双重介质渗流水力特性试验装置研究及应用

研制的双重介质渗流水力特性试验系统,由双重介质试验模块、水循环控制模块和数据采集模块三部分组成,较好地解决了孔隙介质和裂隙介质的模拟、边界条件的模拟、止水工艺的完善和水交换信息采集等关键问题,并具有裂隙宽度可调节,精度高,自动化程度高,试验数据实时全自动采集等特点。该系统可用以研究基于双重介质模型的水量交换以及渗流场的水压分布规律以及双重介质的水力性态和渗流机制。利用该试验系统进行双重法介质渗流水力特性试验,通过趋势图直观分析、极差和方差分析和F-检验等分析手段,得出孔隙-裂隙双重介质水交换影响因子对双重介质水交换的影响能力,从大到小依次为：裂隙宽度、孔隙介质渗透系数、压力梯度、水温。     

基于逾渗模型的盐岩损伤与破坏研究

逾渗是一种分形模型,利用声发射点定位盐岩内部破坏网格来建立逾渗模型,可以分析三轴压缩条件下盐岩逾渗特征以及损伤演变发展。研究发现,各试样逾渗模型团簇(cluster)数与最大团簇占有率关系曲线斜率在70%应力峰值后基本相等,利用最大团簇沿试样轴向的延伸来描述裂缝的扩展规律,并通过最大团簇在轴向的延伸终点得到了各试样逾渗的临界破坏比率,它对于研究渗透陡增点非常重要。提出利用逾渗关联长度(关联长度?代表处于同一团簇中的两个点的平均距离)确定试样损伤起始点,并通过计算逾渗模型破坏比率得到损伤变量,它和基于AE振铃计数以及AE能量计算得到的损伤变量结果相近。研究结果表明,逾渗可以形象地表述岩石内部破裂过程和损伤情况,为研究岩石破坏失效及裂缝衍生发展提供了新的思路。     

应力-温度对低渗透介质渗透率影响研究

在石油/天然气地下储存、核废料地下处置等工程中,低渗透介质的渗透率测试及其随着应力、温度的变化是工程稳定性和环境评价的基础。通过研制的低渗透介质温度-渗流-应力耦合三轴仪（T-M-PTS）,对不同静水压力、不同温度条件下锦屏大理岩的渗透率进行了渗透特性测试,并通过微裂隙模型从理论方面对致密岩石渗透率演化进行了研究。研究结果表明：（1）致密岩石中气体渗透的Klinkenberg效应显著,不能忽略;（2）基于统计理论和逾渗理论的圆盘裂隙模型可以较好地模拟致密岩石扩容和压密过程中渗透率演化特征;（3）静水压力增大,渗透率降低,其降低趋势是开始压缩阶段降低速率较快,随后降低速率越来越小;（4）温度升高,岩石弹性模量降低,岩石被压缩得更加致密,渗透率降低较为明显,从15℃升高至40℃即会降低一个量级,低至3.9×10-21 m2。     

地下空间逾渗与裂缝属性的关系分析

裂缝连通性评价是地下空间研究的重要内容。逾渗分析是研究裂缝网络连通性的一种有效手段。裂缝网络逾渗临界值多使用裂缝间接表征参数(如分形维数)来确定,但存在裂缝网络连通性不同却有相同间接表征参数的情况,这降低了预测的可靠性。为避免此类问题并更加准确快速地表征裂缝网络的连通性,文章在构建逾渗临界值方程时,摒弃了间接参数简化的方式,使用裂缝直接表征参数(如裂缝数量、长度等)进行非线性拟合。通过二维数值模拟建立不同参数的离散裂缝网络模型,分析逾渗低、中、高概率临界条件与裂缝直接表征参数的关系,建立逾渗临界条件预测方程,并对方程在不同尺度研究区的应用进行了讨论和验证。结果表明：预测方程可有效地预测不同概率的逾渗临界值。同时文章在预测方程的基础上,建立了裂缝网络连通性评价标准,这对于裂缝发育地区的地下空间评价工作具有一定的指导和借鉴意义。     

煤岩体采动裂隙网络的逾渗与分形特征关系研究

以平煤八矿己15-22060工作面为背景,引入逾渗和分形理论,分析了煤岩体裂隙网络的演化特征,得到了逾渗概率和分形维数随工作面推进度的变化规律及二者的变化关系。结果表明：逾渗概率的大小受横向离层裂隙和竖向破断裂隙的发育、贯通等因素影响,随工作面推进分阶段线性增大,且增幅有增大的趋势;随工作面推进煤岩体的裂隙网络逐步向工作面前方煤体和上覆岩层扩展,分形维数大小受裂隙的长度、宽度、数量、分布等因素控制,升维过程大致可分为3个阶段;煤岩体裂隙网络的逾渗概率与分形维数的关系可分段拟合为1个幂函数和2个线性函数,且具有很好的相关性,数学模型的建立可为研究煤岩体裂隙网络的等效渗透率奠定一定的基础。     

