Effects of dry-wet cycles on three-dimensional pore structure and permeability characteristics of granite residual soil using X-ray micro computed tomography

Due to seasonal climate alterations,the microstructure and permeability of granite residual soil are easily affected by multiple dry-wet cycles.The X-ray micro computed tomography(micro-CT)acted as a nondestructive tool for characterizing the microstructure of soil samples exposed to a range of damage levels induced by dry-wet cycles.Subsequently,the variations of pore distribution and permeability due to drywet cycling effects were revealed based on three-dimensional(3D)pore distribution analysis and seepage simulations.According to the results,granite residual soils could be separated into four different components,namely,pores,clay,quartz,and hematite,from micro-CT images.The reconstructed 3D pore models dynamically demonstrated the expanding and connecting patterns of pore structures during drywet cycles.The values of porosity and connectivity are positively correlated with the number of dry-wet cycles,which were expressed by exponential and linear functions,respectively.The pore volume probability distribution curves of granite residual soil coincide with the χ^(2)distribution curve,which verifies the effectiveness of the assumption of χ^(2)distribution probability.The pore volume distribution curves suggest that the pores in soils were divided into four types based on their volumes,i.e.micropores,mesopores,macropores,and cracks.From a quantitative and visual perspective,considerable small pores are gradually transformed into cracks with a large volume and a high connectivity.Under the action of dry-wet cycles,the number of seepage flow streamlines which contribute to water permeation in seepage simulation increases distinctly,as well as the permeability and hydraulic conductivity.The calculated hydraulic conductivity is comparable with measured ones with an acceptable error margin in general,verifying the accuracy of seepage simulations based on micro-CT results.

Permeability differences based on three-dimensional geometrical information of void spaces

In this study,the permeabilities of Berea and Otway sandstones were measured under different confining pressures,and porosity was investigated through mercury intrusion porosimetry(MIP).The total porosities of the Berea and Otway sandstones were approximately 17.4%and 25%,respectively.Pore size distributions of each sandstone were almost the same,but the pores in the Otway sandstone were slightly narrower.However,the permeability of the Otway sandstone was smaller than that of the Berea sandstone by one order of magnitude.Three-dimensional(3D)void geometry and geometrical properties of the void spaces relevant to flow were compared to obtain the relation between the permeability differences and porosities of the two sandstones.The 3D geometrical analysis using microfocus X-ray computed tomography(CT)was performed,and the pore geometries of both sandstones were compared using the 3D medial axis(3DMA)method.Pore and throat radii,pore coordination number,tortuosity,number of connecting paths,connecting path volume,and other factors were determined using 3DMA.The Otway sandstone was characterized by a small effective throat/pore radius ratio.Based on the fluid flow mechanism,the lower effective throat/pore radius ratio results in a lower permeability induced by the fluid energy loss,which means that the 3D geometrical shape of void spaces affects the permeability value.

Evaluation of corticospinal tract injury with three-dimensional diffusion tensor tract in patients with acute cerebral infarction

