格子Boltzmann方法研究岩石粗糙裂隙渗流特性

为研究岩石粗糙裂隙水力特性,建立了基于格子Boltzmann的压力模型。通过对3组不同试件（平板光滑裂隙、矩形非吻合裂隙、随机隙宽裂隙）的模拟,拟合出流量与平均隙宽的关系。研究结果表明,在所计算的10种工况中,3组裂隙中的水流都接近于层流。只有平行光滑裂隙中的水流的流量与平均隙宽近似成立方关系,矩形非吻合裂隙和随机隙宽裂隙中的流量与平均隙宽呈现超立方关系。改变平均隙宽和进出口压力都会导致流量与平均隙宽的关系变化。同时,进出口压力的改变也会造成两者之间产生次立方关系。     

多级加载下岩石裂隙渗流分段特性试验研究

利用自行研制的岩石裂隙辐射型渗流系统,试验研究室温下粗晶大理岩、中砂岩、灰岩和细晶大理岩4个岩石张裂隙在法向闭合过程中的渗流分段特性及加载历史的影响。根据闭合裂隙的接触状态及流域分布特征,裂隙渗流可分为群岛流、过渡流、沟槽流3个阶段;单位水头流量与法向应力呈指数函数关系,随法向应力增加而降低,后次加载中相同法向应力下单位水头流量明显较低;单位水头流量与力学隙宽呈幂函数关系,幂指数范围为1.93～2.60,可认为接触型粗糙岩石裂隙渗流量与力学隙宽呈次立方关系;后次加载时,相同力学隙宽下单位水头流量也明显较低;水力等效隙宽与力学隙宽呈分段的线性关系,修正的立方定律在相应分段内成立。研究结果对岩体裂隙渗流计算有一定的理论意义。     

基于宾汉姆流体的微裂隙岩体注浆扩散影响机制分析

基于巴顿公式中的标准节理面轮廓线建立微裂隙几何模型,采用纳维–斯托克斯方程,并将浆液视为宾汉姆流体,建立微裂隙注浆扩散有限元模型,对微裂隙注浆过程中浆液扩散过程展开计算分析,获得裂隙粗糙度、隙宽以及浆液黏度等因素对浆液扩散的影响规律。计算结果表明,当浆液黏度较高时,浆液流动损失主要受黏滞力控制,裂隙等效隙宽随浆液黏度呈非线性增长。对于具有相同粗糙程度的裂隙,等效隙宽随黏度的增长曲线均存在一个明显的拐点,随着裂隙粗糙程度增加,拐点逐渐向前推移,并且突变更加明显。通过多元函数拟合建立等效隙宽与浆液黏度以及裂隙粗糙度的拟合公式。当浆液黏度以及裂隙的粗糙度较低时该公式存在一定误差,而在考虑实际注浆需要所对应的参数变化范围内,该公式均具有较好的拟合效果。研究结果对微裂隙岩体注浆扩散理论具有一定借鉴意义。     

岩体裂隙面粗糙度对其渗流特性的影响研究

为了研究岩体裂隙面的粗糙程度与渗流机制间的相互关系,将裂隙沿长度方向均匀分割成若干段,通过设置每一段末端的随机高度,生成两侧对称的粗糙裂隙面。基于格子Boltzmann方法,采用不可压缩流体的D2G9模型,验证了经典的Poiseuille流,计算了不同相对粗糙度岩体裂隙的渗流特性,讨论了裂隙面粗糙度对渗流流态的影响。研究结果表明:裂隙结构壁面对流体的阻碍作用,使得流体在壁面附近的流动产生急剧调整,同时随着裂隙相对粗糙度的增加,在裂隙隙宽急剧变化的部位,局部伴随着旋涡的形成,导致流体内部摩擦阻力作用增大。在单位时间截面渗流流量及每一段平均隙宽相等的条件下,将本数值解与多平行板理论解进行了对比,对于相对粗糙度δ=0.01674的裂隙方案,由于多平行板理论解忽略了粗糙裂隙隙宽变化而引起的局部压降,其裂隙中线处压降产生的最大误差达到15.2%。当相对粗糙度较小时,裂隙中线处的压力与光滑平板流相类似,近似呈线性变化。随着裂隙相对粗糙度增大,压力变化偏离直线方向,且在断面由窄突然变宽的部位,压力变化偏离线性尤为显著。     

