江汉盆地江陵凹陷二氧化碳地质封存数值模拟

通过建立江汉盆地江陵凹陷新沟嘴组储层和盖层的二维模型,对江陵凹陷进行了CO2地质封存数值模拟研究,分析了注入CO2的运移分布和溶解扩散情况,并对储盖层垂直方向的渗透率进行了敏感性分析.结果表明,所建的二维模型能够较好地描述CO2在储层中的运移过程和分布状况,而且在实际地质封存工作中必须要考虑一些地质上的因素如储盖层的渗透率、毛细管压力、岩石矿物组分丰度等的变化对封存效果的影响.     

混凝土随机骨料模型尺寸效应的细观数值分析

为研究混凝土材料细观不均匀性对试件的静力学性能的影响,采用不同的随机数组,通过固定骨料尺寸,改变试件尺寸的方法,建立了不同的混凝土三维数值模型,都分6步施加相同的均布载荷,来模拟骨料和混凝土试件尺寸比例变化对试件力学性能的影响,研究了骨料随机位置对混凝土试件的应变影响。计算表明,当骨料半径和试件半径的比例系数逐渐增大时,应变误差平方和减小;骨料随机位置对混凝土试件的应变影响逐渐减小,试件的脆性越明显     

VG模型参数选择对咸水层CO_2封存的影响研究

在使用TOUGH2程序模拟咸水层CO2封存时,常使用VG模型计算岩石表面毛细压力,而岩石表面毛细压力的大小则会直接影响CO2在咸水层中运移形式和储存量。总结了不同类型常见砂泥岩的VG模型参数取值范围,基于松辽盆地典型储盖地质结构特征,运用TOUGH2构建了二维CO2封存模型,设计3组算例对比分析了VG参数取值对咸水层CO2封存的影响。研究结果表明:VG模型参数值大小取决于岩石的性质,TOUGH2使用的VG模型缺省值仅适用于小部分岩石;盖层的3个VG模型参数取值对咸水层CO2封存效果影响很大,单纯微调部分VG参数仍不能提高模拟可信度,而对于高、中渗透率储层,VG模型参数取值对模拟结果影响较小。因此,咸水层CO2封存模拟时,应结合储盖层岩性特征赋VG模型参数值。     

连续弯道水流模拟中二次流修正效果评价

为了选择适用于复杂连续弯道的二次流修正方法,选取了线性方法中两种典型的计算模式,通过在正交曲线坐标系二维浅水动量方程中增加扩散应力项开发了考虑二次流影响的平面二维水流模型。基于不同复杂度的4个连续弯道试验的水位和流速分布资料,通过理论分析和数值试验对比定量评估了传统二维模型和两种二次流修正方法在连续弯道水流模拟中的效果。测试结果表明,Delft3D模型的二次流修正方法自由度较高,适用于不同复杂度的连续弯道水流模拟,而Lien模型二次流修正方法适用于微弯或中弯,应用于中弯时需要慎重选用,不适用于急弯连续弯道模拟。两种方法比较,建议连续弯道水流模拟中优选Delft3D模型的二次流修正方法。     

基于TOUGHREACT-MP的苏北盆地盐城组咸水层CO2矿物封存数值模拟

基于TOUGHREACT并行版,建立了苏北盆地盐城组下段砂岩层在碳封存中的CO2-水-岩反应二维径向模型,并评估储层的矿物封存潜力,探讨分析了网格剖分精度和储层各向异性对矿物封存模拟的影响。模拟结果表明:在CO2封存过程中,片钠铝石、方解石和菱铁矿沉淀较明显,在CO2注入5 000a后矿物封存的比例高达34.0%;网格剖分精度的不同对矿物封存反应路径没有影响,但粗网格模型计算得到的矿物封存量偏高;降低储层的kv(垂向渗透率)在短期内会促进CO2的水平运移,有利于溶解和矿物封存;但随着时间延长,降低kv对对流作用的抑制开始体现,表现为1 000a后中等kv值的模型计算出的矿物封存量高于较大kv和较小kv值的模型。     