BACKGROUND:Three-dimensional diffusion tensor tract(DTT)is the newest imaging to describe the structure of white matter fiber in three-dimensions,it has great significance in dividing the concrete anatomic site of gray and white matter lesions,displaying the correlation with fibrous band and judging clinical prognosis,which is incomparable by other imagings.OBJECTIVE:To observe the conditions of corticospinal tract(CST)in acute cerebral ischemic stroke patients,and analyze the relationship between motor function and the severity of CST injury.DESIGN:A case-control observation.SETTING:Department of Medical Imaging,Fuzhou General Hospital of Nanjing Military Area Command of Chinese PLA.PARTICIPANTS:Fifteen patients with acute cerebral infarction were selected from Fuzhou General Hospital of Nanjing Military Area Command of Chinese PLA from February to December in 2005.They all suffered from acute attack and motor disorder of hemiplegic limbs to different extent,and were conformed by CT or MRI.There were 9 males and 6 females,aging 16-87 years old,the median age was 51.7 years,and all were right handed.Fifteen right-handed normal subjects,who were matched by age and sex with the patients in the cerebral infarction group,were selected from the relatives of patients and physicians of the Imaging Department as the control group.All the subjects were informed and agreed with the study.METHODS:The patients with acute cerebral infarction and subjects in the control group received MR diffusion tensor imaging(DTI)with GE 1.5 T nuclear magnetic resonance system,fiber tracking with the software of dTV-Ⅱ.Fractional anisotropy(FA)maps and three-dimensional tractography of bilateral CST of all patients were created.Displacement,continuity and destroy of fibrous bands were observed.At the same time,muscle strength of ipsilateral hand of patients with cerebral infarction was measured with Brunnstrom standard.The correlation between the severity of CST injury and the muscle strength of ipsilateral hand was analyzed with spearman correlation analysis.MAIN OUTCOME MEASURES:①FA values in the infarcted sites and those in the contralateral corresponding sites of patients with cerebral infarction;②CST manifestations in the patients with cerebral infarction and the control group.RESULTS:All the 30 testees were involved in the analysis of results.①The FA values in infarcted sites of white matter were significantly lower than those in the contralateral ones(t=4.570,P<0.001).②In the control group,bilateral CST were reconstructed,they originated from precentral gyrus,went downwards to internal capsule,and extended to pontine and medulla oblongata,each fiber had good uniformity in continuous form.In the patients with cerebral infarction,the forms of contralateral CST were consistent with those in the control group with good continuity.Due to the involvement by the infarcted site to different extents,the ipsilateral CST manifested as continuous interruption and loss of uniformity in anatomic structure and form.The CST involvements were divided into three grades:integrated CST for grade 1(n=2);integrated CST but compressed or displaced for grade 2(n=5);interrupted CST for grade 3(n=8).③The severity of CST injury was obviously correlated with the muscle strength of the ipsilateral hand(r=0.888,P<0.05).CONCLUSION:①CST is injured to different extents in patients with acute cerebral infarction,and the severity of injury is associated with muscle strength.It is indicated that it can be used to judge the prognosis of rehabilitative treatment.②DTT can directly display the status of pyramidal tract more three-dimensionally.

Three-Dimensional Finite Element Numerical Simulation and Physical Experiment for Magnetism-Stress Detecting in Oil Casing

The casing damage has been a big problem in oilfield production. The current detection methods mostly are used after casing damage, which is not very effective. With the rapid development of China's offshore oil industry, the number of offshore oil wells is becoming larger and larger. Because the cost of offshore oil well is very high, the casing damage will cause huge economic losses. What's more, it can also bring serious pollution to marine environment. So the effective methods of detecting casing damage are required badly. The accumulation of stress is the main reason for the casing damage. Magnetic anisotropy technique based on counter magnetostriction effect can detect the stress of casing in real time and help us to find out the hidden dangers in time. It is essential for us to prevent the casing damage from occurring. However, such technique is still in the development stage. Previous studies mostly got the relationship between stress and magnetic signals by physical experiment, and the study of physical mechanism in relative magnetic permeability connecting the stress and magnetic signals is rarely reported. The present paper uses the ANSYS to do the three-dimensional finite element numerical simulation to study how the relative magnetic permeability works for the oil casing model. We find that the quantitative relationship between the stress' s variation and magnetic induction intensity's variation is: Δδ =K* ΔB, K = 8.04×109, which is proved correct by physical experiment.

Three-dimensional stochastic seepage field for embankment engineering

Owing to the complexity of geo-engineering seepage problems influenced by different random factors, three-dimensional simulation and analysis of the stochastic seepage field plays an important role in engineering applications. A three-dimensional anisotropic heterogeneous steady random seepage model was developed on the basis of the finite element method. A statistical analysis of the distribution characteristics of soil parameters sampled from the main embankment of the Yangtze River in the Southern Jingzhou zone of China was conducted. The Kolomogorov-Smirnov test verified the statistical hypothesis that the permeability coefficient tensor has a Gaussian distribution. With the help of numerical analysis of the stochastic seepage field using the developed model, various statistical and random characteristics of the stochastic seepage field of the main embankment of the Yangtze River in the Southern Jingzhou zone of China were investigated. The model was also examined with statistical testing. Through the introduction of random variation of the upstream and downstream water levels into the model, the effects of the boundary randomness due to variation of the downstream and upstream water levels on the variation of simulated results presented with a vector series of the random seepage field were analyzed. Furthermore, the combined influence of the variation of the soil permeability coefficient and such seepage resistance measures as the cut-off wall and relief ditch on the hydraulic head distribution was analyzed and compared with the results obtained by determinate analysis. Meanwhile, sensitivities of the hydraulic gradient and downstream exit height to the variation of boundary water level were studied. The validity of the simulated results was verified by stochastic testing and measured data. The developed model provides more detail and a full stochastic algorithm to characterize and analyze three-dimensional stochastic seepage field problems.

