真三轴条件下原状黄土的非线性本构模型

为了研究复杂应力条件下非饱和原状黄土的非线性本构模型,通过考虑真三轴条件下中主应力和吸力对大主应变的影响,提出了一个考虑中主应力影响的非饱和原状黄土的非线性本构模型。该模型包含12个参数,均可由常含水率非饱和土真三轴试验确定。研究了中主应力对土体变形和强度的影响,探讨了模型参数的变化规律。初始吸力一定时,土体的黏聚力随着中主应力的增大而增大,而内摩擦角随着中主应力的增大呈线性减小趋势。当吸力小于162 kPa时,吸力摩擦角等于21.8°;当吸力大于162 kPa时,吸力摩擦角等于5.5°。初始吸力相同时,不同中主应力条件下土体的体积模量可取为常数。利用该模型预估了常含水率真三轴等向固结试验的吸力变化规律,理论结果与试验结果比较接近。但是对于真三轴剪切试验吸力的预估还需要进行深入探讨。     

NaCl溶液饱和膨胀土的压缩特性及其微观机制

孔隙溶液浓度及组份改变会影响膨胀土颗粒间作用力,改变微观孔隙结构,从而影响土体的物理力学特性。为此,以宁明膨胀土为研究对象,采用不同浓度NaCl溶液制备泥浆预固结重塑样开展一维压缩试验和压汞试验,研究化学作用对重塑天然膨胀土的压缩性和微观孔隙结构的影响规律。结果表明：随着渗透吸力增加,颗粒间水化能力降低,物理化学力使土颗粒由分散状态转变为集聚体状态,形成了集聚体内孔和集聚体间孔,土体表现出双峰孔隙分布特征。预固结样（压力为20 kPa）的初始孔隙比随渗透吸力增加而减小,进而导致固结屈服应力增加;但是渗透吸力对压缩性影响不大,压缩指数和回弹指数基本不变。此外,利用固结系数计算了土体的渗透系数,随着竖向压力增加渗透系数降低;当竖向压力小于200 kPa时,随着渗透吸力增加,渗透系数先增加后减小,但是竖向压力超过200 kPa后,渗透系数变化不大。分析发现,渗透吸力增加导致大孔隙增加,渗透系数增加,但同时密实度增加会导致渗透系数降低,低竖向压力下渗透性受密实度和微观孔隙结构变化耦合作用控制。     

Thermal effects on fluid flow and hydraulic fracturing from wellbores and cavities in low-permeability formations

The coupled heat-fluid-stress problem of circular wellbore or spherical cavity subjected to a constant temperature change and a constant fluid flow rate is considered. Transient analytical solutions for temperature, pore pressure and stress are developed by coupling conductive heat transfer with Darcy fluid flow in a poroelastic medium. They are applicable to low permeability porous media suitable for liquid-waste disposal and also simulating reservoir for enhanced oil recovery, where conduction dominates the heat transfer process. A full range of solutions is presented showing separately the effects of temperature and fluid flow on pore pressure and stress development. It is shown that injection of warm fluid can be used to restrict fracture development around wellbores and cavities and generally to optimize a fluid injection operation. Both the limitations of the solutions and the convective flow effect are addressed. Copyright © 1999 John Wiley & Sons, Ltd.     

Numerical Modeling and Spatial Moment Analysis of Thermal Fronts in a Coupled Fracture-Skin-Matrix System

In this study, the behavior of thermal fronts along the fracture is studied in the presence of fracture-skin in a coupled fracture-matrix system. Cold water is injected into the fracture, which advances gradually towards production well, while extracting heat from the surrounding reservoir matrix. The heat conduction into the fracture-skin and the rock-matrix from the high permeability fracture is assumed to be one dimensional perpendicular to the axis of fluid flow along the fracture. Constant temperature cold water is injected through an injection well at the fracture inlet. The fluid flow takes place along the horizontal fracture which ensures connectivity between the injection and production wells. Since the rock-matrix is assumed to be tight, the permeability of fracture-skin as well as the rock-matrix is neglected. The present study focuses on the heat flux transfer at the fracture-skin interface as against the earlier studies on fracture-matrix interface, and the sensitivity of additional heterogeneity in the form of fracture skin in a conventional fracture-matrix coupled system is studied. The behavior of thermal fronts for various thermal conductivity values of the fracture-skin and rock-matrix is analyzed. Spatial moment analysis is performed on the thermal distribution profiles resulting from numerical studies in order to investigate the impact on mobility and dispersion behavior of the fluid in the presence of fracture-skin. The presence of fracture skin affects the heat transfer significantly in the coupled fracture-matrix system. The lower order spatial moments indicate that the effective thermal velocity increases with increase in skin thermal conductivity and a significant thermal dispersion is observed at the inlet of the fracture owing to the high thermal conductivity of the fracture-skin at the early stages. Furthermore the higher spatial moments indicate that the asymmetricity increases with decrease in skin thermal conductivity unlike the case with half fracture aperture and fluid velocity and the kurtosis is maximum with higher skin thermal conductivity which implies enhanced heat extraction from the fracture-skin into the fracture. Results suggest that the amount of heat extraction by the circulating fluid within the fracture from the reservoir not only depends on the rock-matrix module of the reservoir characteristics but also the fracture-skin characteristics of the system and subsequently influence the reservoir efficiency.     