温变条件下石灰岩裂隙渗透特性实验研究

 在中等温度(25 ℃～90 ℃)和渗透压的条件下,裂隙渗透性对矿物质的溶解表现的很敏感。因此重点通过4个温度平台阶段(25 ℃,50 ℃,70 ℃,90 ℃),以温变对流量和等效水力开度的调节过程来反映温度对石灰岩裂隙渗透特性演化规律的影响。测量反应过程中岩样的渗透流量和渗出液的离子浓度,并将测量的渗流量经过经典的立方定律变换得到裂隙开度随时间的变化情况。实验结果表明：升温过程中流量和等效水力开度表现出明显增大的趋势,恒温过程中等效水力开度逐渐变小并趋于稳定;在不变的有效应力下,经历渗透压增大后,随着温度的升高,裂隙开度总体呈下降趋势。通过分析渗流量、等效水力开度和矿物离子浓度随时间变化的过程,以及它们之间的内部关系,得出温度升高能促进应力作用和渗流作用,渗流通道的局部溶解作用也随着温度的变化而改变,从而影响压力溶解和自由溶解的进程。     

ITASCA系列软件在流固耦合问题中的应用

正流体流动及其作用一直是岩土体工程领域的难点问题之一,除水力条件外,其影响主要与由应力条件、岩土体细观结构和宏观裂隙构造等主要因素决定的渗透特性相关,表现为多尺度(时间和空间)化的复杂非线性流固耦合问题。等效连续介质模型、离散介质模型和综合以上两者特点形成的双重介质模型构成了ITASCA系列软件解决流固耦合问题的理论基础:·等效连续介质模型:以FLAC系列软件为代表,基于经典Boit固结理论描述耦合作用过程;·双重介质模型:利用平板立方定律模拟流体在裂隙中的流动过程,同时纳入Boit理论考虑流体在岩块中的滤失效应。双重介质模型成为DEC、PFC和XSite软件流固耦合模拟技术的模型依据,除合理描述岩体构造特征外,可真实模拟水力作用引致破裂的萌生和扩展过程。     

应力对裂隙岩体渗流影响的研究

研究了应力变化对裂隙岩体渗流特征的影响。假设裂隙网络由等效隙宽相等的相互平行的一组裂隙组成，由于应力变化而导致的隙宽变化控制了裂隙岩体渗流的变化，据此得出了裂隙岩体渗透系数及渗流量与应力的关系式。通过裂隙岩体渗流试验，证实了关系式的正确性。利用此关系式及有限元方法计算，得出了裂隙岩体渗透系数随应力的变化值。     

岩石粗糙裂隙宽配曲线和糙配曲线

 受河流动力学中泥沙粒配曲线的启发, 提出表征岩石粗糙裂隙开度和粗糙度大小及其分布的新方法——宽配曲线和糙配曲线。特别是它们反映出来的中值隙宽、中值凸度和水力隙宽、凸度的代表特征值以及非均匀系数等,对该领域的研究有重要的意义。     

充填砂裂隙在剪切位移作用下渗流规律的实验研究

对充填砂裂隙进行控制位移的剪切渗流实验，结合充填砂的物性，揭示了岩体裂隙发生剪切位移时的渗流规律，并对剪切位移与隙宽、剪切位移与流量、隙宽与流量的关系进行了探讨，提出了砂粒的微小扰动与孔隙比（隙宽）的增加是影响裂隙流量的主要因素。     

裂隙单元修正等效渗透率模型及其验证

 采用裂隙单元表征裂隙网络,引入裂隙单元等效渗透率的概念,按照流量等效的原则计算其大小,然而阶梯状裂隙单元造成渗流流程的增加,同时压差不变导致流量的减少,为解决这一问题,用裂隙在网格中的实际流程长度与裂隙迹长之比来修正裂隙单元等效渗透率,并且针对复杂裂隙网络,对其进行预处理--删除孤立裂隙、死端裂隙、孤立裂隙簇等非连通裂隙。用此修正模型对单裂隙、相交裂隙、复杂裂隙网络进行渗流数值模拟,并与理论解及离散裂隙网络模型方法渗流结果进行比较,结果显示：研究区域下游出口总流量及出口处流量分布均取得较好一致性;同时,此裂隙单元修正等效渗透率模型也能反映出裂隙岩体渗流的非均质和各向异性。     

