考虑不同尺度孔隙压缩敏感性的砂岩渗透特性研究

以往的研究多是从宏观层面建立孔隙压缩敏感性与渗透率之间的关系,缺乏砂岩不同尺度孔隙结构改变对渗透率演化规律的认识。从细观孔隙层面来看,不同尺度孔隙在应力作用下闭合程度差异明显,考虑不同尺度下孔隙压缩敏感性对准确预测渗透率演化至关重要。因此,利用多场耦合核磁共振试验平台开展了砂岩渗流-应力耦合试验,获得了不同应力作用下砂岩渗透率以及孔径分布曲线的变化规律,将孔径分成大孔(>1μm)、中孔(0.1～lμm)、小孔(<0.1μm)3类,计算了砂岩不同尺度孔隙压缩系数及其转化因子,提出了考虑不同尺度孔隙压缩敏感性的砂岩渗透率计算公式。结果发现:砂岩不同尺度孔隙的压缩性差异明显,大尺度孔隙的压缩敏感性更强;考虑不同尺度孔隙压缩敏感性的渗透率公式与试验值具有较好的一致性。     

JHomogenizer: a computational tool for upscaling permeability for flow in heterogeneous porous media

This paper presents an object-oriented programming approach for the design of numerical homogenization programs, called JHomogenizer. It currently includes five functional modules to compute effective permeability and simple codes for computing solutions for flow in porous media. Examples with graphical output are shown to illustrate some functionalities of the program. A series of numerical examples demonstrates the effectiveness of the methodology for two-phase flow in heterogeneous reservoirs. The software is freely available, and the open architecture of the program facilitates further development and can adapt to suit specific needs easily and quickly.     

Analysis of a mixed discontinuous Galerkin method for instationary Darcy flow

We present an a priori stability and convergence analysis of a new mixed discontinuous Galerkin scheme applied to the instationary Darcy problem. The analysis accounts for a spatially and temporally varying permeability tensor in all estimates. The proposed method is stabilized using penalty terms in the primary and the flux unknowns.     

苏里格气田低渗透储层微观孔隙结构特征及其分类评价方法

以苏里格气田盒8段和山1段为例,开展了低渗透储集层微观孔隙结构特征的研究工作。以取心井压汞测试、扫描电镜、铸体薄片等分析化验资料为基础,通过分析储层的孔隙结构和孔隙类型,将储层微观孔隙结构分为4类,分析了各类微观孔隙结构的特征。沉积环境和成岩作用的双重影响使得盒8段和山1段的储层孔隙结构非常复杂,造成本区储层特低渗透率的主要成因机制为成岩期强烈的压实作用及各种自生矿物的充填和胶结作用。主要的储集空间为次生孔隙,孔喉组合主要为低孔小喉型。应用聚类分析和Bayes判别分析方法,选取7种宏观和微观非均质参数,在建立4类微观孔隙结构判别函数的基础上,对苏里格气田进行了微观孔隙结构识别,绘制了主力小层储层综合评价图。分析表明：Ⅰ类、Ⅱ类储层为研究区的优质储层,是今后低渗透盒8段和山1段的主要勘探开发目标。     

A computational method for approximating a Darcy–Stokes system governing a vuggy porous medium

We develop and analyze a mixed finite element method for the solution of an elliptic system modeling a porous medium with large cavities, called vugs. It consists of a second-order elliptic (i.e., Darcy) equation on part of the domain coupled to a Stokes equation on the rest of the domain, and a slip boundary condition (due to Beavers–Joseph–Saffman) on the interface between them. The tangential velocity is not continuous on the interface. We consider a 2-D vuggy porous medium with many small cavities throughout its extent, so the interface is not isolated. We use a certain conforming Stokes element on rectangles, slightly modified near the interface to account for the tangential discontinuity. This gives a mixed finite element method for the entire Darcy–Stokes system with a regular sparsity pattern that is easy to implement, independent of the vug geometry, as long as it aligns with the grid. We prove optimal global first-order L ² convergence of the velocity and pressure, as well as the velocity gradient in the Stokes domain. Numerical results verify these rates of convergence and even suggest somewhat better convergence in certain situations. Finally, we present a lower dimensional space that uses Raviart–Thomas elements in the Darcy domain and uses our new modified elements near the interface in transition to the Stokes elements.     