基于单颗粒受力的均匀推移质颗粒对流和扩散特征

为揭示均匀推移质颗粒对流和扩散特性的控制因素,建立了间歇郎之万方程模型。该模型可在单颗粒尺度充分考虑颗粒的受力特性,模拟颗粒随机、间歇运动过程。通过该模型引入不同的停时分布,对模拟的大量单颗粒运动过程进行统计,从而研究均匀颗粒在大的时空尺度上的对流和扩散特征。结果表明,对于均匀颗粒,受颗粒速度分布的窄尾性限制,即便单步步长是长尾分布,也不一定产生超扩散,扩散特性由停时分布的尾部特征决定,不同分布的停时可导致欠扩散、超扩散和正常扩散。进一步与已有的、复杂程度不同的随机模型进行对比,表明忽略单步时间将影响颗粒的扩散(二阶)特性,但不影响颗粒的对流(一阶)特性,类似地可以推广到更普遍规律,即所研究随机发生的统计矩阶数越高,需要的模型越复杂。     

随机分布裂隙煤岩体模型及其应用

煤层中随机裂隙展布模拟技术是瓦斯抽放数值模拟中的关键技术。利用Monte-Carlo模拟技术模拟实际煤岩裂隙的随机性,建立了随机分布裂隙煤岩体模型,并在Matlab平台下开发了二维随机裂隙煤岩体生成程序RFSS 2D。在煤岩实测数据和统计分析的基础上,利用编制的程序不仅能生成与实际煤岩具有相同统计分布裂隙网络的虚拟煤岩体,而且能实现有限元网格自动剖分。以辽宁某瓦斯抽放试井作为数值算例,依据实测和统计数据建立了随机裂隙分布的煤岩体模型,模拟了瓦斯压和应力的演化规律,结果表明,与瓦斯抽放井连通的裂隙对瓦斯运移影响很大,与瓦斯抽放井不连通的裂隙对瓦斯运移也有影响,但影响有限。     

砂岩含水层CO2封存中考虑盐沉淀反馈作用的数值模拟:以鄂尔多斯盆地为例

盐沉淀是含水层CO2封存中需要关注的问题.当前,大多数数值模拟没有考虑盐沉淀引起的地层孔隙度和渗透率变化对流体流动的反馈作用.以鄂尔多斯盆地刘家沟组地层为例,利用TOUGH2软件建立了一个二维模型.通过修改程序源代码,使得模型能考虑盐沉淀对流体流动的反馈作用.模拟结果表明,刘家沟组地层在CO2注入20 a时,盐沉淀的反...     

岩盐沉淀对咸水层二氧化碳地质封存注入过程的影响:以江汉盆地为例

依托江汉盆地基础地质和水文地质数据,通过建立新沟嘴组一维、二维径向数值模型,对该储层进行CO2封存注入数值模拟研究,分析二氧化碳注入储层后,咸水层中发生的地球物理化学作用.以CO2的溶解作用和岩盐的沉淀作用为主,分析它们对储层压力和注入性的影响.模拟结果表明,二氧化碳注入储层后,近井周围会形成3个分区,即纯液相区、两相混相区、盐沉淀区;盐沉淀会导致压力积累严重,影响CO2可注入性;对盐度的敏感性分析表明,咸水含水层盐度越大,压力积累会越严重.因此,江汉盆地CO2地质封存应考虑盐度的影响,采取相关缓解压力积累的措施.     

弹丸侵彻无钢筋混凝土的数值模拟

采用二维梁-颗粒模型BPM2D（beam-particle model in two dimensions）模拟了刚性弹丸侵彻无钢筋混凝土的过程。离散元法（DEM）和有限元法（FEM）等数值计算方法各有其优势,同时也都存在不足之处：离散元法适于处理由连续介质向非连续介质转化的破坏问题,但对于连续体计算结果精度不高;有限元法适于预测材料破坏的区域,但难以直接用于计算脆性材料破坏过程,因此将两种方法结合可以形成一种较好的混合模型。梁-颗粒模型BPM2D是基于离散元法,结合有限元法开发的二维数值计算模型,采用3种类型梁单元形成混凝土数值试样,每种类型梁单元的力学性质均按韦伯（Weibull）分布随机赋值,以模拟混凝土细观结构的非均匀性,同时梁单元的强度随应变率不同而变化。利用此模型分析了弹丸侵彻下混凝土的破坏过程,并给出侵彻过程弹丸减速度-时间历程曲线。比较计算结果与试验数据表明梁-颗粒模型可有效应用于计算和模拟脆性材料动态破坏问题。     

Discrete element analysis of the rheological characteristics of self-compacting concrete with irregularly shaped aggregate

