裂缝性低渗透油藏流-固耦合数值模拟

根据低渗裂缝性砂岩油藏的储层特征,建立适合裂缝性砂岩油藏渗流的等效连续介质模型;通过实验得出有效压力改变对储层孔隙度、渗透率影响的规律;考虑低渗透油藏渗流时启动压力梯度和低渗储层的流-固耦合特性,将渗流力学与弹塑性力学相结合,建立起适合低渗透油藏的流-固耦合渗流数学模型,并给出其数值解.在黑油模型和弹塑性有限元程序的基础上,编制了计算低渗透油藏流-固耦合渗流的计算软件.通过数值模拟和不考虑流-固耦合时的计算结果相比,可以看出低渗油藏中流-固耦合效应十分明显.     

大型地下洞室群围岩应力-损伤-渗流耦合分析

以渗透体力来考虑渗流场的力学效应,建立了应力-损伤-渗透系数关系方程来考虑应力和损伤对渗流场的影响,结合岩体结构的三维弹塑性损伤有限元分析,建立了大型地下洞室开挖围岩应力-损伤-渗流耦合的计算模型。该计算模型求解的难度主要体现在岩体材料弹塑性、损伤、渗流自由面边界、渗流溢出边界、应力-损伤-渗流相互影响关系等。提出分步迭代法对以上因素进行归纳后按一定顺序分别进行迭代求解,取得了良好的计算效果。将该方法应用于某水电站大型地下洞室群分析,得出了一系列有意义的结论。     

考虑开挖损伤的高放废物地质处置库温度-渗流-应力耦合数值模拟方法

高放废物地质处置库处于温度-渗流-应力（THM）多场耦合环境中,对高放废物处置库进行安全评估时,需进行多场耦合分析。然而高放废物处置库开挖引起硐壁附近围岩应力重分布,产生损伤,导致围岩热学参数（T）、渗流参数（H）和力学参数（M）发生变化,且在空间上分布不均匀,这将会对运营期处置库THM耦合演化过程产生显著影响。通过分析高放废物处置库温度-渗流-应力三场的耦合原理和处置库围岩损伤的分布和演化规律,定义了损伤变量和损伤演化准则,并将损伤变量与热学参数、渗流参数、力学参数以及多场耦合参数（Biot系数、Biot模量和温度排水系数）建立联系,将围岩损伤与温度-渗流-应力建立联系,形成了一个弹塑性损伤温度-渗流-应力多场耦合数值模型,然后利用建立的模型对瑞士Mont Terri高放废物地质处置库围岩加热试验进行模拟,对比了模拟值和试验值,比较了考虑开挖损伤和不考虑开挖损伤对高放废物地质处置库温度-渗流-应力的影响,并分析了在多场耦合作用下开挖损伤的演化规律。     

岩石弹塑性应力-渗流-损伤耦合模型研究(Ⅰ):模型建立及其数值求解程序

在地下水渗流场、应力场、损伤场的耦合作用下更易造成隧道围岩坍塌或涌水等灾害。首先,将围岩材料视作各向同性连续介质,基于Drucker-Prager准则建立岩石弹塑性损伤本构模型,采用完全隐式返回映射算法实现弹塑性损伤本构方程的数值求解。其次,以上述研究为基础根据岩石处于弹塑性状态时渗透系数动态演化公式,建立岩石弹塑性应力-渗流-损伤耦合模型,并给出三场耦合情况下的数值求解迭代方法。针对耦合模型中涉及参数较多且不易测定的问题,基于差异进化算法原理建立智能反分析方法,对耦合模型中的损伤参数进行反演。最后,利用C++语言编制相应的岩石弹塑性应力-渗流-损伤耦合程序和参数反演程序,利用所编程序进行以下计算:(1)对智能反分析程序的性能、正确性进行分析,对比不同差异策略、交叉因子、变异因子的反演精度和收敛速度。(2)分别采用弹性模型和弹塑性损伤模型进行隧道围岩位移场、应力场的计算。(3)不考虑力学作用的情况下进行孔隙水压力、渗流量的计算。(4)采用所建耦合模型计算得到隧道围岩应力场、渗流场以及损伤场的相互影响规律。研究结果表明,基于差异进化算法的智能反分析程序能够较好地解决耦合模型中损伤参数不易确定的难题,为实际工程中获得不易测定的计算参数提供了有效的方法,同时所建立的耦合模型通过应力、渗流和损伤的相互作用更能够真实地反映出岩石材料的宏观破坏现象,所编计算程序能够模拟地下水渗流场、应力场、损伤场之间的耦合特性,为受地下水影响严重的工程建设提供了方法,研究结论为后期对实际隧道工程进行耦合计算奠定基础。     