松散砂粒孔隙结构、孔隙分形特征及渗透率研究

多孔介质孔隙结构特征是决定流体微观运移机制的重要方面。文章用环氧树脂固结松散石英砂磨制出二维多孔介质薄片,并用孔隙结构图像处理方法提取出孔隙结构参数信息;根据孔隙结构参数,建立描述孔隙结构的分形几何模型,分析松散地层孔隙结构的分形特征;进行多孔介质渗透率的实验测量和分形计算。结果表明:多孔介质的孔隙率和孔径与粒径之间存在线性相关性,拟合度分别达到-0.976 5和0.996 6;在方格边长ε和含有孔隙的格子总数N(ε)的双对数坐标系中,lnε-lnN(ε)数据点近似成直线,且不同样品的孔隙分布分维数值比较接近;而在孔径r与大于该孔径的孔隙总数N(r)的双对数坐标系中,lnr-lnN(r)的数据点必须进行分段回归分析,以便能更好地反映孔隙结构的实际情况;孔隙分布分维数和孔径分维数与粒径也存在较高的对应关系;中砂渗透率的实验测得值是5.19×10-5mm2,孔径分维数分段回归法计算的大孔隙多孔介质渗透率为5.75×10-5mm2,测量值与计算值较为接近,孔径分维数可以计算多孔介质的渗透率。     

Application of health risk assessment method for geological environment at national and regional scales

Health risk, defined as possibility or probability of health damage, illness or death of humans due to exposure to risk factors in the environment, was derived for geological environment (soils) and estimated at national scale for the whole Europe and at more detailed regional scale for Slovak Republic. The assessment was based on data obtained from international geochemical mapping of Europe (Geochemical Atlas of the Europe—827 soil samples) and national geochemical mapping programme of the Slovak Republic (9,860 soil samples). The following chemical elements were evaluated: As, B, Ba, Be, Cd, Cu, F, Hg, Mn, Mo, Ni, Pb, Sb, Se and Zn. The health risk assessment method was based on calculations of average daily doses of individual elements analysed in every collected soil sample. Exposure levels were set by using exposure parameters and reference doses from integrated databases of US EPA. The results of calculations were transformed into various sorts of maps (dot, pixel) to delineate areas where increased contents of risk elements can pose risk to human health. The average levels of chronic and carcinogenic risk are presented in the form of tables for single European countries and administrative units of Slovak republic. The results of European mapping (Geochemical Atlas of Europe) indicate that increased levels of potentially toxic elements in soil (mainly As, Pb and Ni) occur primarily in the countries of southern and western Europe. Such elements are associated with increased health risk for resident population. For the countries of northern Europe health risk was estimated at significantly lower level. Relatively high sampling density in Slovak Republic made it possible to calculate health risk at more detailed scale for individual administrative units (municipalities, provinces). The increased health risk level was found in areas well known for high soil contamination (e.g. mining areas).     

Effect of faceting on pore geometry in texturally equilibrated rocks: implications for low permeability at low porosity

The pore geometry of texturally equilibrated rocks is controlled by the interfacial energy ratio between grain boundaries and solid–liquid boundaries. Faceting at pore walls, which is a common feature of pore networks in rocks, strongly affects the liquid distribution. We investigated the effects of faceting on the equilibrium pore geometries based on image analysis of several systems with various degrees of faceting and dihedral angles. The degree of faceting was assessed by the F value, which is the ratio of the flat interface length at the pore wall to the length of total interfacial boundary between solid and liquid. The F values tend to increase with increasing liquid volume fraction. Little-faceted systems show relatively homogeneous liquid distribution. Moderately-faceted systems with a higher dihedral angle (∼55°) are characterized by development of large pores surrounded by faceted walls and complementary shrinkage of triple junction tubes, whereas strongly faceted systems with a low dihedral angle show no evidence of shrinking triple junction tubes, although most pores are surrounded by faceted pore walls. The faceted systems tend to produce more facet boundaries at the pore walls due to the difference of interfacial energies between the flat and curved surfaces. In the systems with the same degree of faceting, heterogeneity of liquid distribution tends to decrease with dihedral angle. For faceting systems, the permeability of texturally equilibrated rocks with low liquid fraction would be significantly decreased by the relative reduction of triple junction volumes or by closure of channels along grain edge due to the truncation of facet walls.     