沉积储层相对渗透率的各向异性特征及其对渗流的影响

沉积过程和成岩过程通常会使储层具有各向异性特征,现有各向异性研究主要针对储层绝对渗透率,缺乏对相对渗透率各向异性的研究,无法反映各相之间干扰程度在不同方向上的差异。为研究各向异性结构对油水两相渗流的影响,本文设计了立体交错取心方法,以降低常规立方体岩心各向异性相渗实验中末端效应的影响,建立了各向异性相渗曲线的获取方法;在此基础上开展天然砂岩油水两相相对渗透率各向异性对比实验,证明了砂岩储层相对渗透率各向异性的存在;分析了各向异性相对渗透率的产生机理及张量表示;研究了相对渗透率各向异性对渗流过程的影响。结果表明,微观层状结构是造成沉积储层相对渗透率各向异性的主要原因;相对渗透率各向异性量化表征了不同方向上水相突进能力、水油流度比的差异;高渗方向的相渗曲线具有残余油饱和度较高、两相渗流区较窄、等渗点含水饱和度较小、等渗点相对渗透率较低的特征;相对渗透率各向异性会导致油水分向渗流,对油藏开发产生显著影响,主要表现为,随开发过程的进行,油水渗流方向会逐渐呈现出明显的差异性,相继出现单向死油区和平面死油区,导致剩余油分布更加复杂。

纤维预制体渗透率测量技术研究进展

纤维预制体渗透率张量作为复合材料液体成型工艺过程中树脂流动数值模拟的决定性输入参数,与树脂流动以及最终成品质量密切相关,是实现复合材料高质量和大批量制造的关键。本文根据流动状态(饱和或非饱和)、流动维度和测量方向(面内或面外)将渗透率测量技术分类,论述了相应条件下的渗透率测量技术以及研究进展,总结归纳了非饱和流动中流动前沿实时监测技术,分析了造成渗透率测量结果分散性大的影响因素,并阐述解决办法,最后提出未来发展趋势。

Anisotropic Variation Law of Rock Permeability with the Burial Depth of Limestone

Permeability tensors of both macrofracture and microfracture Systems weremeasured progressively along the depth of limestone formations at severed sites. It was found thatthe principal permeability values K_x, K_y and K_z in these permeability tensors all decreasesimultaneously and logarithmically with depth. However, the limestone aquifers are composed of anupper region where the larger permeability ellipsoid is upright or prolate and characterized byK_z>K_x and K_z>K_y, a transitional zone, and a lower zone whose smaller permeability ellipsoid ishorizontal or oblate and characterized by K_z>K_x and K_z>K_y. The inversion of the permeabilityellipsoids in direction indicates that the anisotropy of rock permeability with the depth oflimestone formations has evidently changed. The anisotropic variation law of permeability tensors ina macrofracture system displays a similar pattern with that in a microfracture system. It is nextto impossible to examine the rock permeability tensor of the aquifer just by measuring the hydraulicparameters of macrofracture system directly, unless the limestone aquifer is exposed on or near theearth's surface. Therefore, the permeability tensors of a macrofracture system at any depth may beindirectly and roughly determined from the gaugeable permeability tensors of the microfracturesystem by conversion. This anisotropic variation law of rock permeability with depth is of greatsignificance in the study of three-dimensional fracture water flow the huge carbonate formations andin the research on the conditions of karst development and karst distribution.

Characterization of 3D pore nanostructure and stress-dependent permeability of organic-rich shales in northern Guizhou Depression,China

The three-dimensional(3D)pore structures and permeability of shale are critical for forecasting gas production capacity and guiding pressure differential control in practical reservoir extraction.However,few investigations have analyzed the effects of microscopic organic matter(OM)morphology and 3D pore nanostructures on the stress sensitivity,which are precisely the most unique and controlling factors of reservoir quality in shales.In this study,ultra-high nanoscale-resolution imaging experiments,i.e.focused ion beam-scanning electron microscopy(FIB-SEMs),were conducted on two organic-rich shale samples from Longmaxi and Wufeng Formations in northern Guizhou Depression,China.Pore morphology,porosity of 3D pore nanostructures,pore size distribution,and connectivity of the six selected regions of interest(including clump-shaped OMs,interstitial OMs,framboidal pyrite,and microfractures)were qualitatively and quantitatively characterized.Pulse decay permeability(PDP)measurement was used to investigate the variation patterns of stress-dependent permeability and stress sensitivity of shales under different confining pressures and pore pressures,and the results were then used to calculate the Biot coefficients for the two shale formations.The results showed that the samples have high OM porosity and 85%of the OM pores have the radius of less than 40 nm.The OM morphology and pore structure characteristics of the Longmaxi and Wufeng Formations were distinctly different.In particular,the OM in the Wufeng Formation samples developed some OM pores with radius larger than500 nm,which significantly improved the connectivity.The macroscopic permeability strongly depends on the permeability of OM pores.The stress sensitivity of permeability of Wufeng Formation was significantly lower than that of Longmaxi Formation,due to the differences in OM morphology and pore structures.The Biot coefficients of 0.729 and 0.697 were obtained for the Longmaxi and Wufeng Formations,respectively.

