低渗储层微观孔隙结构研究进展

低渗储层微观非均质性强,微观孔隙结构高精度表征成为低渗油藏开发的难点。近年来,国内低渗储层微观孔隙结构的表征技术开始从定性向定量、二维向三维转变,出现了恒速压汞、核磁共振等定量表征技术和X-CT、FIB-SEM等三维重构技术。理论方面也有孔隙结构模拟和分形学的发展。在阅读大量文献的基础上,指出国内低渗储层微观孔隙结构高精度表征处于起步阶段,定量测试和三维重构技术应用还不广泛,孔隙结构模拟和分形理论研究还不深入,有限样品点测试结果难以延伸到整个储层微观孔隙结构的预测。未来低渗储层微观孔隙结构的精细表征主要在于定性和定量相结合、数字岩心和孔隙结构模拟相结合、恒速压汞技术和分形理论相结合等多种理论方法综合应用。     

分形特征在孔渗计算中应用的意义研究

储层孔隙与喉道空间分布具有统计自相似性特点,孔隙结构特征也可以用分形几何学进行描述。与其他方法相比,岩石微观孔隙结构被我们用分形的方法看作复杂无规律的整体进行研究,这样,得到的计算结果更符合真实情况。本文基于压汞实验资料,计算出样品的孔隙结构分形维数。经过深入研究发现,孔隙结构分形维数与储层参数之间具有一定相关性,基于孔隙结构分形维数的储层物性参数建模能够取得较好效果。研究表明基于压汞资料所求取的,孔隙结构分形维数可为储层孔隙结构微观非均质性研究提供有效方法,并为进一步储层评价奠定基础。     

基于REV的孔隙型多孔介质导热分析模型

多孔介质内部组成结构复杂，热质传递过程多变，如何针对多孔介质的微观孔隙分布特性，建立更加精准的分析模型，有待深入研究。基于表征单元体（representative elementary volume，REV）概念，提出了孔隙型多孔介质的两种微观物理模型，即空心骨架基元模型和实心颗粒基元模型，分别建立了相应的热导率计算公式。针对孔隙内的微观结构特征，采用分形方法，对两种基元模型进行了分形修正，更好地表征了微观孔隙结构。基于所建模型，进行了模拟计算，探讨了相关参数对多孔介质导热特性的影响。通过自主研发的实验装置进行了相关实验，验证了模型的准确性。     

基于随机场和数学形态学的水泥基材料三维孔隙结构建模研究

孔隙介质在微米及纳米尺度的孔隙结构决定着材料在宏观尺度的物理和力学特征.构建三维孔隙结构数值模型,对于进一步研究水泥基材料的力学及渗流特性有着重要的意义.考虑到传统的基于简单单元的建模方法无法呈现出孔隙分布的随机性和孔隙形状的复杂性,本文采用偏移集理论将连续的高斯随机场阈值化为二相场,并通过多个具有不同相关长度的二相场联合完成模型构建.采用数学形态学图像分析方法分析模型的孔径分布,验证模型构建出满足试验孔径分布曲线的孔隙结构.研究表明,该方法能够使模型孔隙网络的空间形态随机化,且对孔隙率和孔径分布也有着较好的控制.     

热解温度与煤焦微结构及分形特征关系研究

采用扫描电子显微镜 ( SEM)研究了不同热解温度条件下煤焦的微观结构 .结果表明 ,煤焦结构与煤化程度和热解温度有密切的关系 ,烟煤煤焦的微观结构以孔隙为主 ,无烟煤煤焦以裂隙为主 ,且其微观结构具有良好的统计自相似性 ;并采用分形维数定量表征煤焦在不同温度下的孔隙和裂隙特征 ,研究了煤焦分形维数与热解温度、煤化程度的关系及内在机理 ,发现分形维数随煤化程度增加而减少 ,随热解温度提高而增大 .     

基于核磁共振技术分析生活垃圾焚烧灰渣代砂混凝土的微观结构与抗压强度

孔结构是混凝土微观结构中重要的组成部分,影响着混凝土的宏观性能,为了研究生活垃圾焚烧灰渣代替天然砂对混凝土孔结构和抗压强度的影响,通过核磁共振技术对生活垃圾焚烧灰渣代砂混凝土微观孔结构进行测试,分析了孔隙率和孔隙分布变化规律,并根据分形理论研究了孔隙分形特征,获得各孔径区间的分形维数与整体分形维数,并探讨了各孔径占比、孔隙率和分形维数与抗压强度之间的灰熵关联度。结果表明:随生活垃圾焚烧灰渣代砂率的增加,混凝土孔隙率增加,无害孔占比减小,抗压强度降低;分形维数随着生活垃圾焚烧灰渣代砂率的增加而减小,由于生活垃圾焚烧灰渣具有潜在的水硬性,随龄期的增长,分形维数呈增加趋势,同时孔隙结构得到了优化。灰熵关联度分析发现生活垃圾焚烧灰渣代砂混凝土整体分形维数和无害孔占比对抗压强度影响最大。     