普光气田礁滩相复杂孔隙类型储集层渗透率地震预测方法*

礁滩相碳酸盐岩储集层孔隙类型多样,孔—渗关系复杂,由于缺少相应的岩石物理模型,储集层渗透率地震预测一直是一个难题。作者以普光气田为例,通过分析复杂孔隙类型对储集层渗透率的控制作用及不同孔隙类型储集层在常规测井和声学测井上的响应特征,引入反映孔隙形态和连通性的 “孔构参数”表征相似孔隙度样品的岩石物理参数变化;进而建立基于孔构参数的孔隙类型判别标准和不同孔隙类型储集层孔—渗关系模型,对储集层测井渗透率进行精细解释和评价;并通过分析不同孔隙类型储集层的弹性参数与孔构参数的关系,建立起基于孔构参数的岩石物理量版;采用叠前统计学反演方法首先开展孔隙度、孔构参数的地震反演,进一步开展基于孔隙度和孔构参数的渗透率地震反演,实现了复杂孔隙类型储集层渗透率的定量预测。     

一种预测不同温度下非饱和土相对渗透系数的间接方法

高放核废料地下处置、城市供热管道及埋地高压电缆等工程建设的进行,使得考虑温度影响的非饱和土渗透性能越来越受到人们的重视。在考虑温度效应的非饱和土土-水特征曲线研究基础上,结合利用土-水特征曲线预测非饱和土相对渗透系数的方法,建立了一种预测不同温度下非饱和土相对渗透系数的间接方法。所建立的表达式是针对土-水特征曲线的整个吸力范围,从而使得其在应用上更具一般意义。利用MX-80班脱土和黄土土样的试验结果,对不同温度下相对渗透系数随吸力的变化进行了预测,得到了令人满意的结果     

Influence of damage on pore size distribution and permeability of rocks

The model proposed in this article relates permeability to porosity measurements that can easily be performed in the laboratory. The pore size distribution (PSD) curve is updated with strains and damage. The updated volumetric fractions of natural pores and cracks are introduced in the expression of permeability. Contrary to classical permeability models based on PSD integrations, the model proposed in this article accounts for possible changes in the porosity modes: one mode for undamaged samples and two modes for cracked samples. The proposed approach also accounts for varying states of damage, as opposed to classical fracture network models, in which the cracks pattern is fixed. The only material parameters that are required to describe the microstructure are the lower and upper bounds of the pores size for both natural pores and cracks. All the other PSD parameters involved in the model are related to macroscopic parameters that can easily be determined in the laboratory, such as the initial void ratio. The framework proposed in this article can be used in any damage constitutive model to determine the permeability of a brittle porous medium. Drained triaxial compression tests have been simulated. Before cracks initiation, permeability decreases while the larger natural pores are getting squeezed. After the occurrence of damage, permeability grows due to the increase of cracks density. The model performs well to represent the influence of the confining pressure on damage evolution and permeability variations. Copyright © 2012 John Wiley & Sons, Ltd.     

An upscaling method and a numerical analysis of swelling/shrinking processes in a compacted bentonite/sand mixture

This paper presents an upscaling concept of swelling/shrinking processes of a compacted bentonite/sand mixture, which also applies to swelling of porous media in general. A constitutive approach for highly compacted bentonite/sand mixture is developed accordingly. The concept is based on the diffuse double layer theory and connects microstructural properties of the bentonite as well as chemical properties of the pore fluid with swelling potential. Main factors influencing the swelling potential of bentonite, i.e. variation of water content, dry density, chemical composition of pore fluid, as well as the microstructures and the amount of swelling minerals are taken into account. According to the proposed model, porosity is divided into interparticle and interlayer porosity. Swelling is the potential of interlayer porosity increase, which reveals itself as volume change in the case of free expansion, or turns to be swelling pressure in the case of constrained swelling. The constitutive equations for swelling/shrinking are implemented in the software GeoSys/RockFlow as a new chemo‐hydro‐mechanical model, which is able to simulate isothermal multiphase flow in bentonite. Details of the mathematical and numerical multiphase flow formulations, as well as the code implementation are described. The proposed model is verified using experimental data of tests on a highly compacted bentonite/sand mixture. Comparison of the 1D modelling results with the experimental data evidences the capability of the proposed model to satisfactorily predict free swelling of the material under investigation. Copyright © 2004 John Wiley & Sons, Ltd.     