AN EXPLICIT TENSOR EXPRESSION FOR THE FUNDAMENTAL SOLUTIONS OF A BIMATERIAL SPACE PROBLEM

In this paper, by using the method of tensor operation, the fundamental solutions, given in the references listed, for a concentrated force in a three-dimensional biphase-infinite solid were expressed in the tensor form, which enables them to be directly applied to the boundary integral equation and the boundary element method for solving elastic mechanics problems of the bimaterial space. The fundamental solutions for Mindlin's problem, Lorentz's problem and homogeneous space problem are involved in the present results.

确定裂隙岩体渗透系数张量的一维环单元模型研究

裂隙岩体的渗透张量确定一直是渗流领域研究的重点。本文基于广义达西定律,提出一种新型空间一维环单元模型来计算三维裂隙岩体的渗透张量,该模型简化了以往复杂的三维面状流模型。通过解析法和数值模拟法对一维环单元模型的合理性进行验证,并利用面单元模型校核该模型的精度。结果表明:对于同一裂隙岩体,环单元模型计算得到的渗透张量,与现场压水试验校核过的数据相比,误差在合理范围之内,与面单元模型计算得到的结果相比,基本相同;环单元模型是一种可行、精度高且实用的简化模型,比面状流模型简单,在计算渗透张量时能极大地减小运算规模,优化了算法。研究成果可为确定裂隙岩体的渗透张量提供新思路。

岩体裂隙网络渗流模型及隧道掌子面渗流预测研究

【目的】岩体的裂隙特征直接影响其渗流特性,随着隧道工程中与岩体裂隙渗流有关的突涌水问题日益突显,为提高隧道建设与维护的安全性及稳定性,建立一套隧道掌子面岩体裂隙渗流量的预测计算程序。【方法】基于MATLAB平台,利用Monte-Carlo随机模拟方法编制岩体裂隙二维网络模型生成程序及稳定渗流数值计算程序,研究了裂隙岩体表征单元体积(REV)的尺寸效应及渗透张量的确定方法。将该程序应用于某地铁隧道,在现场调查及裂隙参数统计的基础上,对隧道掌子面裂隙岩体稳定渗流量和渗透系数进行预测。【结果】结果显示:(1)掌子面岩体裂隙方向角服从正态分布,迹长及间距服从负指数分布;(2)裂隙岩体表征单元体(REV)的最佳尺寸为裂隙迹长均值的14倍;(3)计算了裂隙岩体的渗透系数椭圆、渗透张量及渗透主轴,分析结果表明该隧道掌子面裂隙岩体具有强烈的渗透各向异性特征。【结论】结果表明:岩体裂隙的网络模型对于隧道工程岩体的水力学问题研究有不可替代的作用。所编程序可以预测隧道开挖过程中掌子面的渗流量和渗透主方向,为采取经济、合理的地下水控制措施提供依据。

考虑毛管力的全张量裂缝性介质两相流拟有限差分模拟

拟有限差分方法因具有良好的局部守恒性且适用于任意复杂网格系统,在流体力学等领域得到应用。建立基于拟有限差分法的裂缝性介质两相流模型,详细阐述拟有限差分方法的基本原理,并与离散裂缝技术相结合推导得到考虑毛管力的裂缝性油水两相渗流问题拟有限差分计算格式;采用全隐式方法对其两相流问题进行求解,克服传统求解方法不能有效处理全张量渗透率的缺陷。为了验证方法的正确性,对比不同湿润状态下的离散裂缝全隐式拟有限差分方法准确性和收敛性,并将该方法应用于油田现场实例。结果表明:当考虑毛管力时水驱替前缘缓慢穿过基质和裂缝,然后到达生产井;忽略毛管力时水在基质中的流动相对缓慢,流体在裂缝中流动迅速,快速到达生产井;该方法能够正确地模拟不同润湿状态和不同裂缝位置下的毛管力和重力情况。

