The Impacts of Increased Ductility in Organic-Rich Shale on Fracture Network Growth by Hydraulic Fracturing

2021 ◽  
Author(s):  
Chang Huang ◽  
Shengli Chen
Keyword(s):  
Tensile Strength ◽  
Hydraulic Fracturing ◽  
Hydraulic Fracture ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Fracture Network ◽  
Cohesive Crack ◽  
Hydraulic Fractures ◽  
Zone Model ◽  
Fully Coupled

Abstract The difficulty of hydraulic fracturing in organic-rich shale caused by the increased ductility has not been well interpreted quantitatively, although it is well perceived that the increased shale ductility can impede the propagation of hydraulic fractures and enhance the healing of created fractures upon injection shutdown. This study aims to quantitatively study the impacts of increased ductility on the stimulated reservoir volume (SRV) using an advanced XFEM-based simulator. To achieve this goal, a modified cohesive zone model has been integrated into an in-house fully coupled poroelastic XFEM framework. The study continues by extending the functionality of the numerical framework to simulating multiple interacting fractures. The utilization of the object-oriented programming paradigm in the development of the framework makes it an easy extension to include the multi-fracture network by creating more instances of crack segments. A main hydraulic fracture with an arbitrary number of intersected branches can thus be modeled. A series of parametric studies will be conducted to investigate the impacts of increased ductility on the induced SRV by varying four involved material parameters individually. The modified cohesive zone model, which is essentially a traction-separation law (TSL), is characterized by four parameters: the initial tensile strength Tini, ultimate tensile strength Tkrg, the critical separation Dc, and the final crack separation Dmax. It can flexibly model different crack opening scenarios and simulate more realistically the increased shale ductility. The fully coupled poroelastic XFEM framework has been comprehensively verified against the latest semi-analytical solutions on the four well-known propagation regimes. The numerical results show that the shape of TSL does affect the main hydraulic fracture growth as well as the evolvement of the fracture network, given the same cohesive crack energy and tensile strength. It infers that ductility is not only controlled by cohesive crack energy and tensile strength, which further indicates the necessity of the newly proposed cohesive zone model. The magnitude of the initial tensile strength, controlling when the cohesive crack starts propagating, is found to have the greatest impacts on the fracture length, and SRV, among all four TSL parameters. The novelty of this study is two-fold. First, the newly modified cohesive zone model can more realistically represent the increased shale ductility. Second, the advanced XFEM framework that enables the simulation of a fracture network can study the impacts of increased ductility on the whole SRV but not a single crack.

scholarly journals Numerical Investigation of Hydraulic Fracture Propagation Based on Cohesive Zone Model in Naturally Fractured Formations

Processes ◽  
2019 ◽  
Vol 7 (1) ◽  
pp. 28 ◽  
Author(s):  
Jianxiong Li ◽  
Shiming Dong ◽  
Wen Hua ◽  
Xiaolong Li ◽  
Xin Pan
Keyword(s):  
Hydraulic Fracturing ◽  
Hydraulic Fracture ◽  
Cohesive Zone ◽  
Coupled Model ◽  
Fracture Propagation ◽  
Fracture Network ◽  
Zone Model ◽  
Naturally Fractured ◽  
Hydraulic Fracture Propagation ◽  
Multiple Cluster

Complex propagation patterns of hydraulic fractures often play important roles in naturally fractured formations due to complex mechanisms. Therefore, understanding propagation patterns and the geometry of fractures is essential for hydraulic fracturing design. In this work, a seepage–stress–damage coupled model based on the finite pore pressure cohesive zone (PPCZ) method was developed to investigate hydraulic fracture propagation behavior in a naturally fractured reservoir. Compared with the traditional finite element method, the coupled model with global insertion cohesive elements realizes arbitrary propagation of fluid-driven fractures. Numerical simulations of multiple-cluster hydraulic fracturing were carried out to investigate the sensitivities of a multitude of parameters. The results reveal that stress interference from multiple-clusters is responsible for serious suppression and diversion of the fracture network. A lower stress difference benefits the fracture network and helps open natural fractures. By comparing the mechanism of fluid injection, the maximal fracture network can be achieved with various injection rates and viscosities at different fracturing stages. Cluster parameters, including the number of clusters and their spacing, were optimal, satisfying the requirement of creating a large fracture network. These results offer new insights into the propagation pattern of fluid driven fractures and should act as a guide for multiple-cluster hydraulic fracturing, which can help increase the hydraulic fracture volume in naturally fractured reservoirs.