Method and analysis for the upscaling of structural data

3D geological models are created to integrate a set of input measurements into a single geological model. There are many problems with this approach, as there is uncertainty in all stages of the modelling process, from initial data collection to the approach used in the modelling scheme itself to calculate the geological model. This study looks at the uncertainty inherent in geological models due to data density and introduces a novel method to upscale geological data that optimises the information in the initial dataset. This method also provides the ability for the dominant trend of a geological dataset to be determined at different scales. By using self-organizing maps (SOM's) to examine the different metrics used to quantify a geological model, we allow for a larger range of metrics to be used compared to traditional statistical methods, due to the SOM's ability to deal with incomplete datasets. The classification of the models into clusters based on the geological metrics using k-means clustering provides a useful insight into the models that are most similar and models that are statistical outliers. Our approach is guided and can be calculated on any input dataset of this type to determine the effect that data density will have on a resultant model. These models are all statistical derivations that represent simplifications and different scales of the initial dataset and can be used to interrogate the scale of observations.     

颗粒形状对粗粒土孔隙特征和渗透性的影响

粗粒土的颗粒级配、形状和密实度都是影响其孔隙结构,进而影响其渗透特性的重要因素。但长期以来,对颗粒形状影响的关注度较少,其主要原因在于不容易定量描述颗粒形状及其影响的孔隙结构特征。选取已获取长宽比和圆形度的不规则形状碎石、规则形状的球和八面体颗粒分别装填试样,针对颗粒级配和颗粒形状的差异,开展孔隙结构特征和渗透性的对比研究。通过CT扫描试样内部结构图像,重构试样的三维孔隙结构并计算孔隙比表面积。通过渗透试验测得试样的渗透系数。结果表明：试样孔隙比表面积是表达孔隙结构特征的有效参数,在相同级配和孔隙率的条件下,试样孔隙比表面积随颗粒圆形度和趋近球形程度的增加而减小;相同级配和孔隙率的条件下,渗透系数随孔隙比表面积的减小而增大,随颗粒圆形度增大而增大;球形颗粒试样的渗透性最强,试样颗粒越偏离球形,孔隙系统中水流阻力越大,试样渗透性越弱。     

Testing helium equilibrium between quartz and pore water as a method to determine pore water helium concentrations

The effectiveness of carbon capture and geologic storage depends on many factors, including and especially the permeability of the reservoir’s caprock. While caprock integrity is generally assumed if petroleum has been preserved, it is poorly constrained in reservoirs containing only saline waters, and CO₂ leakage poses a potential risk to shallow aquifers. Naturally-occurring He accumulates in pore waters over time with the concentration being strongly dependent on the long term flux of fluid through the caprock. Furthermore, a small fraction of pore-water He diffuses into quartz and this may be used as a proxy for He concentrations in pore water, where dissolved gas samples are difficult to obtain, such as in deep sedimentary basins. In this paper He contained in quartz grains is measured and compared to previously measured pore water concentrations. Quartz was purified from core samples from the San Juan Basin, New Mexico and the Great Artesian Basin, South Australia. Quartz separates were heated at 290 °C to release He from the quartz. The quartz from the San Juan Basin and high purity quartz from the Spruce Pine Intrusion, North Carolina was repeatedly impregnated at varying pressures using pure He, heated and analyzed to build He sorption isotherms. The isotherms appear linear but vary between samples, possibly due to fluid inclusions within the quartz grains as high purity quartz samples partition only 1.5% of He that partitions into San Juan Basin samples. Concentrations of He in the pore water were calculated using the He-accessible volume of the quartz and the air–water He solubility. The mean San Juan Basin He pore water concentration was 2 × 10^–5 cc STP He/g water, ∼400 times greater than atmospheric solubility. Great Artesian Basin samples contain a mean He concentration of 3 × 10^–6 cc STP He/g water or 65 times greater than atmospheric solubility. However, pore water He concentrations in both the San Juan and Great Artesian Basins differ by up to an order of magnitude compared to samples collected with an alternate method. The reason for the offset is attributable to either partial saturation of the pore volume or a lack of He equilibrium between quartz and pore water. Coating of clay or other mineral phases on quartz grains, which tends to reduce the effective diffusion coefficient, may cause the latter. This technique of assessing permeability is promising due to the abundance of existing core samples from numerous basins where carbon sequestration may ultimately occur.     