Concrete is a heterogeneous, multiphase, composite material, and the size and shape of the coarse aggregate used have an important influence on the rheological properties of the concrete. The aggregate is usually simulated with spherical particles in the discrete element method (DEM). However, the shape of real aggregates is uncontrolled and polytropic. Therefore, spherical particles hardly reflect the actual situation. To comprehensively analyze the rheological characteristics of self-compacting concrete (SCC), experimental and simulated tests of slump-flow and L-box tests of different performative SCC are investigated. An efficient and fast random polyhedron particle generation method is proposed to simulate the real shape of the coarse aggregate, which is close to the actual state. The slump-flow and L-box tests of SCC are simulated by using the established discrete element model and the irregular generating particle method. The slump-flow test shows that the generation method could effectively simulate the flow state of concrete, and the L-box test evaluates the passing ability of SCC. The rheological characteristics of the yield stress τ₀ and plastic viscosity η are verified as Bingham model parameters, and the numerical results are perfectly consistent with the experimental results.     

Comparative carbon emission assessments of recycled and natural aggregate concrete:Environmental influence of cement content

This work examines the environmental and geochemical impact of recycled aggregate concrete produc-tion with properties representative for structural applications.The environmental influence of cement content,aggregate production,transportation,and waste landfilling is analysed by undertaking a life cycle assessment and considering a life cycle inventory largely specific for the region.To obtain a detailed insight into the optimum life cycle parameters,a sensitivity study is carried out in which supplementary cementitious materials,different values of natural-to-recycled aggregate content ratio and case-specific transportation distances were considered.The results show that carbon emissions were between 323 and 332 kgCO2e per cubic metre of cement only natural aggregate concrete.These values can be reduced by up to 17％by replacing 25％of the cement with fly ash.By contrast,carbon emissions can increase when natural coarse aggregates are replaced by recycled aggregates in proportions of 50％and 100％,and trans-portation is not included in analysis.However,the concrete with 50％recycled aggregate presented lower increase,only 0.3％and 3.4％for normal and high strength concrete,respectively.In some cases,the rel-ative contribution of transportation to the total carbon emissions increased when cement was replaced by fly ash in proportions of 25％,and case-specific transportation distances were considered.In absolute values,the concrete mixes with 100％recycled aggregates and 25％fly ash had lower carbon emissions than concrete with cement and natural aggregates only.Higher environmental benefits can be obtained when the transportation distances of fly ash are relatively short(15-25 km)and the cement replacement by fly ash is equal or higher than 25％,considering that the mechanical properties are adequate for prac-tical application.The observations from this paper show that recycled aggregate concrete with strength characteristics representative for structural members can have lower carbon emissions than conventional concrete,recommending them as an alternative to achieving global sustainability standards in construction.     

基于细观力学的混凝土劈拉破坏率相关特性研究

提出了一种高效的混凝土细观力学预处理方法,其中包括基于背景网格的骨料投放与细观三组分的识别;建立了颗粒离散元的混凝土细观力学模型。利用该模型对不同应变率下混凝土的劈拉试验进行了数值仿真,仿真结果显示出的混凝土动力强度提高的规律以及不同应变率下混凝土的破坏模式与试验结果吻合较好。研究表明,随着应变率的提高,混凝土破坏时产生更多弥散状分布的裂纹,同时有些裂纹穿过强度较高的骨料,导致混凝土在高应变率破坏过程中消耗更多的能量,是混凝土的动力强度提高的重要原因。此外,在高应变率下,力链产生更多的分叉,空间分布的不均匀性和随机性增加,因此,在高应变率下混凝土动力强度表现出更大的离散性。     

Acidic Rocks as Aggregates in Concrete: Engineering Properties,Microstructures and Petrologic Characteristics

In this research, the possibility of replacing different portions of the normally used aggregate by acidic rocks was investigated. These rock types outcrop at the northern part of Eastern Desert, southwest Ras Gharib area where large quantities of good quality acidic rocks are available their. Portland cement concrete is a composite material made up of the hydrated cement matrix, fine aggregate and coarse aggregate. The scanning electron microscope equipped with an energy dispersive X-ray analysis system (EDX) has been applied to investigate several aspects of Portland cement concrete microstructure. The topics investigated so far include the influence of aggregate composition on the development of the cement paste-aggregate interface and the formation of calcium silicate hydrate CSH/calcium silicate aluminate CSA formation. The silicate gel coated the aggregates in the concrete paste and crystallized into well defined needle like shape, cotton shape as well as euhedral to subhedral crystals of silicate/alumiante and ettringite minerals with free lime librated from the hydrolysis process. The free lime can react again with the aggregates leading to the formation of cementing materials which increase strength and durability of the concrete paste by increasing the interfacial bonds between the used aggregates.     