考虑温度效应的延安新区压实黄土全吸力范围持水和渗透特性研究

干旱和半干旱黄土地区的压实土路基、垃圾填埋场黄土盖层等的持水特性和渗透特性均受温度影响。为了探究温度对全吸力范围内压实黄土水力特性的影响规律和内在机制,基于滤纸法和自行研发的小土柱试验装置对不同温度下延安新区压实黄土的土-水特征曲线(SWCC)以及渗透性曲线(HCC)进行测定。结果表明:在黄土SWCC的高吸力段体积含水率随温度升高而减小,低吸力段受温度影响不明显;黄土渗透曲线则是在低吸力段受温度影响显著,温度越高渗透性越强,当土体吸力增大或体积含水率减小至一定值后,温度对土体渗透性的影响可忽略不计。基于此结果,将改进的van Genuchten(VG)模型作为可考虑温度影响的SWCC预测函数,通过拟合实测数据得出温度相关参数并进行验证;对于渗透性曲线,以90 kPa基质吸力为分段点,在低吸力段和高吸力段分别采用统计模型和幂函数进行分段预测,能够得到与实测数据吻合较好的预测结果。     

Optimizing the design of a geothermal district heating and cooling system located at a flooded mine in Canada

Flooded underground mines are attractive for groundwater heat pump systems, as the voids created during mining operations enhance the subsurface permeability and storage capacity, which allows the extraction of significant volumes of groundwater without requiring extensive drilling. Heat exchange at a flooded mine is, however, difficult to predict because of the complex geometry of the underground network of tunnels. A case study is presented here to demonstrate that numerical simulations of groundwater flow and heat transfer can help assess production temperatures required to optimize the design of a heat pump system that uses mine water. A 3D numerical model was developed for the Gaspé Mines located in Murdochville, Canada, where a district heating and cooling system is being studied. The underground mining tunnels and shafts are represented in the model with 1D elements whose flow and heat transfer contributions are superimposed to those of the 3D porous medium. The numerical model is calibrated to simultaneously reproduce the groundwater rebound that occurred when the mine closed and the drawdown measured during a pumping test conducted in a former mining shaft. Predictive simulations over a period of 50 years are subsequently performed to minimize pumping rate and determine maximum heat extraction rate.     

不同压实度和基质吸力的粉煤灰三轴试验研究

粉煤灰的非饱和抗剪强度是准确分析非饱和状态灰坝的稳定性所必需的,但目前对其研究积累较少。通过控制不同压实度、不同基质吸力、不同净围压对粉煤灰进行一系列非饱和土三轴试验,探讨压实度和基质吸力对粉煤灰应力?应变关系曲线和强度参数的影响。研究结果表明,基质吸力较低时粉煤灰的应力?应变曲线没有峰值,呈硬化型;基质吸力逐渐增大时应力?应变曲线有明显的峰值,呈软化型;压实度越大,非饱和粉煤灰试样的应力?应变曲线的软化特征越明显,总黏聚力和内摩擦角均越大;非饱和粉煤灰的总黏聚力随着基质吸力的增大而逐渐增大,增大速率逐渐变缓,最后趋于稳定;不同基质吸力粉煤灰试样的内摩擦角变化不大,并且都近似等于饱和试样的内摩擦角;随着基质吸力的增大,吸力内摩擦角对粉煤灰试样的抗剪强度的贡献越来越小。研究得出的非饱和粉煤灰的抗剪强度特性对灰坝的设计和稳定性分析具有理论意义和工程实用价值。     

