽  
Layer Plane ◽  
Natural Fractures

Abstract Shale contains a certain amount of natural fractures, which affects the mechanical properties of shale. In this paper, a bonded-particle model in particle flow code (PFC) is established to simulate the failure process of layered shale under Brazilian tests, under the complex relationship between layer plane and natural fracture. First, a shale model without natural fractures is verified against the experimental results. Then, a natural fracture is embedded in the shale model, where the outcomes indicate that the layer plane angle (marked as α) and the angle (marked as β) of embedded fracture prominently interfere the failure strength anisotropy and fracture pattern. Finally, sensitivity evaluations suggest that variable tensile/cohesion strength has a changeable influence on failure mechanism of shale, even for same α or/and β. To serve this work, the stimulated fractures are categorized into two patterns based on whether they relate to natural fracture or not. Meanwhile, four damage modes and the number of microcracks during the loading process are recognized quantitatively to study the mechanism of shale failure behavior. Considering the failure mechanism determines the outcome of hydraulic fracturing in shale, this work is supposed to provide a significant implication in theory for the engineering operation.

scholarly journals Failure Behavior of F Shape Non-Persistent Joint under Experimental and Numerical Uniaxial Compression Test

2021 ◽  
Author(s):  
vahab sarfarazi ◽  
kaveh asgari ◽  
meisam zarei
Keyword(s):  
Numerical Simulation ◽  
Compression Test ◽  
Experimental Testing ◽  
Failure Process ◽  
Particle Flow Code ◽  
Failure Behavior ◽  
Uniaxial Compression Test ◽  
Joint Angles ◽  
Large Joint ◽  
Load Rate

Abstract Experimental and discrete element approaches were used to examine the effects of F shape non-persistent joints on the failure behaviour of concrete under uniaxial compressive test. concrete specimens with dimensions of 200 mm×200 mm×50 mm were provided. Within the specimen, F shape non-persistent joint consisting three joints were provided. The large joint length was 6 cm, and the length of two small joints were 2cm. Vertical distance betwenn two small joints change from 1.5 cm to 4.5 cm with increment of 1.5 cm. In constant joint lengths, the angle of large joint change from 0 to 90 with increments of 30. Totally 12 different models were tested under compression test. The axial load rate on the model was 0.05 mm/min. Cuncurrent with experimental tests, numerical simulation (Particle flow code in two dimension) were performed on the models containing F shape non-persistent joint. Distance between small joints and joint angles were similar to experimental one. the results indicated that the failure process was mostly governed by both of the Distance between small joints and joint angles. The compressive strengths of the samples were related to the fracture pattern and failure mechanism of the discontinuities. Furthermore it was shown that the compressive behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the joint angle. In the first There were only a few AE hits in the initial stage of loading, then AE hits rapidly grow before the applied stress reached its peak. Furthermore, a large number of AE hits accompanied every stress drop. Finally, the failure pattern and failure strength are similar in both approaches i.e. the experimental testing and the numerical simulation approaches.

Numerical Modeling of Multistranded-Hydraulic-Fracture Propagation: Accounting for the Interaction Between Induced and Natural Fractures

SPE Journal ◽  
2011 ◽  
Vol 16 (03) ◽  
pp. 575-581 ◽  
Author(s):  
Arash Dahi-Taleghani ◽  
Jon E. Olson
Keyword(s):  
Hydraulic Fracture ◽  
Fracture Propagation ◽  
Fracture Pattern ◽  
Design Tool ◽  
Propagation Model ◽  
Natural Fracture ◽  
Shear Stresses ◽  
Natural Fractures ◽  
Complex Fracture ◽  
Standard Design

Summary Recent examples of hydraulic-fracture diagnostic data suggest that complex, multistranded hydraulic-fracture geometry is a common occurrence. This reality is in stark contrast to the industry-standard design models based on the assumption of symmetric, planar, biwing geometry. The interaction between pre-existing natural fractures and the advancing hydraulic fracture is a key condition leading to complex fracture patterns. Performing hydraulic-fracture-design calculations under these less-than-ideal conditions requires modeling fracture intersections and tracking fluid fronts in the network of reactivated fissures. Whether a hydraulic fracture crosses or is arrested by a pre-existing natural fracture is controlled by shear strength and potential slippage at the fracture intersections, as well as potential debonding of sealed cracks in the near-tip region of a propagating hydraulic fracture. We present a complex hydraulic-fracture pattern propagation model based on the extended finite-element method (XFEM) as a design tool that can be used to optimize treatment parameters under complex propagation conditions. Results demonstrate that fracture-pattern complexity is strongly controlled by the magnitude of anisotropy of in-situ stresses, rock toughness, and natural-fracture cement strength, as well as the orientation of the natural fractures relative to the hydraulic fracture. Analysis shows that the growing hydraulic fracture may exert enough tensile and shear stresses on cemented natural fractures that the latter may be debonded, opened, and/or sheared in advance of hydraulic-fracture-tip arrival, while under other conditions, natural fractures will be unaffected by the hydraulic fracture. Detailed aperture distributions at the intersection between fracture segments show the potential for difficulty in proppant transport under complex fracture-propagation conditions.

