Cohesive‐frictional model for bond and splitting action of prestressing wire

This paper presents a numerical procedure for bond between indented wires and concrete, and the coupled splitting process of the surrounding concrete. The bond model is an interface, non‐associative, plasticity model. It is coupled with a cohesive fracture model for concrete to take into account the splitting of such concrete. Bond between steel and concrete is fundamental for the transmission of stresses between both materials in precast prestressed concrete. Indented wires are used to improve the bond in these structural elements. The radial component of the prestressing force, increased by Poisson's effect, may split the surrounding concrete, decreasing the wire confinement and diminishing the bonding. The combined action of the bond and the splitting is studied with the proposed model. The results of the numerical model are compared with the results of a series of tests, such as those which showed splitting induced by the bond between wire and concrete. Tests with different steel indentation depths were performed. The numerical procedure accurately reproduces the experimental records and improves knowledge of this complex process. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of a deformable fracture in permeable material

The response of deformable fractures to changes in fluid pressure controls phenomena ranging from the flow of fluids near wells to the propagation of hydraulic fractures. We developed an analysis designed to simulate fluid flows in the vicinity of asperity‐supported fractures at rest, or fully open fractures that might be propagating. Transitions between at‐rest and propagating fractures can also be simulated. This is accomplished by defining contact aperture as the aperture when asperities on a closing fracture first make contact. Locations on a fracture where the aperture is less than the contact aperture are loaded by both fluid pressure and effective stress, whereas locations where the aperture exceeds the contact aperture are loaded only by fluid pressure. Fluid pressure and effective stress on the fracture are determined as functions of time by solving equations of continuity in the fracture and matrix, and by matching the global displacements of the fracture walls to the local deformation of asperities. The resulting analysis is implemented in a numerical code that can simulate well tests or hydraulic fracturing operations. Aperture changes during hydraulic well tests can be measured in the field, and the results predicted using this analysis are similar to field observations. The hydraulic fracturing process can be simulated from the inflation of a pre‐existing crack, to the propagation of a fracture, and the closure of the fracture to rest on asperities or proppant. Two‐dimensional, multi‐phase fluid flow in the matrix is included to provide details that are obscured by simplifications of the leakoff process (Carter‐type assumptions) used in many hydraulic fracture models. Execution times are relatively short, so it is practical to implement this code with parameter estimation algorithms to facilitate interpretation of field data. Copyright © 2006 John Wiley & Sons, Ltd.     

Extension of the Griffith's fracture criteria to saturated clays

Inglis [1] has solved the problem of distribution of stress in an elastic plate around an elliptical hole. His works clarify the role of cracks in the failure of an elastic material. However, his solution cannot be applied to saturated clay because he considers only total stresses, while, in saturated clay, the criterion of rupture should be expressed in terms of effective and not total stresses. The solution of Atkinson and Craster [2] using Biot's poroelasticity theory, shows that there is no high pore pressure in the vicinity of the crack tips for saturated clay. The major difference between this approach and the Biot's theory of is that, in saturated clay, strain is a function of the variation of the effective stress [3], while, in poroelastic media, strain is only a function of the variation of the total stress [4, Equation 2.2]. Also in their solution there is continuity between the pore fluid and the inner fluid in the crack. Their solution is valid for poroelastic media involving a movement of the pore fluid. In our solution there is no movement of the pore fluid (Undrained condition). In this paper we have solved the same problem as Inglis [1], but for the particular case of saturated clay obeying elastic law. By solving this problem we obtained the expressions for pore pressure, effective stress, total stress and displacements. The results show that not only the total stress but also the pore pressure and the effective stress are also high in the vicinity of the crack tips. A new failure criterion, based on Griffith's strain energy principle [5] and maximum tensile stress [6], valid for saturated clay is developed in this paper. Copyright © 2003 John Wiley & Sons, Ltd.     

Microcracks and Overpressure- Induced Fractures

The microfractures and natural fractures studies have a great significance in the hydrocarbon accumulation exploration and can be a good supporter for wide geophysical study on the major structures. This paper is a brief review on microcracks and overpressure- induced fracture. It is to discuss different views on the mechanism of fractures in the subsurface of the earth.     

Numerical Modeling of Natural Fracture Distributions in Shale

The production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures. This study presents a simulation of natural fractures in shale reservoirs, based on a discrete fracture network (DFN) method for hydraulic fracturing engineering. Fracture properties of the model are calculated from core fracture data, according to statistical mathematical analysis. The calculation results make full use of the quantitative information of core fracture orientation, density, opening and length, which constitute the direct and extensive data of mining engineering. The reliability and applicability of the model are analyzed with regard to model size and density, a calculation method for dominant size and density being proposed. Then, finite element analysis is applied to a hydraulic fracturing numerical simulation of a shale fractured reservoir in southeastern Chongqing. The hydraulic pressure distribution, fracture propagation, acoustic emission information and in situ stress changes during fracturing are analyzed. The results show the application of fracture statistics in fracture modeling and the influence of fracture distribution on hydraulic fracturing engineering. The present analysis may provide a reference for shale gas exploitation.     