A Cohesive-Zone Model for Simulating Hydraulic-Fracture Evolution within a Fully Coupled Flow/Geomechanics-Simulation System

SPE Journal ◽  
2020 ◽  
pp. 1-22
Author(s):  
Faruk O. Alpak
Keyword(s):  
Hydraulic Fracture ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Real Life ◽  
Simulation System ◽  
Zone Model ◽  
Fracture Evolution ◽  
Propagation Criterion ◽  
Fully Coupled ◽  
Fracture Growth

Summary A modular multiphysics reservoir-simulation system is developed that has the capability of simulating multiphase/multicomponent/thermal flow, poro-elasto/plastic geomechanics, and hydraulic-fracture evolution. The focus of the work is on the full-physics hydraulic-fracture-evolution-simulation capability of the multiphysics simulation system. Fracture-growth computations use a cohesive-zone model as part of the computation of fracture-propagation criterion. The cohesive-zone concept is developed using energy-release rates and cohesive stresses. They capture the strain-softening behavior of deforming porous material consistent with real-life observations of poro-plastic deformation. Thus, they can be reliably used within both poro-elastic and poro-plastic geomechanics applications, unlike the conventional stress-intensity-factor-based fracture-propagation criterion. The partial-differential equations (PDEs) that govern the Darcy-scale multiphase/multicomponent/thermal flow, poro-elasto/plastic geomechanics, hydraulic-fracture evolution, and laminar channel flow in the fracture are tightly coupled to each other to give rise to a numerical protocol solvable by the fully implicit method. The ensuing nonlinear system of equations is solved by use of a novel adaptively damped Newton-Raphson method. Example fully coupled single-phase isothermal-flow, geomechanics, and hydraulic-fracture-growth simulations are analyzed to demonstrate the predictive power of the simulation system. Numerical-model predictions of fracture length/radius and width are validated against analytical solutions for plane-strain and ellipsoid-shaped fractures, respectively. Results indicate that the simulation system is capable of modeling hydraulic-fracture evolution accurately by use of the cohesive-zone model as the propagation criterion. We also simulate and explore the sensitivities around a real-life hydraulic-fracture-growth problem by fully accounting for the thermal-, multiphase-, and compositional-flow effects.

The quantitative analysis of tensile strength of additively manufactured continuous carbon fiber reinforced polylactic acid (PLA)

2019 ◽  
Vol 25 (10) ◽  
pp. 1624-1636 ◽  
Author(s):  
Hongbin Li ◽  
Taiyong Wang ◽  
Sanjay Joshi ◽  
Zhiqiang Yu
Keyword(s):  
Tensile Strength ◽  
Carbon Fiber ◽  
Fracture Mechanism ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Zone Model ◽  
Fiber Reinforced ◽  
Continuous Fiber ◽  
Content Type ◽  
Continuous Carbon Fiber