考虑三维形貌特征的粗糙节理渗流空腔模型研究

 为研究粗糙节理中的渗流,建立节理渗流空腔模型。将节理平面离散成1 mm×1 mm的单元格,单元格节点处的隙宽值由节理三维空腔组合形貌确定,假设单元格内渗流服从立方定理,逐一对单元格进行分析,用连续性方程求解整个节理面渗流,编制MATLAB程序进行计算。用大理岩人工节理试件沿节理长度方向将同一节理上表面相对下表面分别错开1,2,3,4,5和6 mm,形成节理不同的接触状态,计算这6种情况下的节理三维空腔组合形貌,得到不同接触状态下节理的隙宽分布和平均隙宽值。将节理渗流空腔模型和立方定理在不同接触状态下的渗流计算结果与节理试件在不同接触状态下的室内试验结果进行比较。分析结果表明：节理渗流空腔模型计算结果比立方定理更接近室内渗流试验结果,但是空腔模型计算结果和试验结果还有一定的差距,并对其误差存在的原因进行分析。     

Estimation of fracture flow considering the inhomogeneous structure of single rock fractures

Considering the safe, long-term isolation of energy byproducts, such as radioactive waste, one of the important parameters is the velocity of the groundwater flow through the void of rock masses and/or fractures. Although it is generally known that a natural rock fracture indicates a complex aperture distribution, the fracture is often ideally represented by a parallel plate model. The cubic law is applied to evaluate the hydraulic properties of fractured rock. From several previous research works, it is understood that the cubic law can be applied when the Reynolds number is less than 1.0 and that the inertia term can basically be ignored in such slow fracture flows. In this research work, two-dimensional seepage flow analyses, using the authors' proposed 2D model, in which the inertia term, the pressure term and the diffusion term are incorporated, are carried out for single fracture permeability tests under conditions which allow for the application of the cubic law. In comparing the results of the experiments with the results of the numerical simulation, the results of the simulation employing the 2D model show a good agreement with the experimental results; the 2D model can simulate the water flow in an inhomogeneous fracture more accurately than the simulation based on the local cubic law. From these simulation results, the fracture flow in an inhomogeneous structure is discussed, along with the local Reynolds number, and the resistance through the fracture geometry is considered. Consequently, under the condition of a mean Reynolds number of less than 1.0, the inertia terms do not affect the fracture flow, but the hydraulic resistance does affect the fracture flow.     

Flow on a Single Rock Fracture in the Shear Process and the Validity of the Cubic Law Examined through Experimental Results and Numerical Simulations

In order to discuss the flow through a rough surface rock fracture, direct shear and flow coupling tests on various rock fractures are carried out. Through the experimental results, the flow on the fracture and the validity of the cubic law are discussed and estimated. As a result, it is understood that the cubic law cannot sufficiently explain the experimental results, although it can partially take into account the experimental results in the Re range of 0.02 to 0.2. Moreover, a simulation by a 2D model, including the inertia term, has been carried out. The 2D model has been found to show a good performance of the fracture flow in comparison with the representative fracture flow model, namely, LCL (local cubic law). The flow on the fracture shows the channeling flow and a higher Re value. Consequently, the flow on the fracture is thought to be caused by the influence of the inertia term.     

Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

In this study,the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically.We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling.The geomechanical response of the fractured rock is simulated using a hybrid finitediscrete element model,which can capture the deformation of intact rocks,the interaction of matrix blocks,the displacement of discrete fractures and the propagation of new cracks.Under far-field stress loading,the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures.The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment.We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks.A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening,leading to greater rock mass permeability.However,a disconnected fracture network accommodates much less fracture shearing and opening,and has much lower bulk permeability.We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric(i.e.percolation parameter) of fracture networks,which yields an excellent match to the numerical results.We infer that fluid flow through a well-connected system is governed by traversing channels(forming an"in parallel" architecture) and thus equivalent permeability is sensitive to stress loading(due to stress-dependent fracture permeability),whilst fluid flow through a disconnected system is more ruled by matrix(linking isolated clusters"in series") and has much less stress dependency.     