基于扩展有限元分析的页岩水力压裂裂缝扩展规律探究

水力压裂是开采地下页岩气资源的有效技术手段,探究页岩水力压裂裂缝的扩展规律,可为页岩气的高效开采提供科学的指导依据。通过运用大型有限元软件ABAQUS中的扩展有限元模块,针对不同地应力差工况条件下均质页岩中初始裂缝的位置、方位角、数量和含层理页岩中层理的构造方向、内部倾角、岩性对水力裂缝扩展的影响进行探究。结果表明：对于垂向扩展的水力裂缝,水平主应力增大使裂缝更不易扩展,裂缝扩展长度减小、起裂压力增大;在注液体积流量相同时,向初始裂缝两端同时起裂所形成的水力裂缝长度大于仅向一侧起裂;当初始裂缝处于页岩中部且呈45°方向时,裂缝会向最大水平主应力方向偏转,且偏转程度随最大水平主应力的增大而增大;分时多簇压裂时,裂缝间的扩展会相互干扰,且会较大地影响裂缝扩展的形态和起裂压力,但对裂缝注液点裂缝宽度的影响较小;对于含水平和竖直构造层理的页岩,改变层理内部倾角,水力裂缝会出现不同程度偏转,且其偏转程度随着层理内部倾角的增大而减小;对于含45°方向构造层理的页岩,水力裂缝在层理分别为砂岩、煤岩和泥岩中的偏转程度依次增大,且裂缝偏移比随着最大水平主应力的增大而增大。     

红层泥质胶结物干缩裂缝深度与间距预测模型

红层泥质胶结物在干燥条件下会因含水率变化产生开裂,裂缝进一步发展贯通最终导致红层泥岩崩解破坏。目前对红层泥岩崩解机制的研究多为定性描述,缺乏定量分析方法。因此,定量研究红层泥质胶结物开裂的产生及扩展对深化红层泥岩崩解理论研究具有重要意义。基于线弹性断裂力学及非饱和土力学,建立了考虑含水率变化的红层泥质胶结物裂缝深度及间距计算公式,并对公式参数敏感性进行了分析。应用CT试验数据对PFC2D数值模型进行了校准,并基于干缩开裂数值试验结果,对裂缝深度及间距计算公式进行了对比验证。结果表明：提出的计算公式能较好地预测红层泥质胶结物的开裂深度及间距,裂缝深度随含水率减小呈先快速增加再缓慢增长的特点。研究成果有助于加深对红层泥岩崩解的量化理论分析,并为预测工程中红层泥岩的开裂程度提供参考。     

基于CT三维重建的注水煤岩体裂隙扩展规律研究

针对煤层注水防尘过程中注水压力设置不合理、注水渗流效果差等问题,采用CT扫描技术与RFPA软件相结合的方法,构建了可以表征注水煤岩体内部孔裂隙结构的三维细观非均匀渗流损伤数值模型。通过对经过CT三维重构的煤岩模型进行不同注水压力的渗流损伤模拟,研究了煤层注水压力对煤样的渗流破坏、渗透率演化及声发射特性变化的影响;并通过对重构的煤岩模型进行缩放处理,研究了煤岩尺寸对煤样的渗流破坏和声发射变化的影响。研究结果表明：在煤样微观裂隙扩展过程中,随着注水压力的递增,煤样损伤单元数、渗流运动的渗流场分布范围、渗透率、声发射数目和能量总体呈上升趋势,局部范围内有波动发生,发生波动的原因是由于渗流运动的渗流场与煤样裂隙内部应力场发生临界反应,致使煤样破坏单元位置发生改变;煤样渗透率由3.82×10−5 μm2上升至0.314 μm2,孔隙率由5.45%上升至48.45%,揭示了煤岩体裂隙总体上随注水压力增大而不断扩展贯通,局部上随注水压力增大而扩展趋势有所下降的影响规律;随着煤样尺寸的增加,注水破坏后煤样的孔隙率呈现先下降后逐渐平稳的趋势,声发射特性变化趋势正好与之相反,表明煤样的尺寸对注水煤岩渗流破坏有显著影响,但当煤样尺寸超过40 mm时,煤样尺寸对注水煤岩渗流破坏的影响趋于稳定。CT扫描技术与RFPA软件相结合的方法能够有效模拟注水煤岩的裂隙渗流扩展行为。