透水混凝土制备及性能研究综述

综述了透水混凝土的原材料特性和配合比设计方法;阐述了研究透水混凝土抗压强度和孔隙结构的数学模拟和软件建模的方法;对比分析了透水混凝土与普通混凝土在抗压强度计算、尺寸效应和应力-应变关系等方面的区别;归纳总结了透水混凝土孔隙分布的二维和三维图像,为研究透水混凝土孔隙结构提供了新思路.最后,提出今后应注重对透水混凝土微观结构的研究等建议.     

基于CT技术的沥青混合料微细观结构研究进展

综述了CT技术在沥青混合料微细观结构研究中的应用。介绍了CT技术在沥青混合料细观空隙研究中的应用，总结了表征空隙特征的参数；介绍了CT技术在二维及三维集料均匀性评价中的应用，分析了现有均匀性评价方法的局限性；从疲劳、永久变形及水损害等方面阐述了基于CT技术的微观损伤研究，归纳了损伤阶段空隙的三种发展方式；介绍了基于CT技术的虚拟力学试验研究，表明了CT技术应用于数值仿真的可行性；最后分析了CT技术面临的问题并展望其前景，为沥青混合料微细观结构研究提供参考。     

四种炼焦煤热解成焦过程微观结构演变

实验选取了兖州气煤(YZ)、霍州肥煤(HZ)、范矿肥煤(FK)、新峪焦煤(XY)4种不同变质程度炼焦煤为样品，利用坩埚焦制备了不同热解温度的焦样，利用X射线衍射仪、扫描电子显微镜表征了焦样微观结构的变化，探究了不同变质程度炼焦煤热解成焦过程的微观结构演化。结果显示，不同热解产物石墨片层间距及堆叠高度主要受热解温度影响，热解温度越高，焦样的石墨化程度越有序。随热解温度的升高，热解产物的表面越来越光滑，孔隙和裂缝越来越多，孔径逐渐增大；挥发分含量较高的炼焦煤，热解产物表面的孔隙较多。     

机械搅拌驱动下三维空间混合效果实时测量及表征

为精确实时测量机械搅拌驱动下三维空间多相混合效果，提出了一种三维空间的混合效果精确测量方法，通过类比势能的概念，计算总体平均质量度量Q来表征混合效果，以克服颗粒重叠二维点集伪均匀性度量的片面性。对三维空间混合效果精确测量方法进行了实验验证，计算了总体平均质量度量Q，用以表征颗粒分布的均匀性；分析了电机转速与红色小球相对运动速度均值关系。结果表明，不同转速和变频对示踪颗粒相对运动速度变化有明显的影响；特别是对二维与三维混合均匀性进行了比较，二维的混合均匀度测量方法计算结果明显不够精准，存在较大偏差，比三维的平均偏差高75%，三维空间混合均匀度测量方法计算结果更能反映出真实的混合效果；比较了中心化偏差(CD)和可卷偏差(WD)两种常用的混合均匀性测量方法，CD和WD方法对混合均匀性的表征均存在失效的情况，进一步验证了本方法的可行性以及准确性。     

Local porosity analysis of pore structure in cement paste

Three-dimensional (3-D) local porosity theory (LPT) was originally proposed by Hilfer and recently used for the analysis of pore space geometry in model sandstone. LPT pursues to define the probability density functions of porosity and porosity connectivity. In doing so, heterogeneity differences in various sandstone samples were assessed. However, fundamental issues as to the stochastic concept of geometric heterogeneity are ignored in Hilfer's LPT theory. This paper focuses on proper sampling procedures that should be based on stochastic approaches to multistage sampling and geometric heterogeneity. Standard LPT analysis provides a 3-D microscopic modeling approach to materials. Traditional experimental techniques yield two-dimensional (2-D) section images, however. Therefore, this paper replaces the method for assessing material data in standard LPT theory to a more practical one, based on stereological, 3-D interpretation of quantitative image analysis data. The developed methodology is used to characterize the pore structure in hardened cement paste with various water/cement ratios (w/c) at different hydration stages.     