压实黄土非饱和渗透系数试验研究

压实黄土非饱和渗透特性具有重要的工程意义。对不同干密度的压实黄土试样,采用非饱和导水率测定系统进行非饱和渗透系数量测,得到不同干密度压实黄土的渗透系数与吸力的关系。试验结果表明：渗透系数随吸力的增大而成非线性减小,随干密度的减小而增大;在低吸力时,干密度对渗透系数的影响较大;在较高吸力时,干密度对渗透系数的影响较小。进一步给出了压实黄土的渗透系数模型并进行回归分析,得到各Wind幂函数模型参数与干密度的关系,建立了基于干密度的压实黄土渗透系数的确定方法。     

基于极限平衡法的现浇X形桩群桩负摩阻力计算分析

软弱土及湿陷性黄土等地区桩基负摩阻力计算是桩基础设计与施工必须考虑的问题之一,目前针对群桩负摩阻力计算方面的研究相对较少。采用有效应力法计算单桩负摩阻力,基于极限平衡原理计算负摩阻力群桩效应系数,从而得到群桩负摩阻力计算公式。结合现场试验结果,对现浇X形桩群桩负摩阻力及桩身下拽力进行计算分析,并讨论了群桩基础中各基桩位置对桩身下拽力的影响,以及群桩效应系数随相关影响因素的变化规律。研究结果表明,文中计算结果与现场实测结果具有良好的一致性;地面堆载等级越高,群桩所受的桩身下拽力越大,且群桩效应系数随着桩间距的增大而增大,随着负摩阻力系数和中性点深度比的增大而减小。     

岩石渗流-流变耦合的试验研究

通过对试验设备与工艺的改进,发展出一套比较有效的渗流-流变耦合试验方法。以多孔隙石灰岩为对象,研究了不同应力及水压作用下岩石试件的流变力学特性。试验结果显示,试件的应变总体上随偏应力及水压的提高而加大。在低应力作用下,垂向应变的变化量最大。提高应力后,渗流方向应变的增加趋势显著超过垂向。试件的渗透系数与体积变化具有相似规律,在低应力作用下,试件体积不断缩小,渗透系数也随之减小,此时增大水压会使试件进一步压缩。应力增大到某值后,体积逐步扩大而出现扩容,渗透系数也随之增大,此时增大水压会使扩容进一步增大。在各级应力作用下,试件的流变速率均有减速及稳定阶段。当应力较大时,会出现加速流变阶段导致破坏。     

Performances of two instrumented laboratory models for the study of rainfall infiltration into unsaturated soils

Rainfall infiltration and suction variation in unsaturated soils must be taken into consideration in the analysis of most slope stability problems, particularly in the tropical regions where the annual precipitation is high. The process of rainfall infiltration into unsaturated soils is an extremely complex problem attributed to the non-linearity of the hydraulic property functions of the unsaturated soils. This paper describes in detail two instrumented laboratory models, i.e. one-dimensional soil column, and two-dimensional slope model used to provide experimental evidences for the transient suction variations in the unsaturated soils under certain rainfall conditions. The performances of the laboratory models were tested on four typical types of residual soils, i.e. sand-gravel, silty gravel, sandy silt, and silt (kaolin), and a two-layered soil system, i.e. sandy silt underlain by silty gravel. The results showed that the suction distributions for the single-layered homogeneous soils obtained from the simpler one-dimensional soil column were almost identical to that of two-dimensional slope model. However, the two-dimensional slope model should be employed for the two-layered soil system because of the dominant effect of the lateral flow mechanism. The capillary barrier effect was observed when a less permeable soil layer was underlain by a more permeable soil layer. The minimum suction value in soil is governed by the rainfall intensity, rainfall duration, and the saturated permeability of soil. The infiltration rate of the fine-grained soils that subjected to shrink and crack was independent of the soil permeability, but was significantly governed by the preferential flows developed in the soils.     

Consolidation around a heat source buried deep in a porous thermoelastic medium with anisotropic flow properties

A solution is derived for the heat flow and consolidation which occur when a heat source is buried deep in a porous thermoelastic soil having anisotropic flow properties. This solution is used to examine the pore pressure generation and dissipation near both point and cylindrical heat sources. An increase in temperature will tend to generate an increase in excess pore pressure. However, the pore water will tend to flow from regions of high excess pore pressure to regions of low excess pore pressure, and so consolidation will occur, and temperature-generated excess pore pressures will tend to dissipate. Many natural soils exhibit horizontal layering and so have a higher horizontal than vertical permeability. It is shown that in soils the excess pore pressure generated by a heat source is significantly less than that in an isotropic soil having an equal vertical permeability.     