Purpose Continuous fiber-reinforced thermoplastic composites are being widely used in industry, but the fundamental understanding of their properties is still limited. The purpose of this paper is to quantitatively study the effects of carbon fiber content on the tensile strength of continuous carbon fiber-reinforced polylactic acid (CCFRPLA) fabricated through additive manufacturing using the fused deposition modeling (FDM) process. Design/methodology/approach The strength of these materials is highly dependent on the interface that forms between the continuous fiber and the plastic. A cohesive zone model is proposed as a theoretical means to understand the effect of carbon fiber on the tensile strength properties of CCFRPLA. The interface formation mechanism is explored, and the single fiber pulling-out experiment is implemented to investigate the interface properties of CCFRPLA. The fracture mechanism is also explored by using the cohesive zone model. Findings The interface between carbon fiber and PLA plays the main role in transferring external load to other fibers within CCFRPLA. The proposed model established in this paper quantitatively reveals the effects of continuous carbon fiber on the mechanical properties of CCFRPLA. The experimental results using additively manufacturing CCFRPLA provide validation and explanation of the observations based on the quantitative model that is established based on the micro-interface mechanics. Research limitations/implications The predict model is established imagining that all the fibers and PLA form a perfect interface. While in a practical situation, only the peripheral carbon fibers of the carbon fiber bundle can fully infiltrate with PLA and form a transmission interface. These internal fibers that cannot contract with PLA fully, because of the limit space of the nozzle, will not form an effective interface. Originality/value This paper theoretically reveals the fracture mechanism of CCFRPLA and provides a prediction model to estimate the tensile strength of CCFRPLA with different carbon fiber contents.

scholarly journals Effect of Joint Geometrical Parameters on Hydraulic Fracture Network Propagation in Naturally Jointed Shale Reservoirs

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-23 ◽  
Author(s):  
Zhaohui Chong ◽  
Qiangling Yao ◽  
Xuehua Li
Keyword(s):  
Hydraulic Fracturing ◽  
Hydraulic Fracture ◽  
Fracture Network ◽  
Hydraulic Fractures ◽  
Geometrical Parameters ◽  
Breakdown Pressure ◽  
Opening Mode ◽  
Hydraulic Fracture Network ◽  
Shale Reservoirs ◽  
Network Pattern

The presence of a significant amount of discontinuous joints results in the inhomogeneous nature of the shale reservoirs. The geometrical parameters of these joints exert effects on the propagation of a hydraulic fracture network in the hydraulic fracturing process. Therefore, mechanisms of fluid injection-induced fracture initiation and propagation in jointed reservoirs should be well understood to unleash the full potential of hydraulic fracturing. In this paper, a coupled hydromechanical model based on the discrete element method is developed to explore the effect of the geometrical parameters of the joints on the breakdown pressure, the number and proportion of hydraulic fractures, and the hydraulic fracture network pattern generated in shale reservoirs. The microparameters of the matrix and joint used in the shale reservoir model are calibrated through the physical experiment. The hydraulic parameters used in the model are validated through comparing the breakdown pressure derived from numerical modeling against that calculated from the theoretical equation. Sensitivity analysis is performed on the geometrical parameters of the joints. Results demonstrate that the HFN pattern resulting from hydraulic fracturing can be roughly divided into four types, i.e., crossing mode, tip-to-tip mode, step path mode, and opening mode. As β (joint orientation with respect to horizontal principal stress in plane) increases from 0° to 15° or 30°, the hydraulic fracture network pattern changes from tip-to-tip mode to crossing mode, followed by a gradual decrease in the breakdown pressure and the number of cracks. In this case, the hydraulic fracture network pattern is controlled by both γ (joint step angle) and β. When β is 45° or 60°, the crossing mode gains dominance, and the breakdown pressure and the number of cracks reach the lowest level. In this case, the HFN pattern is essentially dependent on β and d (joint spacing). As β reaches 75° or 90°, the step path mode is ubiquitous in all shale reservoirs, and the breakdown pressure and the number of the cracks both increase. In this case, β has a direct effect on the HFN pattern. In shale reservoirs with the same β, either decrease in k (joint persistency) and e (joint aperture) or increase in d leads to the increase in the breakdown pressure and the number of cracks. It is also found that changes in d and e result in the variation in the proportion of different types of hydraulic fractures. The opening mode of the hydraulic fracture network pattern is observed when e increases to 1.2 × 10−2 m.

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