岩溶区土壤有机碳和团聚体的关系研究——以重庆市中梁山岩溶槽谷为例

研究了重庆市中梁山岩溶槽谷不同土地利用方式对土壤有机碳和团聚体稳定性的影响,在此基础上引入通径分析方法探讨有机碳与各粒径团聚体之问的关系.研究表明,土地利用方式对土壤有机碳、团聚体稳定性指数和团聚体保存几率影响明显:坡耕地有机碳含量仅为17.544(g/kg),分别比灌丛、橘林地、草地和菜地低61.14%、71.66%、74.43%和100.83%;团聚体稳定性指数依次为草地(2.394)>灌丛(2.383)>橘林地(2.085)>坡耕地(1.953)>菜地(0.865);团聚体稳定性与有机碳含量的相关性与土地利用方式有关,在灌丛和草地中明显正相关,在橘林地和菜地中呈负相关,而在坡耕地中相关性不明显;大粒径团聚体保存几率是决定有机碳含量的主要园子,5～10mm和3～5mm粒级的土壤团聚体对有机碳累积与稳定的决定系数分别为0.61和0.50.     

Experimental studies relating to microfracture in sandstone

A fundamental understanding of the relation between stress concentrations at grain contacts and microfractures in granular aggregates is obtained through two-dimensional photomechanical model studies and is tested through observational studies of experimentally deformed sandstone discs, glass beads, and quartz sand.In uncemented aggregates, the state of stress in each grain is controlled by the manner in which the applied load is transmitted across grain contacts. The angles between lines connecting pairs of contacts and the axis of the principal load acting at the boundaries of the aggregate determine which of all contacts will be most highly stressed or “critical”. Microfractures follow that maximum principal stress trajectory which connects critical contacts, and they propagate through those points where the magnitude of the local maximum stress difference is the greatest. Microfractures, therefore, are extension fractures. It then follows that both the locations and orientations of fractures can be predicted if the state of stress in the grains is known.Positioning of critical contacts depends primarily on sorting, packing, grain shapes, and the boundary load conditions applied to the aggregate. Some critical contacts and, therefore, microfractures tend to join together in a series or “chain”. Orientations of chains are most strongly influenced by the direction of the maximum compressive load at the boundary of the aggregate. A hydrostatic load applied on the boundaries of an aggregate can cause microfracturing within grains. Orientations for microfractures and contact lines are random in poorly sorted aggregates, but they are influenced by packing in well sorted aggregates.Grains of cemented aggregates are more highly stressed at their centers than at contacts. By analogy, microfracture orientations depend strongly on the position of the greatest load axis and only slightly on the low-magnitude stress concentrations at contacts. These microfractures parallel the greatest principal stress trajectory in regions where the magnitude of the maximum stress difference is greatest. These observations lead to the conclusion that fractures in grains of cemented aggregates are also extension fractures and should exhibit a higher degree of preferred orientation than in uncemented counterparts.These conclusions hold when cementing materials have about the same elastic moduli as the grains. Cements may be so weak that the aggregate behaves as if it were uncemented in terms of microfracture fabric, or so stiff that the major part of the load is transmitted by the cement, and the composite is no longer an aggregate in the mechanical sense.     

Natural aggregates in the performance and durability of concrete: physical characteristics

The production of rock aggregates for construction is the largest of the extractive industries in Britain. Natural aggregates form the main component, by volume, in the manufacture of concrete but the part played by aggregates in the durability and performance aspects of concrete is still only moderately understood. This feature summarizes the main characteristics of common aggregate in concrete, especially those leading to cracking and deterioration. Physical and mechanical causes of concrete cracking are of immense importance but are omitted from the following. The feature expands on the earlier one: ‘Concrete: a man‐made rock?’ (Geology Today, 2010, v.26, n.2). To follow is another that will continue the story by looking at chemical reactivity in concretes.     