单裂隙岩体非饱和临界状态渗流特性初步研究

探讨了裂隙非饱和渗流机制,认为裂隙尺度上的非饱和渗流实际上是由许多局部开度尺度上的饱和渗流构成的。用重正化理论的方法,对裂隙发生逾渗相变时的临界特性进行了研究,指出裂隙非饱和渗流存在一临界饱和度值,当饱和度小于该值时,裂隙中无渗流存在。对裂隙非饱和水力传导度进行了讨论。     

基于重整化群方法的红层软岩损伤破坏逾渗阈值研究

逾渗现象广泛存在于大自然中,当物体中起决定性作用的关键变量变化至某一阈值时,其状态发生突变,此时关键变量的临界值称为逾渗阈值。岩石在水的作用下的损伤破坏过程中存在着逾渗现象,可通过重整化群的方法来研究。本文针对华南地区广泛分布的红层软岩,探讨了其在损伤过程中的逾渗现象与破坏的本质关联。通过对典型红层软岩的微观结构分析,概化出红层软岩的骨架构件和胶结物构件,建立红层软岩的重整化群模型的基本单元,将基本单元进行组合,建立出红层软岩的重整化群模型。根据重整化变换规则,推导计算出其逾渗阈值。研究表明:当红层软岩在荷载条件下的破坏临近逾渗阈值时,其内部结构发生大量破坏,软岩内部从无序损伤突变为有序损伤,继之损伤破坏加剧,由稳定破坏向非稳定破坏突变,最终导致其失稳破坏。研究对揭示红层软岩损伤破坏机理具有重要意义。     

双重介质模型研究现状——裂隙-孔隙含水层的数学模型

和多孔介质中流体运动规律研究所得到的进展相比,裂隙介质中流体运动规律的研究仍处于发展阶段。近年来,由于裂隙含水层试验资料的解释,核废料及有毒化学废料的地质处理,海水入侵以及裂隙油田开发的需要,引起了人们对裂隙介质中流体运动规律研究的极大兴趣,本文介绍了描述裂隙含水层水力特性的双重介质模型的理论、方法及应用范围。文中所介绍的四种模型可以根据实际的地质—水文地质条件直接应用于裂隙-孔隙含水层试验的解释。     

基于随机孔隙网络模拟的渗透率应力敏感机理

储层岩石物性应力敏感研究主要停留在实验测试与数据处理方法上,无法通过单一形状孔隙的理论模型从机理上得到解释;因此,本研究基于随机孔隙网络模型实现渗透率应力敏感感机理解释。以逾渗理论、水电相似原理、Kirchoff定律为理论基础,采用迭代法,基于QT Creator平台编程生成了三维随机孔隙网络模型,并对无因次半径与有效应力的关系进行了模拟。模拟结果表明:纵横比越小,无因次半径受到有效应力的影响越明显。因此,通过网络模型建立渗透率与有效应力关系,能够实现渗透率应力敏感的机理解释。     

裂隙贯通率对双重孔隙介质热-水-应力耦合现象影响的有限元分析

为探讨裂隙的贯通率对于耦合的温度场、渗流场和应力场的作用,应用所建立的遍有节理岩体双重孔隙-裂隙介质热-水-应力耦合模型,以一个假定的位于非饱和地层中的高放废物地质处置库为算例,针对裂隙和孔隙的贯通率不同的4种工况进行了二维有限元数值分析,考察了围岩中的温度、负的孔隙和裂隙水压力、地下水流速、孔隙及裂隙的渗透系数修正因子和应力的变化、分布情况。结果显示,由于裂隙贯通率的差异使得双重介质的刚度不同,引起岩体中应力状态及水平的改变,从而影响到孔隙、裂隙的孔隙率及渗透系数的量值,并导致孔隙水和裂隙水的压力大小、分布以及水流速度的变化     

Modelling of steady‐state fluid flow in 3D fractured isotropic porous media: application to effective permeability calculation

In this paper, 3D steady‐state fluid flow in a porous medium with a large number of intersecting fractures is derived numerically by using collocation method. Fluid flow in the matrix and fractures is described by Darcy's law and Poiseuille's law, respectively. The recent theoretical development presented a general potential solution to model the steady‐state flow in fractured porous media under a far‐field condition. This solution is a hypersingular integral equation with pressure field in the fracture surfaces as the main unknown. The numerical procedure can resolve the problem for any form of fractures and also takes into account the interactions and the intersection between fractures. Once the pressure field and then the flux field in fractures have been determined, the pressure field in the porous matrix is computed completely. The basic problem of a single fracture is investigated, and a semi‐analytical solution is presented. Using the solution obtained for a single fracture, Mori‐Tanaka and self‐consistent schemes are employed for upscaling the effective permeability of 3D fractured porous media. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of Stochastic Fracture Network with Numerical Fluid Flow Simulations to Groundwater Flow Modeling in Fractured Rocks