Novel preparative method for porous hydrogels using overrun process

Porous poly(vinyl alcohol) (PVA) hydrogels were prepared using the overrun process which is usually used in manufacturing ice cream. The pores in the hydrogel formed exhibit dual‐pore structure due to the injection of air bubbles and ice recrystallization. Morphological investigation revealed that overrun‐processed hydrogels had closed pore structures and that their pore size and size distribution had been influenced by the impeller rate and concentration of polymer solution. The closed‐pore structure was reformed into interconnected open‐pore structure at lower concentrations of the solution. The freeze–thawing process, which takes place in PVA cross‐linking, has no effect on the bubble structure, but removes the small pores formed during ice recrystallization. Besides the swelling ratio of overrun‐processed PVA hydrogels is increased tenfold in comparison with non‐porous hydrogels. Overrun‐processed hydrogels showed more rapid swelling kinetics than freeze‐dried and film‐like hydrogels due to their larger surface area. In the future, the overrun process can be applied to prepare porous scaffolds containing proteins, such as growth factors and other cytokines, without denaturation, because it operates at a low temperature. Copyright © 2004 Society of Chemical Industry     

催化剂孔道结构设计及孔内反应-扩散耦合模拟

采用硬球-拟颗粒(HS-PPM)耦合方法并结合简化的集总反应模型,模拟了碳四(C4)烯烃催化裂解过程中反应物和产物在孔道内的反应?扩散耦合过程,提出了一种多级孔道结构可控设计方法,实现了对孔隙率、孔径和孔体积占比等参数的独立量化调控,建立了包含三种不同孔径分布的催化剂孔道模型并定义了量化参数对反应?扩散耦合进行表征。结果表明,各组分在复杂孔道内的扩散过程显著受限,反应?扩散耦合性能较差,导致催化剂总体性能降低,达不到本征动力学允许的高反应速率。提出的可控设计孔道模型结合HS-PPM耦合模拟方法有助于催化剂材料的孔尺度结构的设计。     

Pore Network Modeling of Nanoporous Ceramic Membrane for Hydrogen Separation

Pore network modeling of porous media has this advantage that can consider the pore structure incorporating any desired details, but it has not been studied sufficiently. In addition, most studies are limited to mathematical modeling only which need validation. In the present study, this approach was applied to hydrogen separation from syngas by nanoporous ceramic membrane to predict the membrane permeance theoretically based on its pore structure. Gas transport through nanoporous membrane was modeled with the aim of a 2D network model. A dusty gas model was used for gas transport in the individual pores. Model validation showed that the model predictions are in good agreement with the experimental data with the coordination number of 2.5 and the pore length of about 20 nm. A parametric study indicated that hydrogen permeance through the membrane increases with the average and minimum pore size and decreases with temperature and pressure. Also, hydrogen selectivity increases slightly with temperature and decreases with pressure and average pore size.     

Computational Simulations for the Assessment of the Mechanical Properties of Glass with Controlled Porosity

Porous glass with closed controlled porosity is used as a model system in order to numerically assess the effect of pores on the macroscopic mechanical and fracture behavior of brittle solids. A computational code called OOF, which converts digitalized two-dimensional (2-D) images of materials microstructures into finite element meshes, is adopted, so that the effect of 2-D microstructural features (e.g. pore size and shape) on the global mechanical response of the material can be determined. Firstly, microstructures of porous glass bodies containing isolated pores were considered. These specimens were numerically investigated in terms of fracture initiation and propagation: the numerical model predicted that larger pores initiate fracture, in agreement with experimental results. Then, the effect of porosity on the elastic and fracture properties was thoroughly investigated by means of model two-dimensional microstructures consisting of selected area fractions of pores (equivalent to pore volume fractions in three dimensions) and with prescribed pore shape, orientation and dimensions. In particular, the effect of pore dimension and shape was studied, finding that the critical stress for crack initiation scales with pore dimension and aspect ratio, i.e. oblate and larger pores oriented perpendicularly to the stress direction cause a higher reduction of strength of the specimen. Finally, several 2-D microstructures characterized by different values of area fraction of pores of the same shape were investigated, in order to determine the variation of elastic properties and the fracture response of porous glasses with pore content. The study confirms the suitability of the 2-D OOF code to investigate the mechanical and fracture behavior of porous materials. Issues regarding the limitation of the model due to its 2-D character are also discussed where appropriate.     