考虑开挖损伤的高放废物地质处置库温度-渗流-应力耦合数值模拟方法

高放废物地质处置库处于温度?渗流?应力（THM）多场耦合环境中，对高放废物处置库进行安全评估时，需进行多场耦合分析。然而，高放废物处置库开挖引起硐壁附近围岩应力重分布，产生损伤，导致围岩热学参数（T）、渗流参数（H）和力学参数（M）发生变化，且在空间上分布不均匀，这将会对运营期处置库THM耦合演化过程产生显著影响。通过分析高放废物处置库温度?渗流?应力三场的耦合原理和处置库围岩损伤的分布和演化规律，定义了损伤变量和损伤演化准则，并将损伤变量与热学参数、渗流参数、力学参数以及多场耦合参数（Biot系数、Biot模量和温度排水系数）建立联系，将围岩损伤与温度?渗流?应力建立联系，形成了一个弹塑性损伤温度?渗流?应力多场耦合数值模型，然后利用建立的模型对瑞士Mont Terri高放废物地质处置库围岩加热试验进行模拟，对比了模拟值和试验值，比较了考虑开挖损伤和不考虑开挖损伤对高放废物地质处置库温度?渗流?应力的影响，并分析了在多场耦合作用下开挖损伤的演化规律。     

用流固耦合方法研究油藏压裂后应力应变和孔渗特性变化

油藏压裂后将引起地应力场发生变化，使岩石变形，导致孔隙度和渗透率变化，进而影响产量，为研究这一问题，作者建立了油藏压裂后流－固耦合渗流模型，考虑了以下因素：油藏岩石变形，地应力，孔隙度和渗透率变化，人工裂缝，流体渗流与岩石应变耦合，储藏渗流与裂缝渗流耦合，非达西效应等。较详细地给出了耦合方程及推导过程，控制方程包括的未知变量有压力，饱和度及位移，11个变量，和11个方程，用有限差分方法将流体渗流和岩石应变方程离散成主对角占优的七对角矩阵，可在修改已有三维二相渗流和三维固体力学程序的基础上，采用隐式迭代方法求解，示例分析表明，用此模型可以研究储层应力变变，孔隙度和渗透率随时间和空间变化规律，为开发方案制定，整体压裂设计，压后生产管理等方面提供定量分析技术。     

基于渗流-损伤-应力耦合作用下考虑力学参数弱化的巷道围岩变形破坏分析

地下工程岩体渗流-损伤-应力耦合问题的研究对于巷道围岩的稳定性分析具有重要意义。本文在总结分析了巷道变形破坏类型影响因素的基础上,基于弹塑性力学、渗流力学以及损伤理论建立了岩体渗流-损伤-应力耦合模型。该模型充分考虑了多物理场耦合过程中,工程岩体的非均质性,岩体力学参数发生的动态弱化过程,围岩塑性屈服的峰后特性以及渗透系数在损伤过程中的突变性。基于多物理场耦合软件,数值模拟结果分析得到,使用该模型能更好地反映巷道围岩的屈服破坏程度和渐进破坏过程。应用该模型分析不同深度下的巷道围岩渐进性破坏过程可以得出:水平地应力为主导的地层中的巷道,屈服破坏主要发生在顶拱和底板,竖直地应力为主导的地层中的巷道,屈服破坏主要发生在两侧边墙,水平地应力和竖直地应力相近的地层中,巷道四周均发生不同程度的破坏,这与工程实际有很好的符合。     