Hydraulic fracturing in an unconventional naturally fractured reservoir: a numerical and experimental study

2011 ◽  
Vol 51 (1) ◽  
pp. 507 ◽  
Author(s):  
Mohammad Sarmadivaleh ◽  
Vamegh Rasouli ◽  
Noufal Kakode Shihab
Keyword(s):  
Hydraulic Fracturing ◽  
Numerical Models ◽  
Interaction Mechanism ◽  
Triaxial Stress ◽  
Natural Fracture ◽  
Particle Flow Code ◽  
Filling Material ◽  
Fractured Reservoir ◽  
Natural Fractures ◽  
True Triaxial

Natural fractures play a vital role in the production of low permeability reservoirs when no stimulation techniques are used. The characteristics of natural fractures, together with their pattern that defines how they communicate with each other and to the wellbore, will govern how effectively they can contribute in production enhancement. In most occasions, however, hydraulic fracturing must be used as a remedy to have an economical production rate. Fraccing itself is a complicated process, but would be further complicated when it is practiced in a discontinuous medium. Depending on the properties of the natural fracture(s) and operational condition of the fraccing job, opening, offsetting, crossing or arresting are possible interactions that may happen when an induced fracture reaches a natural discontinuity. In this study, the simplest interaction case with an angle of approach of 90° was studied through both laboratory experiments and numerical modelling. The experiments were carried out under real-triaxial stress conditions using a true-triaxial stress cell (TTSC). Two cement blocks of 20 cm with artificially-made natural fractures were used in this study. The cuts in one sample were filled with weak glue, whereas stiff cement was used in the second sample. The results indicate the importance of interface filling material properties in dominating the interaction mechanism. The numerical models built to simulate these two lab scenarios used particle flow code 2D (PFC2D). The model was tuned and validated against the experimental observations and a good agreement observed between the results of the two approaches.

scholarly journals Investigating the Failure Mechanism of Jointed Rock Slopes Based on Discrete Element Method

2020 ◽  
Vol 2020 ◽  
pp. 1-19
Author(s):  
Chao Peng ◽  
Qifeng Guo ◽  
Zhenxiong Yan ◽  
Minglong Wang ◽  
Jiliang Pan
Keyword(s):  
Rock Mass ◽  
Failure Mechanism ◽  
Failure Process ◽  
Structural Complexity ◽  
Jointed Rock ◽  
Research Area ◽  
Open Pit ◽  
Particle Model ◽  
Rock Slopes ◽  
Synthetic Rock

This paper presents a comprehensive engineering method to investigate the failure mechanism of the jointed rock slopes. The field geology survey is first carried out to obtain the slope joint data. A joint network model considering the structural complexity of rock mass is generated in the PFC software. The synthetic rock mass (SRM) approach for simulating the mechanical behavior of jointed rock mass is employed, in which the flat-jointed bonded-particle model (FJM) and smooth joint contact model (SJM) represent intact rock and joints, respectively. Subsequently, the effect of microparameters on macromechanical properties of rock is investigated for parameter calibration. Moreover, the scale effect is analyzed by multiscale numerical tests, and the representative elementary volume (REV) size in the selected research area is found as 16 m × 16  m × 16 m. The microparameters of the SRM model are calibrated to match the mechanical properties of the engineering rock mass. Finally, an engineering case from Shuichang open-pit mine is analyzed and the failure process of the slope during the excavation process from micro- to macroscale is obtained. It has been found that failure occurs at the bottom of the slope and gradually develops upwards. The overall failure of the slope is dominated by the shallow local tension fracture and wedge failure.

Shear Failure Mechanism of Infilling Rock Joints and its PFC Simulation

2015 ◽  
Vol 723 ◽  
pp. 317-321 ◽  
Author(s):  
Lei Xu ◽  
Qing Wen Ren
Keyword(s):  
Failure Mechanism ◽  
Shear Test ◽  
Shear Failure ◽  
Failure Process ◽  
Rock Joint ◽  
Rock Joints ◽  
Particle Flow Code ◽  
Rock Masses ◽  
Natural Rock ◽  
Simulation Results

Infilling rock joints widely exist in natural rock masses, and the shear failure of infilling rock joints plays an important role in the instability of rock masses. In order to study the shear failure mechanism of infilling rock joints, Particle Flow Code is used to simulate the direct shear test of infilling rock joints. The PFC models with different infilling thickness are established firstly, and then the procedures of PFC simulation are described. In the end, the shear failure process of infilling rock joints with different infilling thickness is simulated. Based on the PFC simulation results, it can be concluded that the shear failure mode changes with increasing infilling thickness, and the shearing of the infilling rock joint rarely gives birth to microcracks in rock due to the existence of the infilling material.

