Papers by Keyword: Fracture Propagation

Abstract: Information of in situ stress field is crucial for predicting the response of rock masses to the disturbance associated with underground constructions. Hydraulic fracturing is an efficient method for determining the stress field and is suitable at the early stages of projects when no underground access exists. For this, a series of new improved techniques and equipments are developed for the deep boreholes to increase the reliability of system. For a better understanding of the stress test, the ideal HF pressure-time curve is given and the fracturing procedure is analyzed. Based on the theory of HF stress calculation, two implicit inequations are deduced.

Abstract: In this paper, experimental and finite element modeling methods were adopted to investigate the effects of microporosity on the tensile properties and fracture behavior of high-pressure die-casting (HPDC) AM50 alloy. By specimen-to-specimen fractographic analysis, the variability in tensile properties could be quantitatively correlated with the areal fraction of the porosity presented in the corresponding fracture surfaces by using a simple power law equation. Numerical models of synthetic microstructures with different pore sizes, areal fractions of pores and pore distributions were established. Based on the experimental and numerical simulation results, it could be concluded that the fracture will initially occur in the region where has the highest intensity of equivalent stress field (i.e., contains the most highly localized cluster of pores and shrinkage), and then, fracture crack will fast propagate through the adjacent regions which have the relatively high intensity of stress field.

Abstract: In this paper, under the assumption of homogeneous rock conditions, the fracture opening and propagating theories, the boundary element method was used to simulate the interaction between the fracture in the processing of the simultaneous multi-frac treatments. The research shows that simultaneous multiple fracturing change the direction of crack extension, which lead to direction change of the in-situ principle stresses. The fractures attract or repel each other in different horizontal wellbores /fractures configuration.

Abstract: The Acoustic Emission (AE) monitoring technique is widely used in mechanical and material research for detection of plastic deformation, fracture initiation and crack growth. However, the influence of AE features (such as signal amplitude, frequency, rise time and duration) on the fracture parameters (such as brittle or ductile mode of propagation and fracture propagation speed) is not completely understood. In this paper, the effect of loading conditions on fracture behavior was studied using AE monitoring during tensile testing of an aluminum alloy specimen. The fracture development was observed using a high speed video camera and was analyzed using the finite element method. The hardware and software produced by Physical Acoustics Corporation (USA) was used. Variations in AE parameters were analyzed and correlated to the stress-strain curves obtained during testing. It is shown that the strain rate and the presence of a crack (modeled by a notch on the sample), affect the fracture mode (brittle or ductile) and a relative amount of the mode dependent AE signatures.

Abstract: We develop a novel constitutive modeling approach for the analysis of fracture propagation in quasi-brittle materials using the Material Point Method. The kinematics of constitutive models is enriched with an additional mode of localized deformation to take into account the strain discontinuity once cracking has occurred. The crack details therefore can be stored at material point level and there is no need to enrich the kinematics of finite elements to capture the localization caused by fracturing processes. This enhancement also removes the drawback of classical smeared crack approach in producing unphysical snapping back constitutive responses when the spatial resolution is not fine enough. All these facilitate the implementation of the new approach in the Material Point Method for analysis of large scale problems. Numerical examples of fracture propagation are used to demonstrate the effectiveness and potentials of the new approach.

Abstract: The fracture distribution around circular cavities has been widely observed in experiments and well documented in literature from 1980. Most of the works are based on experiments’ results, which cost considerable time and money. With varied numerical methods developed more and more researcher employ numerical experiments on computers instead of physical experiments. Firstly, the nodal enrichment functions for Extended Finite Element Method in conjunction with additional degrees are presented. Moreover, we describe the cohesive segments method, which is followed by the damage initiation and evolution laws. In the last a borehole numerical model is built up and the simulation results of the primary fracture propagation are presented.

Abstract: The fractured low permeability reservoirs develop complex fracture network. As the of waterflooding recovery heightens, excessive high injection pressures and excessive water injection rate will result in open, initiation, propagation and coalescence of micro-fracture, connecting injection with production form the high permeability zone, which results in a one-way onrush of waterflooding, water cut in oil well water rise quickly, causing a severe oil well flooding and channeling, thereby reducing the ultimate oil recovery efficiency. The effect of the waterflooding seepage within natural fracture on fracture initiation is studied and analyzed here, applying the theory of rock fracture mechanics to analyze the interaction of fracture system for naturally fractured reservoirs in waterflooding developing process, studying the mechanical mechanism of opening, initiation, propagation and coalescence of natural fracture under injection pressure, which is important theoretical significance for studying the distribution law of fracture and defining appreciate water injection mode and injection pressure in the process of injection development of the naturally fractured reservoir and for delaying the directivity water break-through and water flooding rate of oil well in the process of injection development.

Abstract: Two methods for the extraction of Stress Intensity Factors (SIFs) from three-dimensional (3-D)problems are presented: the Contour Integral Method and the Cutoff Function Method. The formula-tions are tailored for the Generalized Finite ElementMethod andmixed-mode 3-D propagating cracks.The case of crack faces loaded by prescribed tractions is also considered. Another contribution of thispaper is a procedure to control the noise of extracted SIFs based on theMoving Least SquaresMethod.The proposed approach provides a continuous and smooth approximation of 3-D SIF functions foreach fracture mode. Numerical experiments demonstrating the accuracy and robustness of the pro-posed methodology are presented. They include 3-D mixed-mode fatigue crack growth simulationsand the case of a pressurized crack.

Abstract: Hydraulic fracturing is one of the effective means to enhance coal bed methane production for vertical wells. This paper presents an approach that uses pseudo-3D fracture propagation model to study the influence of petrophysical properties, differential stress, treatment conditions, etc. on fracture geometry. It is shown that differential stress, pump rate is proportional to fracture length and width; elastic modulus, Poisson`s ratio, pump rate, etc. is proportional to fracture height. The finding is of great importance for acquiring ideal fracture geometry.

Abstract: In order to describe hydraulically created fracture propagation’s characteristics of rock matrix exactly, in this paper, establishing a stress field ‘s calculation model of fracturing propagation tip , obtaining numerical method of fracturing propagation’s characteristics based on damaging and describing fracturing propagation’s characteristics combined with method of finite element. Research shows that the corrigendum between stress field ‘s calculation model of fracturing propagation tip and practical engineering are 0.64 percent and 1.43 percent respectively. Compared with the traditional method, the result is more exactly.