低渗透煤层气藏中气-水两相不稳定渗流动态分析

针对低渗透煤层渗流问题,考虑启动压力梯度及其引起的动边界和动边界内吸附气解吸作用的渗流模型研究目前仅限于单相流,而更符合实际的气-水两相渗流动边界模型未见报道.本文综合考虑了煤层吸附气的解吸作用、气-水两相渗流、非达西渗流、地层应力敏感等影响因素,进行了低渗透煤层的气-水两相渗流模型研究.采用了试井技术中的"分相处理"方法,修正了两相渗流的综合压缩系数和流度,并基于含气饱和度呈线性递减分布的假设,建立了煤层气藏的气-水两相渗流耦合模型.该数学模型不仅可以描述由于低渗透煤层中渗流存在启动压力梯度而产生的可表征煤层有效动用范围随时间变化的移动边界,还可以描述煤层有效动用范围内吸附气的解吸现象以及吸附气解吸作用所引起的煤层含气饱和度的上升;为了提高模型精度,控制方程还保留了二次压力梯度项.采用了稳定的全隐式有限差分方法进行了模型的数值求解,并验证了数值计算方法的正确性,获得了模型关于瞬时井底压力与压力导数响应的双对数特征曲线,由此分析了各渗流参数的敏感性影响.本文研究结果可为低渗透煤层气藏开发的气-水两相流试井技术提供渗流力学的理论基础.     

考虑非饱和土基质吸力作用的土石坝渗流分析

传统的渗流分析主要考虑饱和区而忽略非饱和区内的渗流，本文首先对影响非饱和土渗流特性的重要物理量基质吸力进行了分析，阐述了其形成机理并推出其理论计算公式，然后在考虑基质吸力的作用下，基于饱和．非饱和渗流计算原理，采用有限元法考虑了渗流场非饱和区的影响，并以各向同性均质坝和心墙土石坝为算例，进行了计算分析。算例表明，由于基质吸力产生的虹吸作用，使得浸润线上部的非饱和区内也存在着连续的水流，否定了传统分析中浸润线是最上部流线的假设，通过分析这种水流的特点，定性的得出其对土石坝渗流稳定性的影响，对土石坝渗流分析具有一定的指导意义。     

露天矿边坡饱和渗透系数随机场数值模型研究

为分析强降雨入渗及渗透系数空间变异性对闭坑露天矿边坡渗流场的影响程度，基于非饱和渗流理论和随机场理论，采用非侵入式随机方法，通过FISH语言编写非饱和区单元饱和度、渗透系数与基质吸力的修正函数，建立饱和-非饱和渗流随机场模型，开展强降雨作用的高大陡深岩质边坡渗流特征研究。研究结果表明，修正饱和-非饱和渗流随机场模型能够准确地描述露天矿边坡的降雨入渗过程。且降雨入渗主要影响到露天矿边坡浅层渗流场。随降雨持时变化，在坡面与地下水位线之间形成包围的且逐渐缩小的非饱和区。坡面最早出现暂态饱和区且厚度逐渐增加，但增幅逐渐放缓。同时坡面点孔隙水压力最早达到稳定值零，离坡面越远的点孔隙水压力达到稳定值零的用时会越长。该结论可为闭坑露天矿边坡的地质灾害风险预测提供参考意义。     

非饱和渗流下土石坝坝坡可靠性分析

将饱和-非饱和渗流场统一起来,建立了饱和-非饱和渗流的微分方程.根据极限平衡原理,考虑基质吸力对抗剪强度的影响,采用延伸的摩尔-库仑破坏准则,建立边坡在非饱和土条件下的安全系数公式.运用饱和-非饱和渗流有限元程序,对土石坝工程进行了计算和分析,探讨了稳态和瞬态下库水位变化时,坝体内浸润面的变化、瞬态孔隙水压力分布情况.进一步采用蒙特卡罗法把渗流计算所得的瞬态孔隙水压力分布用于坝坡可靠性分析,计算坝坡的瞬态可靠指标,对土石坝在非饱和、非稳定渗流作用下,不同水位下降速度对上游坝坡可靠性的影响进行了研究.结果表明:库水位下降对坝坡产生饱和-非饱和渗流作用,渗流场受降速的影响大,库水位下降速度增大,坝坡稳定性降低;坝坡可靠指标与库水位的关系呈抛物线变化趋势,渗流浸润线开始比较陡,最后趋于平缓,而坝坡可靠指标表现为开始减小,经过一段时间达到最小值,然后逐渐增大.     

