Pore-scale contact angle measurements at reservoir conditions using X-ray microtomography

Contact angle is a principal control of the flow of multiple fluid phases through porous media; however its measurement on other than flat surfaces remains a challenge. A new method is presented for the measurement of the contact angle between immiscible fluids at the pore scale at reservoir conditions (10 MPa and 50 °C) inside a quarry limestone through the use of X-ray microtomography. It is applied to a super-critical CO₂–brine–carbonate system by resampling the micro-CT data onto planes orthogonal to the contact lines, allowing for vectors to be traced along the grain surface and the CO₂–brine interface. A distribution of contact angles ranging from 35° to 55° is observed, indicating that the CO₂–brine–carbonate system is weakly water-wet. This range of contact angles can be understood as the result of contact angle hysteresis and surface heterogeneity on a range of length scales. Surface heterogeneity is examined by comparison of micro-CT results with optical thin sections and SEM images.     

A multiple-point statistics algorithm for 3D pore space reconstruction from 2D images

Fluid flow behavior in a porous medium is a function of the geometry and topology of its pore space. The construction of a three dimensional pore space model of a porous medium is therefore an important first step in characterizing the medium and predicting its flow properties. A stochastic technique for reconstruction of the 3D pore structure of unstructured random porous media from a 2D thin section training image is presented. The proposed technique relies on successive 2D multiple point statistics simulations coupled to a multi-scale conditioning data extraction procedure. The Single Normal Equation Simulation Algorithm (SNESIM), originally developed as a tool for reproduction of long-range, curvilinear features of geological structures, serves as the simulation engine. Various validating criteria such as marginal distributions of pore and grain, directional variograms, multiple-point connectivity curves, single phase effective permeability and two phase relative permeability calculations are used to analyze the results. The method is tested on a sample of Berea sandstone for which a 3D micro-CT scanning image is available. The results confirm that the equi-probable 3D realizations obtained preserve the typical patterns of the pore space that exist in thin sections, reproduce the long-range connectivities, capture the characteristics of anisotropy in both horizontal and vertical directions and have single and two phase flow characteristics consistent with those of the measured 3D micro-CT image.     

吉木萨尔凹陷芦草沟组致密油储层微观孔喉结构研究?

通过微纳米 CT、FIB 双束扫描电镜等实验手段,对吉木萨尔凹陷二叠系芦草沟组致密油储层进行详细的探讨。研究发现,吉木萨尔凹陷二叠系芦草沟组储层为中孔-低渗、特低渗、低孔特低渗储层,孔隙以微纳米级为主,类型多样,主要有粒间孔（缝）、粒间溶孔、晶间孔、层间缝及微裂隙等。不同类型储集段,储层物性和孔隙结构差异较大：上甜点区砂屑云岩孔隙以粒间孔（缝）和溶孔为主,多为微米级;下甜点区灰质粉砂岩以粒间孔和晶间孔为主,孔隙多为纳米级,0.05～0.1μm;非甜点区的灰质炭质泥岩主要发育晶间孔、层间缝和微裂隙,裂隙宽约5～6μm。     

应用CT技术研究瘦煤在不同温度下孔隙变化特征

应用µCT225kvFCB型高精度(µm级)CT试验分析系统,对直径1 mm的瘦煤从18 ℃到600 ℃高温下的热破裂过程进行了显微CT观测和分析.发现了瘦煤的热破裂规律:300 ℃煤样中大量小的孔隙贯通成大的孔隙团,因此瘦煤的热破裂阈值在300 ℃附近,从常温到300 ℃煤样的孔隙率增加了0.5倍左右.研究了煤样在100~600 ℃范围内热破裂孔隙分布的分形规律,研究发现:从100 ℃到600 ℃,煤样的分形维数D随温度升高呈现先减小、后增加的趋势.孔隙的演化规律:由初始较规则的小孔隙变形,过渡到大量较规则、小孔隙贯通成不规则的大的孔隙团,之后孔隙团缩合减小趋于规则.分布初值的对数随温度升高呈现先降低后增加的趋势;从18 ℃到600 ℃孔隙数量的演化规律为先减小后增加.     

基于微CT技术的砂岩数字岩石物理实验

数字岩石物理技术可弥补传统岩石物理实验的诸多不足,为岩石物理学研究提供一个新平台.本文以常规砂岩为研究对象,利用微CT扫描结合先进的图像处理技术建立了具有真实孔隙结构特征的三维数字岩芯模型;应用Avizo软件内含的多种形态学算法进行数字岩芯孔隙结构量化及表征研究,统计获取了孔隙度、孔隙体积分布及孔径分布特征,建立了等价孔隙网络模型;将Avizo与多场耦合有限元软件Comsol完美对接,实现了孔隙尺度的渗流模拟并计算获得绝对渗透率,对于考虑固相充填孔隙的情况,模拟计算了岩石有效弹性参数,并与近似Gassmann方程良好验证.本文所提出的将Avizo与Comsol结合使用的方法丰富了现有的数字岩石物理研究手段,为其大规模发展提供了一条新途径.     