裂隙岩体等效渗透系数张量数值法研究

由于核废料地质储存、地热开采、深部油气开采的工程需求,裂隙岩体渗透性及其随着应力、温度的影响受到广泛关注。通过温度-渗流-应力耦合三轴仪对大理岩人工裂隙渗透率随应力及温度变化规律进行了试验研究,获得了大理岩闭合裂隙渗透率随应力、温度的变化趋势及受影响程度。在试验基础上,通过数值方法研究了裂隙岩体等效渗透系数的尺寸效应及各向异性,获得了该裂隙岩体的等效渗透系数REV及渗透张量。

各向异性材料散射的FDTD分析

从时域有限差分法基本的原理出发 ,利用矩阵运算的特点 ,推导出了电各向异性、磁各向异性以及导电率各向异性的材料中的FDTD方程。验证了算法 ,并计算了各种单轴和双轴各向异性材料的雷达散射截面RCS ,讨论了各向异性材料的散射特性。

非结构网格油藏数值模拟方法研究

根据所建立的单个控制体离散形式的物质平衡方程 ,基于 PEBI和 CVFE网格 ,采用任意形式的渗透率张量 ,建立了相应的数值算法。PEBI网格是一种局部正交网格 ,其离散形式与有限差分方法相似 ,流动系数的取值原则和界面渗透率的取法均与有限差分相同。 CVFE网格是非正交网格 ,本文混合运用有限差分和有限元方法计算折算压力梯度 ,得到与有限差分和 PEBI网格不同的离散形式 ,提出了根据流量符号判定上游的方法 ,使模型更为合理。模型计算表明 ,对于形式简单的对角渗透率张量 ,本文方法可以与九点差分格式相比 ,精度是可靠的 ,并且该方法还能模拟完全各向异性渗透率张量 ,对于精细油藏数值模拟 。

裂隙岩体渗流损伤耦合模型的理论分析

基于自洽理论推导了复杂应力状态下含水裂隙岩体的本构关系及损伤演化方程 ,提出了考虑断裂损伤效应的裂隙岩体渗透张量表达式 ,综合以上两个方面 。

基于离散裂隙网络模型的节理岩体渗透张量及特性分析

节理岩体几何结构非常复杂,研究其渗流特性对于指导含水岩层稳定性分析具有重要价值。应用离散裂隙网络模型DFN方法,基于VC++6.0软件平台,建立了平面渗流分析方法,分析了节理岩体不同几何分布情况下的渗透率张量特征,通过定义渗流定向性系数对岩体渗流的定向性特征进行了定量分析。结果表明:单组节理岩体渗流具有明显的各向异性特征,渗流定向性随着节理角度变化显著;节理随着节理贯通性增加,节理渗透率呈现对数增加趋势;两组节理情况下,各向异性特征随着节理组间夹角变化;两组节理岩体渗流特征研究中,正交分布下,岩体仍存在各向异性,但渗流定向性系数较低;当节理倾角服从正态分布时,随着节理倾角标准差增大,渗透率增加;两组节理夹角不同时,节理渗透主方向倾角随着夹角增大而相应增大,基本沿两组节理夹角方向的角平分线方向。

考虑渗透率张量的非均质油藏有限元数值模拟方法

针对具有混合边界的非均质油藏,考虑全张量形式的渗透率,建立弹性微可压缩单相流体不稳定渗流问题的数学模型.根据变分原理,将压力微分方程的边值问题转化为泛函的极值问题,建立渗流模型的有限元方程.针对典型的均质和非均质渗流问题进行模拟计算,得到油藏内压力动态分布曲线,并分析曲线特征.研究表明,有限元法计算精度很高,适用于求解利用渗透率张量表征的非均质油藏渗流问题.为非均质油藏的开发和精细油藏数值模拟提供了理论依据.

双场耦合条件下裂隙岩体的渗透张量

根据渗流能量迭加原理提出了确定裂隙岩体渗透张量的新模型。该模型采用多种变量描述岩体不连续面及其网络的地质特征，并考虑了渗流与变形的耦合效应。研究表明，新模型与其它模型相比更能全面地揭示裂隙岩体复杂的渗透特性，并适用于渗流场与应力场的耦合分析。