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基于CT的数字岩心三维建模

为研究储层微观孔隙结构和建立微观渗流模型,本文通过对东营凹陷沙三中亚段H152井区的典型低渗透储层岩样进行CT（computed tomography）扫描及图像处理,建立了微观尺度的数字岩心模型;继而经过对该模型进行分析和计算,提取了储层孔喉网络模型,在三维空间上直观、清晰地显示了不同尺度的孔隙及喉道的形态、大小和分布;最后通过对孔隙结构特征、孔隙度、渗透率和压降等动静态参数的分析和计算,建立了储层岩样的微观渗流模型。根据算法对比和参数分析结果认为：与传统中值滤波相比,非局部均值滤波算法可在相似性比较的基础上进行滤波处理,从而提高模型的准确性;基于CT的数字岩心建模可为地质研究提供可靠的数字模型;根据近似等压面假设的微观渗流数值模拟分析了流体渗流特征,为揭示低渗透储层流体渗流规律提供了一种新的途径。     

Analysis of fracture network connectivity using percolation theory

Connectivity aspects of fracture networks are analyzed in terms of percolation theory. These aspects are of fundamental importance in characterization, exploitation, and management of fractured formations. General connectivity and power law relationships are determined that characterize the density of fractures and average number of intersections per fracture necessary to ensure network connectivity, the likelihood of a fractured formation being hydraulically connected, and the probability that any specific fracture is connected to the conducting portion of the network. Monte Carlo experiments with a two-dimensional fracture network model confirm the percolation theory predictions. These relationships may prove useful in formulating theoretically tractable approximations of fracture nerworks that capture the essential system properties.     

A 2D mixed fracture–pore seepage model and hydromechanical coupling for fractured porous media

A novel two-dimensional mixed fracture–pore seepage model for fluid flow in fractured porous media is presented based on the computational framework of finite-discrete element method (FDEM). The model consists of a porous seepage model in triangular elements bonded by unbroken joint elements, as well as a fracture seepage model in broken joint elements. The principle for determining the fluid exchange coefficient of the unbroken joint element is provided to ensure numerical accuracy and efficiency. The mixed fracture–pore seepage model provides a simple but effective tool for solving fluid flow in fractured porous media. In this paper, examples of 1D and 2D seepage flow in porous media and porous media with a single fracture or multiple fractures are studied. The simulation results of the model match well with theoretical solutions or results obtained by commercial software, which verifies the correctness of the mixed fracture–pore seepage model. Furthermore, combining FDEM mechanical calculation and the mixed fracture–pore seepage model, a coupled hydromechanical model is built to simulate fluid-driven dynamic propagation of cracks in the porous media, as well as its influence on pore seepage and fracture seepage.

     

Modeling of subsidence and stress-dependent hydraulic conductivity for intact and fractured porous media

Summary This study investigates the changes in deformation and stress dependent hydraulic conductivities that occur as a result of underground mining in intact and fractured porous media. The intact porous medium is assumed to be comprised of regularly packed spherical grains of uniform size. The variation in grain size or pore space due to the effect of changing intergranular stresses results in a change in rock hydraulic conductivity. A model is developed to describe the sensitivity of hydraulic conductivity to effective stresses through Hertzian contact of spherical grains. The fractured porous medium is approximated as an equivalent fracture network in which a single fracture is idealized as a planar opening having a constant equivalent thickness or aperture. Changes in fracture aperture as a result of changes in elastic deformation control the variation of hydraulic conductivity. A model is presented to illustrate the coupling between strain and hydraulic conductivity. Subsidence induced deformations that result from mining induced changes in hydraulic conductivity in both intact and fractured media. These changes are examined and compared with results from a mining case study.     

模拟裂隙多孔介质中变饱和渗流的广义等效连续体方法

描述了一种计算裂隙多孔介质中变饱和渗流的广义等效连续体方法。这种方法忽略裂隙的毛细作用,设定一个与某孔隙饱和度相对应的综合饱和度极限值,并假定：（1）如果裂隙多孔介质的综合饱和度小于该极限值,水只在孔隙中存在并流动,而裂隙中则没有水的流动;（2）如果综合饱和度等于或大于该极限值,水将进入裂隙,并在裂隙内运动。分析比较了等效连续体模型的不同计算方法,并给出了一个模拟裂隙岩体中变饱和渗流与传热耦合问题的应用算例。结果表明,所述方法具有一般性,可以有效地模拟裂隙多孔介质中变饱和渗流的基本特征。     

A Connectivity Index for Discrete Fracture Networks

Connectivity is an important measure for assessing flow transport in rock, especially through fractures. In this paper, rock fracture systems are modelled by a discrete fracture model simulated by a marked point process. A connectivity index is then introduced to quantify the connectivity between any two points in space. Monte Carlo simulation is used to evaluate the connectivity index for stationary cases and relationships between the connectivity index and the parameters of the discrete fracture model are analysed. The average number of intersections per fracture, X_f, and the fracture intensity, P12 (P32), are calculated and the relationships between these parameters and the connectivity index are investigated, concluding that X_f is the more suitable parameter for the classification of rock mass flow properties. The relationships between the connectivity index and the percolation state of the fractured medium are also discussed. An edge correction is briefly discussed and a practical example is used to demonstrate the method of computing the connectivity index.