Fracture permeability normal to bedding in layered rock masses

A two-dimensional model of fracture permeability normal to bedding in layered rock with orthogonal, bed-delimited fractures is investigated. Steady-state, saturated laminar flow is assumed. Permeability is assumed to be a function of apertures and flow path lengths in an otherwise impermeable matrix. Bedding planes or other separations between layers are modeled as fictitious interlayers to render a layered fractured rock mass equivalent to a layered porous medium. Layer, or bed-normal permeability is quantified in terms of readily available field data, using a probability model to represent fracture connectivity between layers. The bed-normal permeability is shown to depend on fracture spacing and aperture, layer spacing and bedding plane aperture, and to be scale-dependent. Numerical investigation and comparison with field data indicates that the derived expression is useful for estimating layer-normal permeability.     

An analytical solution for transient flow of Bingham viscoplastic materials in rock fractures

We present below an analytical solution to model the one-dimensional transient flow of a Bingham viscoplastic material in a fracture with parallel walls (smooth or rough) that is subjected to an applied pressure gradient. The solution models the acceleration and the deceleration of the material as the pressure gradient changes with time. Two cases are considered: a pressure gradient applied over a finite time interval and an applied pressure gradient that is constant over time. The solution is expressed in dimensionless form and can therefore be used for a wide range of Bingham viscoplastic materials. The solution is also capable of capturing the transition that takes place in a fracture between viscoplastic flow and rigid plug flow. Also, it shows the development of a rigid central layer in fractures, the extent of which depends on the fluid properties (viscosity and yield stress), the magnitude of the pressure gradient, and the fracture aperture and surface roughness. Finally, it is shown that when a pressure gradient is applied and kept constant, the solution for the fracture flow rate converges over time to a steady-state solution that can be defined as a modified cubic law. In this case, the fracture transmissivity is found to be a non-linear function of the head gradient. This solution provides a tool for a better understanding of the flow of Bingham materials in rock fractures, interfaces, and cracks.     

Experimental and numerical study of the geometrical and hydraulic characteristics of a single rock fracture during shear

Coupled shear-flow tests were conducted on two artificial rock fractures with natural rock fracture characteristics under constant normal loading boundary conditions. Numerical simulations using the 3-D Navier–Stokes equations taking account of the inertial effects of fluid were conducted using the void space geometry models obtained from the coupled shear-flow tests. The test and numerical simulation results show that the evolutions of geometrical and hydraulic characteristics of rock fracture exhibit a three-stage behavior. Transmissivity of a certain void space geometry within a fracture is related to the Reynolds number of fluid flow due to the inertial effects of fluid, which can be represented by the Navier–Stokes equations, but cannot be represented by some simplified equations, such as the cubic law, the Reynolds equation or the Stokes equations. The mechanical aperture is usually larger than the hydraulic aperture back-calculated from measured flow rate, and the difference between them is found strongly related to the geometrical characteristics of the fractures. A mathematical equation is proposed to describe the relation between hydraulic aperture and mechanical aperture by means of the ratio of the standard deviation of local mechanical aperture to its mean value, the standard deviation of local slope of fracture surface and Reynolds number.     

High resolution, non-destructive measurement and characterization of fracture apertures

We measure at high resolution the fracture aperture of several consolidated materials and statistically characterize the aperture distribution for future studies on single and multiphase flow and transport through fractured porous media. The images of the real fractures in granite and sandstone rocks are obtained with a computer aided tomography (CAT) X-ray scanner. The minimum pixel size is 0.27×0.27 mm, but fractures as small as 35 μm can be accurately measured, using a calibration standard for the variation in CT number due to the “missing rock mass” in a given pixel. The distribution of fracture apertures is best described by a lognormal function. This was also corroborated by comparing the theoretical to actual ratio of geometric mean to arithmetic mean apertures. The mechanical roughness of the fractures ranged from 0.17 to 1.5. The variability in fracture aperture displayed only short range structure, with correlation lengths of 0.8 to 7 mm at maximum, which is at most one-tenth of the minimum dimension (width) of the fracture plane. Fracture aperture distribution is rather heterogeneous. Potential flow channels can be deduced from the fracture aperture distribution.