黄土-古土壤饱和渗透性与孔隙分布特征关系研究

由于沉积环境的差异,古土壤较上覆黄土致密,其饱和渗透系数应低于黄土,但试验结果却显示二者的饱和渗透系数相近。为揭示这一现象的机理,在陕西泾阳南黄土塬开挖33 m的探井,沿井壁按1 m间距取黄土和古土壤原状试样,进行变水头渗透试验,测定试样的饱和渗透系数。同时用压汞试验（mercury intrusion porosimetry, MIP）及扫描电镜（scanning electron microscope, SEM）测试分别获取试样的孔隙分布曲线和微观结构图像,以分析黄土-古土壤饱和渗透性与孔隙分布特征的关系。结果表明：（1）黄土-古土壤地层的饱和渗透系数整体上沿深度方向规律性减小,但相邻黄土和古土壤层的饱和渗透系数无明显差异;（2）MIP及SEM测试结果表明,黄土结构均匀、孔隙大小较为一致,而古土壤具有不均匀的团块-裂隙结构,虽然团块内部较黄土致密,但团块间存在裂隙;（3）饱和渗透系数的大小取决于透水孔隙的体积分数,其中黄土的透水孔隙主要为较大孔隙（孔径>2 μm）,而古土壤的透水孔隙主要为团块间的微裂隙,虽然二者渗透系数相近,但渗透机理完全不同。为研究黄土与古土壤的孔隙分布特性和解决黄土区工程建设中的问题提供了理论依据。     

用积分法求取岩石孔隙结构特征值

针对图解法求取孔隙结构特征值使用条件严格,同时误差较大,而矩法饱和度范围又受限于试验。采用积分法求取特征值,即对孔隙半径进行积分求取特征值。积分法必须先获取孔隙半径数学表达式,即毛管压力数学模型。对毛管压力数学模型进行了探究,在双对数坐标系下推导并建立了毛管压力幂函数模型,采用最小二乘法求取模型参数,并通过对比Sami论文数据验证了推导的模型。深入分析矩法与积分法的联系可知,代入孔隙半径数学表达式即可用积分法求取岩石孔隙结构特征值。最后,对3种方法求取结果进行分析讨论,得出积分法算法严谨,更加接近真实值。这一研究加深了对岩石微观孔隙结构认识。     

砂西油田低渗透储层成岩储集相及储集空间演化模式

综合应用取心井的岩石铸体薄片、图像分析、扫描电镜、电子探针能谱、阴极发光、粘土矿物X衍射分析和镜质体反射率等实验测试资料,研究了E₃¹储层的成岩作用、成岩演化序列和成岩阶段划分。通过分析低渗透储层成岩作用对储集物性的影响,认为造成本区储层砂岩较低渗透率的主要成因机理是成岩期各种自生矿物的充填和胶结作用。根据成岩作用及其特有的储集空间组合,将E₃¹储层划分为4种成岩储集相类型:A相——不稳定组分中强溶解次生孔隙成岩储集相、B相——中等胶结中强压实残余粒间孔成岩储集相、C相——碳酸盐强胶结成岩储集相、D相——早期硬石膏强胶结成岩储集相。详细阐述了低渗透储层中各类成岩储集相的特征及其形成机制,并探讨了其储集空间类型及演化模式。