How Natural Fractures Could Affect Hydraulic-Fracture Geometry

SPE Journal ◽  
2013 ◽  
Vol 19 (01) ◽  
pp. 161-171 ◽  
Author(s):  
Arash Dahi Taleghani ◽  
Jon E. Olson
Keyword(s):  
Compressive Stress ◽  
Hydraulic Fracture ◽  
Fracture Propagation ◽  
Fracture Pattern ◽  
Natural Fracture ◽  
Barnett Shale ◽  
Natural Fractures ◽  
Extended Finite Element ◽  
Fracture Geometry

Summary Hydraulic fracturing is recognized as the main stimulating technique to enhance recovery in tight fissured reservoirs. These fracturing treatments are often mapped by use of hypocenters of induced microseismic events. In some cases, the microseismic mapping shows asymmetry of the induced-fracture geometry with respect to the injection well. In addition, the conventional theories predict fracture propagation along a path normal to the least compressive in-situ stresses, whereas in some cases the microseismic data suggest fracture propagation parallel to the minimum compressive stress. In this paper, we present an extended-finite-element-method (XFEM) model that can simulate asymmetric fracture-wing development as well as diversion of the fracture path along natural fractures. Simulation results demonstrate the sensitivity of the fracture-pattern geometry to differential stress and natural-fracture orientation with respect to the in-situ maximum compressive stress. We examine the properties of sealed natural fractures that are common in formations such as the Barnett shale and show that they may still serve as weak paths for hydraulic-fracture beginning and/or diversion. The presented model predicts faster fracture propagation in formations where natural fractures are favorably aligned with the tectonic stresses.

scholarly journals Dynamic Fracture and Energy Evolution Characterization of Naturally Fractured Granite Subjected to Multilevel Cyclic Loads

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-16
Author(s):  
Ning Guo ◽  
Changhong Li ◽  
Hao Liu ◽  
Yu Wang
Keyword(s):  
Fractured Rock ◽  
Failure Process ◽  
Volume Growth ◽  
Fatigue Loading ◽  
Natural Fracture ◽  
Dissipated Energy ◽  
Energy Evolution ◽  
Natural Fractures ◽  
Naturally Fractured ◽  
Fractured Granite

Naturally fractured rock mass is susceptible to stress disturbance and could result in the stimulation of natural fractures and even serious geological hazards. In this work, multilevel uniaxial fatigue loading experiments were carried out to reveal the fracture and energy evolution of naturally fractured granite using stress-strain descriptions and energy evolution analysis. Results reveal the influence of natural fracture on mechanical properties of granite, regarding the fatigue lifetime, fatigue deformation characteristics, fatigue damage, energy evolution, and fatigue failure pattern. Volumetric and shear processes caused by the sliding and shearing along the natural fracture control the whole failure process. The energy dissipation and release characteristics are strongly impacted by natural fractures. The elastic energy and dissipated energy both decrease with increasing natural fracture volume, growth of the dissipated energy becomes faster for rock near to failure. It is proved that the dissipated energy is mainly used to activate the preexisting natural fractures.

scholarly journals A numerical investigation on the performance of hydraulic fracturing in naturally fractured gas reservoirs based on stimulated rock volume

2020 ◽  
Vol 10 (8) ◽  
pp. 3333-3345
Author(s):  
Ali Al-Rubaie ◽  
Hisham Khaled Ben Mahmud
Keyword(s):  
Hydraulic Fracturing ◽  
Hydraulic Fracture ◽  
Natural Fracture ◽  
Shear Stresses ◽  
Individual Element ◽  
Natural Fractures ◽  
Gas Reservoirs ◽  
Stimulated Reservoir Volume ◽  
Reservoir Volume ◽  
Naturally Fractured

Abstract All reservoirs are fractured to some degree. Depending on the density, dimension, orientation and the cementation of natural fractures and the location where the hydraulic fracturing is done, preexisting natural fractures can impact hydraulic fracture propagation and the associated flow capacity. Understanding the interactions between hydraulic fracture and natural fractures is crucial in estimating fracture complexity, stimulated reservoir volume, drained reservoir volume and completion efficiency. However, because of the presence of natural fractures with diffuse penetration and different orientations, the operation is complicated in naturally fractured gas reservoirs. For this purpose, two numerical methods are proposed for simulating the hydraulic fracture in a naturally fractured gas reservoir. However, what hydraulic fracture looks like in the subsurface, especially in unconventional reservoirs, remain elusive, and many times, field observations contradict our common beliefs. In this study, the hydraulic fracture model is considered in terms of the state of tensions, on the interaction between the hydraulic fracture and the natural fracture (45°), and the effect of length and height of hydraulic fracture developed and how to distribute induced stress around the well. In order to determine the direction in which the hydraulic fracture is formed strikethrough, the finite difference method and the individual element for numerical solution are used and simulated. The results indicate that the optimum hydraulic fracture time was when the hydraulic fracture is able to connect natural fractures with large streams and connected to the well, and there is a fundamental difference between the tensile and shear opening. The analysis indicates that the growing hydraulic fracture, the tensile and shear stresses applied to the natural fracture.

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