隧道开挖问题中的流固耦合模型及数值模拟

隧道工程中的地下水问题是富水地层中普遍存在的重要问题,地下水流动对隧道围岩稳定性有重要影响。给出了描述隧道开挖过程中力学与水力特征及表征方法,根据岩体的基本结构特征及代表性单元体（REV）是否存在提出了流固耦合模型的建立方法;提出了隧道水力耦合数值分析中的耦合计算模型的建立方法;利用数值法研究了隧道开挖渗流与应力耦合问题,得到变形和渗流场的变化规律。结果表明,隧道开挖引起的渗流影响边界大于力学影响边界,由于渗流引起的渗流力增加了围岩的应力、位移,从围岩-支护结构共同作用原理考虑,进行隧道支护结构设计时是应该考虑渗流效应的。     

考虑Klinbenberg效应的煤中应力-渗流耦合数学模型

已有研究表明,瓦斯在煤层中渗流存在明显的Klinbenberg效应,建立考虑Klinbenberg效应的煤中瓦斯渗流应力耦合模型是煤层瓦斯抽放数值模拟的关键。通过试验研究了煤中的气渗透率与瓦斯压和围压的关系,试验数据的拟合分析表明,赵阳升公式能很好地模拟瓦斯渗流的Klinkenberg效应及围压对煤的渗透率的影响。将赵阳升公式引入到建立的煤层瓦斯渗流应力弹塑性耦合模型中,建立了考虑Klinkenberg效应的煤层渗流-应力弹塑性耦合数学模型,在数值计算程序Coupling Analysis中予以实现,并利用已有解析解对程序的可靠性进行了验证。按考虑和不考虑“Klinbenberg“效应两种工况进行了数值计算。结果表明,按前者计算的瓦斯压降要大于后者。这与理论研究的结论一致,表明所建立的考虑Klinbenberg效应的模型具有理论和工程价值。     

泥岩核废料处置库温度-渗流-应力耦合参数敏感性分析

针对比利时HADES地下实验室PRACLAY现场加热试验,应用温度-渗流-应力耦合弹塑性模型,模拟现场加热过程中泥岩核废料处置库的水力学响应特征。采用单因素分析法,就泥岩热、水、力学参数对核废料处置库围岩孔压、温度、有效应力的影响进行了三维有限元分析。并基于参数敏感性分析结果,就温度、渗流、应力三场两两耦合作用对处置库围岩水力学响应的影响程度进行了系统分析。研究结果表明：泥岩热、水、力学参数中,渗透系数、弹性模量以及导热系数对加温所导致的超孔压的值影响较大;凝聚力、内摩擦角以及热膨胀系数对孔压的影响较小,但会显著影响围岩的有效应力;导热系数对围岩温度场的分布有决定性影响,温度传递的差异会显著影响围岩的孔压和有效应力;不同的热、水、力学参数对孔压、温度以及有效应力的影响机制是不同的,温度、渗流、应力三场两两耦合作用对围岩水力学响应的影响程度也存在显著的差异性。温度场对应力场、温度场对渗流场的耦合效应十分显著,加热后,围岩超孔压的产生以及热膨胀导致的有效应力变化会显著影响处置库的稳定。该研究结果在一定程度上可以为核废料处置库泥岩的热、水、力学参数的确定及耦合机制分析提供科学依据。     

非饱和土的水力和力学特性及其弹塑性描述

简单回顾了非饱和土本构模型研究的发展历程,总结了近几年非饱和土弹塑性本构模型最新研究成果,重点介绍了能统一模拟非饱和土水力性状和力学性状耦合的弹塑性本构模型。通过对建立模型过程中的几个核心问题讨论,较详细地说明该类模型的结构、性能以及相关问题。非饱和土水力性状的滞回性用假定存在饱和度弹性区间的弹塑性过程来模拟;该类耦合模型不仅考虑了吸力对非饱和土水力性状和力学性状的影响,还考虑了饱和度对应力-应变关系和强度的影响以及土体变形对土-水特征曲线的影响。用同一套模型参数,耦合模型可统一预测在吸力控制或含水率控制下沿各种应力路径下非饱和土的水力-力学特性,并简单介绍了膨胀性非饱和土的弹塑性本构模型以及耦合模型在有限元数值计算中的应用。     