地下水位上升下黄土斜坡稳定性分析

黄土高原一些地区,由于塬上引水灌溉使得地下水位不断抬升,造成黄土滑坡频繁发生。地下水位变化严重影响着黄土斜坡的稳定性。基于饱和-非饱和渗流理论和延伸的摩尔-库伦破坏准则,结合室内饱和和非饱和试验结果,针对泾阳南塬一典型黄土斜坡,考虑地下水位上升情况下,对其进行了瞬态饱和-非饱和渗流分析;然后将计算得到的瞬态孔隙水压力分布用于斜坡的极限平衡分析。结果表明:地下水位上升对暂态渗流场和斜坡稳定性有明显影响;考虑非饱和渗流和吸力强度的边坡稳定分析方法更加符合实际情况。     

非饱和粘性土中气体渗透特征

由于气体的进入形成了非饱和带,饱和土层中的气体渗透实质上是非饱和渗透问题。本文借助基于多孔介质气体渗流理论并考虑气体压缩性的、描述非饱和土中气体渗流运动的本构方程,试验研究了上海地区饱和粘性土层中的气体渗透规律。结果表明,饱水粘性土层中,气体渗透存在起始压力值,数值为100～200kPa, 与上海地区粘土的非饱和进气值较为接近;气体渗透速度与外界所施加的"压力平方差梯度"之间存在明显的分段特征。后者可能与在充满结合水的微孔隙通道中,只有一部分结合水在气压梯度的作用下发生流动,从而形成气体通道,施加的外力越大,形成的气体通道也越大,气体渗流也越明显。非饱和粘性土中气体渗透规律的研究,对于饱和粘土中的气压法施工,具有重要的理论与工程实践意义。     

饱和多孔介质中的混合有限元法和有限应变下应变局部化分析

对基于Biot理论的饱和多孔介质中动力-渗流耦合分析提出了一个耦合场混合元．固相位移、应变和有效应力以及流相压力、压力梯度和Darcy速度在单元内均处理为独立变量分别插值．基于胡海昌-Washizu三变量广义变分原理给出的饱和多孔介质动力-渗流耦合问题控制方程的单元弱形式，导出了单元公式．进一步导出了考虑压力相关非关联塑性的非线性单元公式和发展了相应的一致性算法．对几何非线性分析，采用了共旋公式途径．数值结果例题显示所发展耦合场混合元模拟大应变下由应变软化引起以应变局部化为特征的渐进破坏现象的性能．     

页岩基质解吸-扩散-渗流耦合实验及数学模型

采用川南地区龙马溪组页岩样品,设计了页岩基质解吸-扩散-渗流耦合物理模拟实验,揭示了页岩基质气体流动特征以及压力传播规律.推导了页岩气解吸-扩散-渗流耦合数学模型并且利用有限差分法对数学模型进行数值求解,与实验结果相比较表明该数学模型能够很好地描述气体在页岩基质中的流动规律.同时对页岩基质气体流动的影响因素进行了分析,认为页岩基质的渗透率、扩散系数、解吸附常数等因素均能影响页岩基质气体的流量和压力传播规律,在页岩气藏的开发过程中需要考虑这些参数的影响,该数学模型为页岩气井产能计算提供了更准确的计算方法.     