Some Geological Considerations and Durability Analysis on the Use of Crushed Pyroclastics from Abakaliki (Southeastern Nigeria) as Concrete Aggregate

Samples of pyroclastic rocks from Abakaliki area (Nigeria) were subjected to geological and geochemical tests to determine the suitability and durability of the rocks as concrete aggregate. Petrographic analysis shows fine-grained texture and predominance of plagioclase (21–60 %) and shaley-muddy lithic fragment, while geochemical analysis classifies the rocks as alkaline. Analysis indicates that the pyroclastics would likely perform marginally well as concrete aggregates, especially in heavy Portland cement concrete and bituminous concrete. This is due to appreciably high values of natural moisture content, low bulk density, relative density and specific gravity, which suggests low durability, high voids and less aggregate absorption in concrete and bitumen mixes. The rocks are rated as moderate to high strength material on the basis of their unconfined compressive strength values, but are very likely to have low durability as buttressed by poor sulphate soundness test results, probably due to high amount of weak minerals. Analyses reveal that the Abakaliki Pyroclastics are not completely satisfactory as concrete aggregates source, despite their origin, texture, strength and economic availability. Hence, caution is required when they are used in concrete projects.     

Meso‐scale particle modeling of concrete deterioration caused by alkali‐aggregate reaction

A meso‐scale particle model is presented to simulate the expansion of concrete subjected to alkali‐aggregate reaction (AAR) and to analyze the AAR‐induced degradation of the mechanical properties. It is the first attempt to evaluate the deterioration mechanism due to AAR using the discrete‐element method. A three‐phase meso‐scale model for concrete composed of aggregates, mortar and the interface is established with the combination of a pre‐processing approach and the particle flow code, PFC2D. A homogeneous aggregate expansion approach is applied to model the AAR expansion. Uniaxial compression tests are conducted for the AAR‐affected concrete to examine the effects on the mechanical properties. Two specimens with different aggregate sizes are analyzed to consider the effects of aggregate size on AAR. The results show that the meso‐scale particle model is valid to predict the expansion and the internal micro‐cracking patterns caused by AAR. The two different specimens exhibit similar behavior. The Young's modulus and compressive strength are significantly reduced with the increase of AAR expansion. The shape of the stress–strain curves obtained from the compression tests clearly reflects the influence of internal micro‐cracks: an increased nonlinearity before the peak loading and a more gradual softening for more severely affected specimens. Similar macroscopic failure patterns of the specimens under compression are observed in terms of diagonal macroscopic cracks splitting the specimen into several triangular pieces, whereas localized micro‐cracks forming in slightly affected specimens are different from branching and diffusing cracks in severely affected ones, demonstrating different failure mechanisms. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of pressure on closure temperature of a trace element in cooling petrological systems

Closure temperature is important to many diffusion-related problems involving cooling. The classic model of Dodson and its modifications for cooling petrological systems are formulated at constant pressure. Many petrologic processes involve changes in both temperature and pressure. The effect of changing pressure on diffusional loss in cooling petrological systems has not been considered in Dodson’s model. During upwelling, the decompression rate is related to the cooling rate through the slope of the upwelling path. Simple analytical expressions for the average or mean closure temperature and closure pressure in cooling-upwelling mono-mineralic and bi-mineralic systems are obtained by noting that both temperature and pressure decrease as a function of time along the upwelling path. These pressure-adjusted equations are nearly identical to closure temperature equations for isobaric cases if one replaces the activation energy and pre-exponential factor for diffusion in the isobaric formulations by the path-dependent activation energy and pre-exponential factor. The latter also depend on the slope of the upwelling path. The competing effects between pressure and temperature on diffusion during upwelling result in reductions in the effective activation enthalpy for diffusion and exchange enthalpy for partitioning, which in turn leads to systematic deviations in closure temperatures from cases of constant pressure. For systems with large activation volume for diffusion, it may be possible to deduce upwelling path and upwelling rate from closure temperatures and closure pressures of selected elements. Examples of closure temperature and closure pressure for REE diffusion in garnet and clinopyroxene and in garnet–clinopyroxene aggregates are presented and discussed in the context of the minor’s rule and the REE-in-garnet–clinopyroxene thermobarometer. Closure temperatures for middle-to-heavy REE in garnet–clinopyroxene aggregates are controlled primarily by diffusion in clinopyroxene unless the modal abundance of garnet is very small or the effective grain size of clinopyroxene is considerably smaller than that of garnet.