The flow and heat transfer characteristics of compressed air in high-pressure air injection wells

High-pressure air injection (HPAI) is a significant enhanced oil recovery (EOR) technology of light oils especially in deep, thin, low-permeability reservoirs. The flow and heat transfer behaviors of compressed air in wellbore is essential to maximize performance of air in EOR. Due to strong compressibility of air and high injection pressure, wellbore temperature and pressure are greatly affected by friction and gas compression. However, the available models of wellbore flow and heat transfer are only accurate for thermal fluid, such as saturated steam and superheated steam, injected at relatively low pressure and high temperature. In this paper, a novel model is proposed to characterize wellbore pressure and temperature distribution for HPAI wells with consideration of dynamic behaviors of injected air. Flow and heat transfer in depth direction are coupled with air properties by iterative technique, and heat transfer in radial direction is treated as steady state in wellbore and transient state in formation. The mathematical model is solved by employing finite difference method and it is validated by field data. Then, integrated analyses of flowing pressure, heat transfer mechanism, and interaction between pressure and temperature are conducted. Results indicate that (1) as well depth increases, temperature difference between formation and air tends to become constant, and the radial heat transfer tends to reach an equilibrium state. The higher the flow rate is, the deeper the equilibrium depth is. (2) Air temperature is dominated by heat transmission from formation at low flow rates and dominated by frictional heat and gas compression effect at high flow rates. Fictional heat begins to affect air temperature at an injection rate beyond the critical value, while gas compression effect can increase air temperature in the whole calculated injection rate range. (3) Interaction between wellbore temperature and pressure is mainly achieved by altering air density. The effect of injection pressure on air temperature can be negligible, while the influence of injection temperature shows strong dependency on injection rate.     

路堤下刚性桩复合地基桩-土应力比计算

路堤下刚性桩复合地基普遍存在的负摩阻力对桩-土相互作用特性影响显著。基于极限剪切位移受应力水平影响较小而受试样尺寸影响较大的试验结果,提出了桩-土界面剪切刚度随应力水平变化的等单位长度极限剪切位移理想弹塑性模型,针对桩-土相互作用的上部负摩阻力塑性区、中部协调变形弹性区和下部摩擦承载塑性区3区段模式,建立了塑性区非均匀、弹性区非线性的摩阻力分布图式;以均布路堤荷载下等桩长复合地基中单桩等效加固单元体为研究对象,利用单元体荷载传递微分方程,结合桩-土-垫层压缩变形协调条件,推导出了中性面位置及桩-土应力比的解析表达式,分析了路堤荷载及垫层柔度系数的影响。研究表明：中性面位置随垫层柔度系数增加而下移,随路堤荷载增大先下移后上抬;桩-土应力比随垫层柔度系数增加而减小,随路堤荷载提高先增大后降低,据此提出了采用最大桩-土应力比进行桩体布置和垫层设计的技术思路。     

Analytical solutions for transient temperature distribution in a geothermal reservoir due to cold water injection

An analytical solution to describe the transient temperature distribution in a geothermal reservoir in response to injection of cold water is presented. The reservoir is composed of a confined aquifer, sandwiched between rocks of different thermo-geological properties. The heat transport processes considered are advection, longitudinal conduction in the geothermal aquifer, and the conductive heat transfer to the underlying and overlying rocks of different geological properties. The one-dimensional heat transfer equation has been solved using the Laplace transform with the assumption of constant density and thermal properties of both rock and fluid. Two simple solutions are derived afterwards, first neglecting the longitudinal conductive heat transport and then heat transport to confining rocks. Results show that heat loss to the confining rock layers plays a vital role in slowing down the cooling of the reservoir. The influence of some parameters, e.g. the volumetric injection rate, the longitudinal thermal conductivity and the porosity of the porous media, on the transient heat transport phenomenon is judged by observing the variation of the transient temperature distribution with different values of the parameters. The effects of injection rate and thermal conductivity have been found to be profound on the results.     