库水位上升条件下边坡渗流场模拟

库水位的上升将导致边坡体内渗流场的变化,进而引起边坡的失稳破坏。边坡体内的渗流场是饱和渗流与非饱和渗流共同作用的结果,通过对库水位上升过程中坡体内地下水位线的位置变化及相应的渗流场参数的计算表明:由于上部松散堆积体与基岩渗透系数的差异,基岩内地下水位线的抬升速度明显滞后于上部松散堆积体;体积含水量、压力水头、流速和水力梯度等渗流参量在基岩中的变化幅度较小,而在上部松散堆积体中的变化幅度则相对较大。这说明库水位上升对上部松散堆积体的影响相对来说还是比较明显的。     

海陆过渡相页岩气藏不稳定渗流数学模型

海陆过渡相页岩常与煤层和砂岩呈互层状产出, 储层连续性较差、横向变化快、非均质性强, 水力压裂技术是其获得经济产量的关键手段. 然而, 目前缺乏有效的海陆过渡相页岩气藏不稳定渗流数学模型, 对其渗流特征分析及储层参数评价不利. 针对这一问题, 首先建立海陆过渡相页岩气藏压裂直井渗流数学模型, 其次采用径向复合模型来反映强非均质性, 采用Langmuir等温吸附方程来描述气体的解吸和吸附, 分别采用双重孔隙模型和边界元模型模拟天然裂缝和水力裂缝, 建立并求解径向非均质的页岩气藏压裂直井不稳定渗流数学模型, 分析海陆过渡相页岩气藏不稳定渗流特征, 并进行数值模拟验证和模型分析应用. 分析结果表明, 海陆过渡相页岩气藏不稳定渗流特征包括流动早期阶段、双线性流、线性流、内区径向流、页岩气解吸、内外过渡段、外区径向流及边界控制阶段. 将本模型应用在海陆过渡相页岩气试井过程中, 实际资料拟合效果较好, 其研究成果可为同类页岩气藏的压裂评价提供一些理论支撑, 具有较好应用前景.      

Nonlinear Coupled Mathematical Model for Solid Deformation and Gas Seepage in Fractured Media

Based on detailed investigation into the interactional physical mechanism of solid deformations and gas seepage in rock matrix and fracture, a nonlinear coupled mathematical model of solid deformation and gas seepage is put forward and the FEM model is built up to carry out numerical analysis. The coupled interaction laws between solid deformations and gas seepage in rock matrix and fractures has been emphasized in the model, which is a vital progress for coupled mathematical model of solid deformation and gas seepage of rock mass media. As an example, the methane extraction in fractured coal seam has been numerically simulated. By analyzing the simulation results, the law of methane migration and exchange in rock matrix and fractures is interpreted.     

FLUID-SOLID COUPLING MATHEMATICAL MODEL OF CONTAMINANT TRANSPORT IN UNSATURATED ZONE AND ITS ASYMPTOTICAL SOLUTION

IntroductionThetransportofcontaminantsinunsaturatedzonehascausedmuchattention .Inearly1960s,contaminationproblemsofsoilandgroundwaterhadbeenstudiedathomeandabroad[1].Andinrecentyears ,thetransformationandtransportationofcontaminantshavebeendeeplystudiedinthefieldsofhydrogeology ,petroleumengineering ,environmentalengineeringandsoon[2 ,3].Somecontaminanttransportmodelshavebeenpresentedsofar.Forexample ,Paker[4 ]etal.presentedaconstitutivemodelgoverningparametersofwater,gasandcontaminantswhenth…     

Flow of Coal-Bed Methane to a Gallery

Coal-seam methane reservoirs have a number of unique feature compared to conventional porous or fractured gas reservoirs. We propose a simplified mathematical model of methane movement in a coal seam taking into account the following features: a relatively regular cleat system, adsorptive methane storage, an extremely slow mechanism of methane release from the coal matrix into cleats and a significant change of permeability due to desorption.Parameters of the model have been combined into a few dimensionless complexes which are estimated to an order of magnitude. The simplicity of the model allows us to fully investigate the influence of each parameter on the production characteristics of the coal seam. We show that the reference time of methane release from the coal matrix into cleats – the parameter which is most poorly investigated – may have a critical influence on the overall methane production.     