Finite element modelling of three-phase flow in deforming saturated oil reservoirs

A numerical simulation is presented for three-dimensional three-phase fluid flow in a deforming saturated oil reservoir. The mathematical formulation describes a fully coupled governing equation systen which consists of the equilibrium and continuity equations for three immiscible fluids flowing in a porous medium. An elastoplastic soil model, based on a Mohr–Coulomb yield surface, is used. The finite element method is applied to obtain simultaneous solutions to the governing equations where displacement and fluid pressures are the primary unknowns. The final discretized equations are solved by a direct solver using fully implicit procedures. The developed model is used to investigate the problems of three-phase fluid flow in a deforming saturated oil reservoir.     

异常高压气藏储层参数应力敏感性研究

异常高压气藏开采过程中,由于流体的产出,使储层岩石受力发生改变并使储层岩石发生弹塑性变形;而弹塑性变形反过来又影响到储层的孔隙度和渗透率,因此研究储层孔隙度和渗透率应力敏感性具有极其重要的意义。基于岩石力学的基本理论,推导出异常高压气藏岩石变形规律及变形方程,以此理论推导指导试验,将理论研究与实验规律相结合,在模拟地层条件下,对实际岩心样品进行了储层应力敏感性实验研究。实验研究表明,该方法能精确的描述储层孔隙度和渗透率应力敏感性,实验结果与理论推导结果完全吻合,进一步证明了理论推导的正确。     

Modelling of elastoplastic damage in concrete due to desiccation shrinkage

We present a numerical modelling of elastoplastic damage due to drying shrinkage of concrete in the framework of mechanics of partially saturated porous media. An elastoplastic model coupled with isotropic damage is first formulated. Two plastic flow mechanisms are involved, controlled by applied stress and suction, respectively. A general concept of net effective stress is used in take into account effects of capillary pressure and material damage on stress‐controlled plastic deformation. Damage evolution depends both on elastic and plastic strains. The model's parameters are determined or chosen from relevant experimental data. Comparisons between numerical simulations and experimental data are presented to show the capacity of model to reproduce mains features of concrete behaviour under mechanical loading and during drying shrinkage of concrete. An example of application concerning drying of a concrete wall is finally presented. The results obtained allow to show potential capacity of proposed model for numerical modelling of complex coupling processes in concrete structures. Copyright © 2002 John Wiley & Sons, Ltd.     

On the use of effective stress in three-dimensional hydro-mechanical coupled model

In the last decades, a number of hydro-mechanical elastoplastic constitutive models for unsaturated soils have been proposed. Those models couple the hydraulic and mechanical behaviour of unsaturated soils, and take into account the effects of the degree of saturation on the stress–strain behaviour and the effects of deformation on the soil–water characteristic response with a simple reversible part for the hysteresis. In addition, the influence of the suction on the stress–strain behaviour is considered. However, until now, few models predict the stress–strain and soil–water characteristic responses of unsaturated soils in a fully three-dimensional Finite Element code. This paper presents the predictions of an unsaturated soil model in a Three-dimensional Framework, and develops a study on the effect of partial saturation on the stability of shallow foundation resting on unsaturated silty soil. Qualitative predictions of the constitutive model show that incorporating a special formulation for the effective stress into an elastoplastic coupled hydro-mechanical model opens a full range of possibilities in modelling unsaturated soil behaviour.     

Delayed plastic model for time‐dependent behaviour of materials

A delayed plastic model, based on the theory of plasticity, is proposed to represent the time‐dependent behaviour of materials. It is assumed in this model that the stress can lie outside the yield surface and the conjugate stress called static stress is defined on the yield surface. The stress–strain relation is calculated based on the plastic theory embedding the static stress. Thus, the stress–strain relation of the model practically corresponds to that of the inviscid elastoplastic model under fairly low rate deformation. The delayed plastic model is coupled with the Cam‐clay model for normally consolidated clays. The performance of the model is then examined by comparing the model predictions with reported time‐dependent behaviour of clays under undrained triaxial conditions. It is shown that the model is capable of predicting the effect of strain rate during undrained shear and the undrained creep behaviour including creep rupture. Copyright © 2001 John Wiley & Sons, Ltd.     