井内嵌入同轴多孔与固体圆柱地下水渗流流型分析

对二维无限大多孔介质内单向均匀水平流垂直绕过“固体小圆柱-多孔介质环-水环-多孔介质”复杂四层结构下的流场进行了解析求解。内、外多孔介质区域均采用Brinkman模型,纯流体水环采用Stokes模型,通过耦合界面间的质量、动量守恒关系得到了各区域流函数的通用表达式。在此基础上分析了不同几何参数,不同内、外多孔介质渗透系数情况下,圆柱外绕流的流型变化;着重研究了水环间隙以及内、外多孔介质渗透系数的变化对流型及横向、纵向速度分布的影响。结果表明:外部多孔区流型主要受控于外部渗透系数;水环间隙宽度对水环内速度峰值影响较大;内部渗透系数增加到某一临界值情况下,横截面速度分布从阶梯形变为抛物形,即“穿透”现象。研究结果对有类似结构的地埋管换热器、地下水污染物吸收装置、地下水测速装置等的设计研发有理论指导意义。     

A three‐dimensional staggered finite element approach for random parametric modeling of thermo‐hygral coupled phenomena in porous media

The aim of this paper is to present a three‐dimensional (3D) finite element modeling of heat and mass transfer phenomena in partially saturated open porous media with random fields of material properties. Randomness leads to transfer processes within the porous medium that naturally need a full 3D modeling for any quantitative assessment of these processes. Nevertheless, the counterpart of 3D modeling is a significant increase in computations cost. Therefore, a staggered solution strategy is adopted which permits to solve the equations sequentially. This appropriate partitioning reduces the size of the discretized problem to be solved at each time step. It is based on a specific iterative algorithm to account for the interaction between all the transfer processes. Accordingly, a suitable linearization of mass convective boundary conditions, consistent with the staggered algorithm, is also derived. After some validation tests, the 3D numerical model is used for studying the drying process of a cementitious material with regard to its intrinsic permeability randomness. Copyright © 2011 John Wiley & Sons, Ltd.     

非饱和重塑黏土渗透性试验研究

工程中常常遇到非饱和土渗流问题,其渗透性是非饱和土的重要研究内容。采用常规压力板仪详细研究了重塑黏土在3种不同击实功和5种不同击实含水率下的土-水特征曲线,并用变水头法测量了相应的饱和土渗透系数。结合土水曲线和饱和渗透系数,基于Van Genuchten模型和Fredlund模型,详细探讨了重塑非饱和重塑黏土在不同击实条件下的渗透性。结果表明,土样饱和渗透系数随着击实含水率的增大而减小,且随着基质吸力的增大土样渗透性差别逐渐减小。当试样吸力达到1 MPa时,可认为不同击实含水率下试样的渗透性基本相同;在轻型击实和简化轻型击实下,试样渗透性很相似。重型击实功下试样的渗透系数和轻型击实下土样渗透系数的比值 随着吸力的增大而逐渐增大,在饱和状态时比值约为1/100,当吸力达到1 MPa时比值逐渐稳定在1/10右。     

花岗岩粗糙单裂隙对流换热特性的数值模拟

在干热岩储层中开采地热能,往往需要对储层进行人工水力压裂以形成贯穿的换热通道。然而,热储中的对流换热对干热岩的采热率有重要影响,经过人工刺激的储层会形成几何形态各异的裂隙面,而裂隙粗糙程度的不同则会引起换热性能的显著差异。因此,选取4条Barton的经典岩石裂隙粗糙度曲线,在试验室条件下建立一个单裂隙对流换热模型。详细分析了花岗岩粗糙裂隙中热工质的换热特性。结果表明：局部对流换热系数沿着裂隙长度方向逐渐降低;节理粗糙系数JRC值越大,平均对流换热系数就越大,表明换热性能越好;局部对流换热系数的分布与JRC曲线的几何轮廓形态有很好的相关性,波峰波谷的变化趋势相一致;相对于温度而言,高流速对局部对流换热系数具有放大效应,流速越大,局部对流换热系数波动越大。     

基质吸力对非饱和重塑黄土抗剪强度的贡献

非饱和土抗剪强度随饱和度降低、基质吸力增大而提高.然而,不同类型非饱和土中,基质吸力对抗剪强度的贡献却显著不同.本文通过室内试验,探讨了基质吸力对非饱和重塑黄土抗剪强度贡献的基本特征.研究结果表明,非饱和重塑黄土抗剪强度随着基质吸力的增大,表现为非线性的和明显的4个阶段性增、降和逐渐稳定特征,且分段特征值与土-水特征曲...