Comprehensive Seepage Simulation of Fluid Flow in Multi-scaled Shale Gas Reservoirs

Shale gas seepage behaviour is a multi-field/-scale problem and makes transient pressure analysis a very challenging task. Non-Darcy flow in nanopores is prominent due to the broken of continuity hypothesis. Slippage effect and Knudsen diffusion are two important seepage mechanisms in nanopores, while recent studies show surface diffusion is another important transporting mechanism on surface of nanopores. Porous kerogen system contains large amounts of dissolved gas, which should not be overlooked. In this study, a comprehensive mathematical model was established by pseudo-quadruple porosity medium conception, coupling the effects of slippage flow, Knudsen diffusion, surface diffusion, ad-/desorption and gas transferring from kerogen to nanopore system, while fluid flow in fractures/macropores is described by Darcy’s law. Transient pressure behaviours of a multiple fractured horizontal well in box-shaped shale gas reservoir were studied, with nine possible flow regimes divided and parameters sensitivity analysed. Adsorbed constant and dissolved constant were defined to reflect the amount of adsorbed gas and dissolved gas, respectively. Research shows that adsorbed gas and dissolved gas are two important gas storage forms, neither of which should be neglected. The study can not only help us understand fluid flow mechanisms in nanopores from microscopic perspective, but enable us to analyse production performance and determine key operational parameters from macroscopic perspective.     

双重孔隙介质THMM耦合模型及其有限元分析

建立了一种饱和-非饱和遍有节理岩体的双重孔隙-裂隙介质热-水-应力-迁移耦合模型,其特点是应力场和温度场是单一的,但具有不同的孔隙渗流场、裂隙渗流场和孔隙浓度场、裂隙浓度场,以及可考虑裂隙的组数、间距、方向、连通率和刚度对本构关系的影响;并开发了相应的二维有限元程序.针对一个假定的高放废物地质处置库,就岩体为非饱和双重孔隙-裂隙介质和放射性核素泄漏的情况进行了数值分析,考察了岩体中的温度、负孔隙水压力、饱和度、地下水流速、核素浓度和主应力的状态.结果显示:孔隙和裂隙中的负水压力及核素浓度呈现不同的变化、分布;尽管裂隙水饱和度平均仅为孔隙水饱和度的1/10,但因裂隙的渗透系数比孔隙的渗透系数大4个数量级,故裂隙中地下水的流速约是孔隙中相应值的3倍;孔隙和裂隙中核素浓度的量值接近.     

Multi-scale Fractured Coal Gas–Solid Coupling Model and Its Applications in Engineering Projects

With coal mining entering the geological environment of “high stress, rich gas, strong adsorption and low permeability,” the difficulty of joint coal and gas extraction clearly augments, the risk of solid–gas coupling dynamic disasters greatly increases, and the underlying mechanisms become more complex. In this paper, based on the characteristics of coal’s multi-scale structure and spatiotemporal variation, the multi-scale fractured coal gas–solid coupling model (MSFM) was built. In this model, the interaction between coal matrix and its fractures and the mechanical characteristics of gas-bearing coal were considered, as well as their coupling relationship. By MATLAB software, the stress–damage–seepage numerical computation programs were developed, which were applied into Comsol Multiphysics to simulate gas flow caused by coal mining. The simulation results showed the spatial variability of coal elastic modulus and cross-flow behaviors of coal seam gas, which were superior to the results of traditional gas–solid coupling model. And the numerical results obtained from MSFM were closer to the measured results in field, while the computation results of traditional model were slightly higher than the measured results. Furthermore, the MSFM in a large scale was verified by field engineering project.     

高温高压气体在低含水率介质中迁移的数值模拟

针对气液两相非等温渗流模型高度非线性的特点，发展了适宜的数值离散方法。根据相态转换准则和控制方程的性质，采用最低饱和度法简化算法。空间离散方面，使用有限体积法；时间离散方面，设计了一套包含合理求解顺序的Picard迭代法，解决了方程组强耦合的问题。利用上述数值方法对高温高压气体的迁移行为进行数值模拟，证明了气体在低含水率介质和等效孔隙度的干燥介质内的运动基本一致，并分析了空腔内的气液相态转变过程。在此基础上，研究了多孔介质孔隙度和渗透率对气体压强演化和示踪气体迁移的影响。研究表明，孔隙度越小（相同渗透率）、渗透率越高（相同孔隙度），示踪气体的迁移距离越远，并给出了估算不同孔隙度和渗透率下迁移距离的半经验公式。     