Time homogenization for clays subjected to large numbers of cycles

This paper discusses the reliability and the efficiency of a time homogenization method employed to reduce the computational time during cyclic loading for two common geotechnical tests and two elastoplastic models for clays. The method of homogenization is based upon splitting time into two separate scales. The first time scale relates to the period of cyclic loading and the second to the characteristic time of the fatigue phenomenon. The time homogenization method is applied to simulate an undrained triaxial test (homogeneous stress state) and a pressuremeter test (nonhomogeneous stress state) under one‐way cyclic loading on normally consolidated clay. This method is coupled with two elastoplastic models dedicated to cyclic behavior of clay (a bounding surface plasticity model and a bubble model). Both linear and nonlinear elasticities are considered. The difficulty encountered when applying this method to models introducing nonlinear elasticity and kinematic hardening is pointed out. The performance of time homogenization related to the main parameters is numerically investigated by comparison with conventional finite element simulations. Copyright © 2012 John Wiley & Sons, Ltd.     

Finite element modelling for simulating the surface subsidence above a compacting hydrocarbon reservoir

The surface subsidence above a compacting saturated oil reservoir is the main topic of this paper. From a literature review, it is obvious that extensive efforts have been conducted for investigating this phenomenon in various situations. Herein, a numerical model, based on the finite element method, was used for simulating three-dimensional three-phase fluid flow in a deforming saturated oil reservoir. The mathematical formulation describes a fully coupled governing equation system which consists of the equilibrium and continuity equations for three immiscible fluids flowing in a porous media. An elastoplastic soil model, based on a Mohr Coulomb yield surface, was utilized. The finite element method was applied to obtain simultaneous solutions to the governing equations where the displacements and the fluid pressures are the primary unknowns. The final discretized equations are solved by a direct solver using fully implicit procedures. A linear analysis was used to study the stability conditions of the present model. Finally, a series of simulations were conducted to indicate the validity and the utility of the developed model.     

Implementation of a hydromechanical elastoplastic constitutive model for fully coupled dynamic analysis of unsaturated soils and its validation using centrifuge test results

Prediction of unsaturated soil behavior during earthquake loading has received increasing attention in geotechnical engineering research and practice in recent years. Development of a fully coupled analysis procedure incorporating a coupled hydromechanical elastoplastic constitutive model for dynamic analysis of unsaturated soils has, however, been limited. This paper presents the implementation of a coupled hydromechanical elastoplastic constitutive model into a fully coupled dynamic analysis procedure and its validation using a centrifuge test. First, the fully coupled finite element equations governing the dynamic behavior of unsaturated soils with the solid skeleton displacement, pore water pressure, and pore air pressure as nodal unknowns are briefly presented. The closest point projection method is then utilized to implement the coupled hydromechanical elastoplastic constitutive model into the finite element equations. The constitutive model includes hysteresis in soil–water characteristic curves, cyclic elastoplasticity of the solid skeleton, and the coupling mechanisms between the SWCCs and the solid skeleton. Finally, the analysis procedure is validated using the results from a dynamic centrifuge test on an embankment constructed of compacted unsaturated silt subjected to base shaking. Reasonable comparisons between the predicted and measured accelerations, settlements, and deformed shapes are obtained.

     

Analysis of borehole failure modes and pore pressure effects

The theoretical background and finite element simulations of Red Wildmoor sandstone hollow-cylinder experiments performed under various axial and radial loading conditions are presented together with borehole failure predictions. A non-linear, stress dependent elastic, Mohr–Coulomb elastoplastic constitutive model generalized for three-dimensional Cosserat continua is employed. The failure predictions are based on the bifurcation theory and incorporate the effect of microstructure. Two bifurcation instability criteria are formulated, one for lateral and one for axial failure modes. The effect of fluid flow on borehole failure is investigated with reservoir depletion and drawdown simulations and the results are corroborated through experimental data.