Nano-TiO2-Coated Unidirectional Porous Glass Structure Prepared by Freeze Drying and Solution Infiltration

A unidirectional porous glass structure coated with a nanometer TiO₂ thin film on pore walls was successfully fabricated by a two-step process. Firstly, a water-based slurry of sheet glass was frozen while controlling the growth direction of ice crystals, and then sublimation of the ice under vacuum formed the green body. The green body was subsequently sintered to produce porous glass with unidirectional pores. Secondly, the sintered porous glass was infiltrated by a titanium (IV) butoxide solution, and then dried and calcined at a moderate temperature to generate a nanometer TiO₂ film on pore walls. The present porous glass with nanometer TiO₂ has more total surface area than the TiO₂ film on glass plates, which is believed to be more effective in the phtotocatalysis application for air or water purification.     

Porous WO3 formed by anodization in oxalic acid

This work reports the investigation of the anodic growth of porous tungsten oxide (WO₃) in oxalic acid. It was found that pore diameter increases from approximately 50 to 90 nm while the layer thickness of porous structures increases from approximately 100 to 400 nm as the voltage varies from 40 to 100 V. Extended anodization duration improves the homogeneity of the porous structure at 40 V but it has no effect on the pore layer thickness. During the pore formation process, nucleation of small pores in the existing pore is observed while several layers of pores with voids underneath the surface layer are formed especially at high anodization voltage. A possible growth mechanism of the pores is proposed.     

Stress-induced anisotropy during sintering of hierarchical porosity ceramics

There is significant interest in the design and processing of porous ceramics due to their use in a variety of applications including energy storage, catalysis, adsorption, separation, and life science applications. For many of these applications, it is desirable to have a hierarchical porous structure in which there is a distinct difference between sizes of pores. Our previous study has shown that microstructure and properties of porous materials become anisotropic after sinter-forging. In particular, the small interparticle pores (intrinsic pores) orient parallel to the applied compressive stress, in contrast to large pores from pore formers (extrinsic), which orient perpendicular to the applied stress. However, the pore size, for transition from extrinsic to intrinsic behavior, (transient pore size) has not been quantified. In this study, we report on the effect of applied stresses during sinter-forging on the morphology (shape and size) of pores of different size. Based on these results, we propose a two-step approach to predict transient pore size for hierarchically porous ceramics. We use this approach to quantify the effect of applied stresses on the transient pore size. Finally, we postulate that the stress dependence of the transient pore size may be related to sintering stress—a fundamental quantity in continuum models of sintering. In addition, it can be used to calculate the effective surface energy of complex sintering systems.     

Numerical calculations of the thermal conductivity of porous ceramics based on micrographs

The overall thermal conductivity of a porous material is strongly sensitive to the volume fraction and spatial distribution of the pores. For this second aspect analytical models predicting thermal conductivity as a function of pore volume fraction are obliged to make a simplifying assumption concerning the pore shape. In order to describe the effects of the microstructure on heat transfer in greater detail, we have developed a method involving 2D finite element calculations based on real micrographs of the porous solid. The approach was tested on micrographs of tin oxide samples with pore contents from 10% to 50%. Quantitative results obtained for pore contents up to 20% give very good agreement to Rayleigh's model. Higher pore contents lead to a number of difficulties but the qualitative results are used to support the choice of Landauer's effective medium expression as an appropriate general analytical model for the thermal conductivity of a porous ceramic material.     

Coupling the porous conditional moment closure with the random pore model: applications to gasification and CO2 capture

Gasification of coal or biomass with in situ CO₂ capture simultaneously allows production of clean hydrogen at relatively low cost and reduced emission of CO₂ into the atmosphere. Clearly, this technology has a great potential for a future carbon constrained economy. Therefore, the development of a comprehensive, physically-based gasifier model is important. The sub-models that describe reactive transport processes in coal particles as well as in particles of CO₂ sorbent material are among the key sub-models, which provide a necessary input for an overall gasifier model. Both coal and sorbent are materials that have complicated pore structures. The porous conditional moment closure (PCMC) model proves to be adequate for modeling reactive transport through porous media with fixed pore structure. Consumption of coal in the heterogeneous gasification reaction, however, widens the pores and reduces the surface area available for this reaction. At the same time, formation of a carbonate layer narrows the pores in the sorbent material and reduces the reaction rate of CO₂ sorption. In both cases the pore structures are affected. Such changes are not taken into account in the existing PCMC model. In this study, we obtain the parameters of the diffusive tracer distribution based on the pore size distribution given by the widely applied random pore model (RPM), while coupling PCMC with RPM. Such coupling allows taking into account changes in pore structure caused by heterogeneous reactions and thus improves the accuracy of these key sub-models.