A Model for Flow and Deformation in Unsaturated Swelling Porous Media

A thermomechanical theory for multiphase transport in unsaturated swelling porous media is developed on the basis of Hybrid Mixture Theory (saturated systems can also be modeled as a special case of this general theory). The aim is to comprehensively and non-empirically describe the effect of viscoelastic deformation on fluid transport (and vice versa) for swelling porous materials. Three phases are considered in the system: the swelling solid matrix s, liquid l, and air a. The Coleman–Noll procedure is used to obtain the restrictions on the form of the constitutive equations. The form of Darcy’s law for the fluid phase, which takes into account both Fickian and non-Fickian transport, is slightly different from the forms obtained by other researchers though all the terms have been included. When the fluid phases interact with the swelling solid porous matrix, deformation occurs. Viscoelastic large deformation of the solid matrix is investigated. A simple form of differential-integral equation is obtained for the fluid transport under isothermal conditions, which can be coupled with the deformation of the solid matrix to solve for transport in an unsaturated system. The modeling theory thus developed, which involves two-way coupling of the viscoelastic solid deformation and fluid transport, can be applied to study the processing of biopolymers, for example, soaking of foodstuffs and stress-crack predictions. Moreover, extension and modification of this modeling theory can be applied to study a vast variety of problems, such as drying of gels, consolidation of clays, drug delivery, and absorption of liquids in diapers.     

GAS-WATER TWO-PHASE DIFFUSION MODEL FOR UNSATURATED LAYERED COVER SYSTEMS

为了研究垃圾填埋场内部生物降解产生的气体(以甲烷为主) 和氧气等在非饱和覆盖层中的运移规律,建立了气体在成层非饱和覆盖层中的一维扩散模型. 该模型考虑了覆盖层含水量瞬态变化和气体扩散的耦合作用. 采用有限元多物理场耦合分析软件COMSOL Multiphysics 4.3进行求解. 其计算结果与前人实验结果能符合得很好. 研究结果表明,对典型垃圾填埋场中厚度为1m的非饱和黏土覆盖层,含水量变化对覆盖层中气体污染物的扩散运移有显著影响. 与含水量稳态变化情况相比,20 d到100 d的运移时间内同一覆盖层深度处耦合含水量作用下,气体相对浓度可以相差接近8倍. 含水量是气体在非饱和覆盖层中不稳定运移的重要影响因素.     

Simulation of Pressure Transient Behavior for Asymmetrically Finite-Conductivity Fractured Wells in Coal Reservoirs

Based on Fick’s law in matrix and Darcy flow in cleats and hydraulic fractures, a new semi-analytical model considering the effects of boundary conditions was presented to investigate pressure transient behavior for asymmetrically fractured wells in coal reservoirs. The new model is more accurate than previous model proposed by Anbarci and Ertekin, SPE annual technical conference and exhibition, New Orleans, 27–30 Sept 1998 because new model is expressed in the form of integral expressions and is validated well through numerical simulation. (1) In this paper, the effects of parameters including fracture conductivity, coal reservoir porosity and permeability, fracture asymmetry factor, sorption time constant, fracture half-length, and coalbed methane (CBM) viscosity on bottomhole pressure behavior were discussed in detail. (2) Type curves were established to analyze both transient pressure behavior and flow characteristics in CBM reservoir. According to the characteristics of dimensionless pseudo pressure derivative curves, the process of the flow for fractured CBM wells was divided into six sub-stages. (3) This paper showed the comparison of transient steady state and pseudo steady state models. (4) The effects of parameters including transfer coefficient, wellbore storage coefficient, storage coefficient of cleat, fracture conductivity, fracture asymmetry factor, and rate coefficient on the shape of type curves were also discussed in detail, indicating that it is necessary to keep a bigger fracture conductivity and fracture symmetry for enhancing well production and reducing pressure depletion during the hydraulic fracturing design.