基于全局低分辨率CT扫描的合成层析成像技术

合成层析成像可由有限几个投影数据重建三维物体,与传统CT相比,该方法所需的射线剂量少得多.由于投影数据不完备,导致的重建图像伪影是合成层析成像的一个主要问题.本文提出一种对感兴趣区域进行合成层析成像方法,和其他方法相比可以得到更好的重建图像质量.该方法首先对目标进行低分辨率全局扫描,然后对感兴趣区域进行高分辨率局部扫描,最后,利用两次扫描的投影数据集合重建感兴趣区域的图像.数值仿真结果表明,用本方法重建感兴趣区域图像,要明显好于没有使用全局扫描信息的方法.     

多孔介质微观孔隙结构三维成像技术

详细介绍目前国际上最先进的多孔介质微观结构三维成像技术,包括系列切片技术、聚焦离子束技术(FIB)、激光共聚焦扫描显微镜、Micro-CT技术,分析了各自的优越性及局限性,探讨了Micro-CT技术的应用现状及其在地质学领域的应用前景.分析结果表明,Micro-CT及今后的Nano-CT技术是一种无损伤的3D成像技术,不但可获得足够分辨率的多孔介质微观孔隙结构3D图像,而且还可以现场实时检测孔隙中流体的渗流状态,在多孔介质孔隙结构及渗流机理研究中具有重要的应用价值.该技术必定成为今后的主流技术,不仅将在石油工程、水文工程等领域发挥重要的作用,而且还将广泛应用于岩石学、矿物学、古生物学、材料学、资源科学及环境科学等领域.该技术的研究和应用在我国还处在起步阶段,应加大设备引进和研究力度.     

美国爱荷华大学CT/显微CT实验室科研成果与学术交流

爱荷华大学CT／显微CT实验室是美国该专业中颇有名望的研究单位，它在CT／显微CT理论，技术及床应用等领域研究成果丰硫，作为访问学者的作用曾在该实验室工作一年，为了帮助国内同行了解国外CT／显微CT科学研究状况，促进国际学术交流，特撰写此文与读者共享。     

显微CT的锥束重建技术

显微CT技术一直是Iowa大学CT/显微CT实验室研究的焦点。这篇文章报告了我们在锥束显微CT方面最近取得的一些进展。我们利用Feklkamp类算法了多X光源和检测器偏置条件下的近拟图象重建；解释了Grangeat精确重构框架下的图象伪影。最后重点介绍了最近有关一般分块迭代Landweber格式的收敛性结果；给出了模拟和实验结果，探讨了进—步的研究方向。     

Crushing and embedment of proppant packs under cyclic loading:An insigh to enhanced unconventional oil/gas recovery

Crushing and embedment are two critical downhole proppant degradation mechanisms that lead to a significant drop in production outputs in unconventional oil/gas stimulation projects.These persistent production drops due to the non-linear responses of proppants under reservoir conditions put the future utilization of such advanced stimulation techniques in unconventional energy extraction in doubt.The aim of this study is to address these issues by conducting a comprehensive experimental approach.According to the results,whatever the type of proppant,all proppant packs tend to undergo significant plastic deformation under the first loading cycle.Moreover,the utilization of ceramic proppants(which retain proppant pack porosity up to 75％),larger proppant sizes(which retain proppant pack porosity up to 15.2％)and higher proppant concentrations(which retain proppant pack porosity up to 29.5％)in the fracturing stimulations with higher in-situ stresses are recommended to de-escalate the critical consequences of crushing associated issues.Similarly,the selection of resin-coated proppants over ceramic and sand proppants may benefit in terms of obtaining reduced proppant embedment.In addition,selection of smaller proppant sizes and higher proppant concentrations are suggested for stimulation projects at depth with sedimentary formations and lower in-situ stresses where proppant embedment pre-dominates.Furthermore,correlation between proppant embedment with repetitive loading cycles was studied.Importantly,microstructural analysis of the proppant-embedded siltstone rock samples revealed that the initiation of secondary induced fractures.Finally,the findings of this study can greatly contribute to accurately select optimum proppant properties(proppant type,size and concentration)depending on the oil/gas reservoir char-acteristics to minimize proppant crushing and embedment effects.