Mass transfer controlled by fracturing in micritic carbonate rocks

The fractured Coniacian chalk from the Omey area (Paris Basin, France) displays strong evidence of modifications controlled by brittle deformation. Fracturing is associated with important changes in pore space (decrease in total porosity and pore interconnection, change in distribution of pore access diameters and capillary characteristics), nannofacies (gradual evolution from a point-contact fabric to a welded, interlocked or coalescent fabric) and chemical composition (Sr concentration decrease). These modifications result from fluid–rock interaction that control significant mass transfer (percentage of secondary calcite >50%). Sr is a remarkable indicator of these mass transfers. Sr analyses allowed us to prove that the deformed zone (26.7 m) is wider than the fractured zone (11.3 m). They also indicate that the footwall block is less affected than the hanging wall block. A physicochemical model of the deformation mechanism is proposed. It shows that a cyclic process of fracturing controls the temporal evolution of the fluid saturation and fluid pressure and, consequently, the mass transfer.     

Mass transport in porous rocks

The physical processes by which materials may move within porous rocks are briefly reviewed. In impermeable rocks diffusion through the solid rock limits mass transfer. Diffusion coefficients in solids are typically of the order 10^–15 to 10^–8 cm²/sec and temperature-dependent. When there is no fluid convection in a permeable rock, diffusion through liquid-filled pores, with diffusion coefficients of the order 10^–6 cm²/sec, will be the dominant mass transport process. Fluid convection in permeable rocks can increase mass transfer rates by many orders of magnitude. The pressure gradient in the fluid, fluid viscosity, and the specific permeability of the rock are dominant in determining the fluid velocity and mass transfer rates. The role of these transport processes in the deposition and leaching of orebodies in porous rocks is discussed.

---

Zusammenfassung Massenbewegung in undurchlässigem Gestein ist durch die Diffusion durch das feste Gestein begrenzt. Die temperaturabhängigen Diffusionskoeffizienten in Festkörpern sind von der Größenordnung 10^–15 bis 10^–8 cm²/sek. Wo keine Flüssigkeitskonvektion in einem durchlässigen Gestein stattfindet, ist Diffusion durch flüssigkeitshaltige Poren, mit Diffusionskoeffizienten von der Größenordnung 10^–6 cm²/sek., der wichtigste Massentransport-Prozeß. Der Massentransport durch durchlässiges Gestein kann durch Flüssigkeitskonvektion um viele GrÖßenordnungen vergrößert werden. Der Druckgradient in der Flüssigkeit, die Viskosität der Flüssigkeit und die spezifische Permeabilität des Gesteins sind die wichtigsten Faktoren, welche die Geschwindigkeit und Menge des Massentransportes bestimmen. Das Wesen dieser Transportprozesse bei der Ablagerung und Auslaugung von Erzlagerstätten wird diskutiert.

     

Optimal design of hydraulic fracturing in porous media using the phase field fracture model coupled with genetic algorithm

We present a framework for the coupling of fluid-filled fracture propagation and a genetic inverse algorithm for optimizing hydraulic fracturing scenarios in porous media. Fracture propagations are described by employing a phase field approach, which treats fracture surfaces as diffusive zones rather than of interfaces. Performance of the coupled approach is provided with applications to numerical experiments related to maximizing production or reservoir history matching for emphasizing the capability of the framework.     

A new type of open fracturing in the rocks

The finest open fractures (ultramicrofractures) were found from the study of surface density of open fractures in the rocks from the super-deep Uralian borehole SG-4. The high-resolution method of the registration of open fractures in the rocks is described. The typical structures of systems formed by open fractures are shown and their genesis is proposed.     

Chemically induced fracturing in alkali feldspar

Fracturing in alkali feldspar during Na⁺–K⁺ cation exchange with a NaCl–KCl salt melt was studied experimentally. Due to a marked composition dependence of the lattice parameters of alkali feldspar, any composition gradient arising from cation exchange causes coherency stress. If this stress exceeds a critical level fracturing occurs. Experiments were performed on potassium-rich gem-quality alkali feldspars with polished (010) and (001) surfaces. When the feldspar was shifted toward more sodium-rich compositions over more than about 10 mole %, a system of parallel cracks with regular crack spacing formed. The cracks have a general (h0l) orientation and do not correspond to any of the feldspar cleavages. The cracks are rather oriented (sub)-perpendicular to the direction of maximum tensile stress. The critical stress needed to initiate fracturing is about 325 MPa. The critical stress intensity factor for the propagation of mode I cracks, K _Ic, is estimated as 2.30–2.72 MPa m^1/2 (73–86 MPa mm^1/2) from a systematic relation between characteristic crack spacing and coherency stress. An orientation mismatch of 18° between the crack normal and the direction of maximum tensile stress is ascribed to the anisotropy of the longitudinal elastic stiffness which has pronounced maxima in the crack plane and a minimum in the direction of the crack normal.     

Microcrack-based coupled damage and flow modeling of fracturing evolution in permeable brittle rocks

Microcracks in brittle rocks affect not only the local mechanical properties, but also the poroelastic behavior and permeability. A continuum coupled hydro-mechanical modeling approach is presented using a two-scale conceptual model representing realistic rock material containing micro-fractures. This approach combines a microcrack-based continuous damage model within generalized Biot poroelasticity, in which the tensors of macroscopic elastic stiffness, Biot effective stress coefficient and of overall permeability are directly related to microcrack growth. Heterogeneity in both mechanical and hydraulic properties evolves from an initially random distribution of damage to produce localized failure and fluid transmission. A significant advantage of the approach is the ability to accurately predict the evolution of realistic fracturing and associated fluid flow in permeable rocks where pre-existing fractures exert significant control. The model is validated for biaxial failure of rock in compression and replicates typical pre- and post-peak strength metrics of stress drop, AE event counts, permeability evolution and failure modes. The model is applied to the simulation of hydraulic fracturing in permeable rocks to examine the effects of heterogeneities, permeability and borehole pressurization rate on the initiation of fracturing. The results indicate that more homogenous rocks require higher hydraulic pressure to initiate fracturing and breakdown. Moreover, both the fracturing initiation pressure and breakdown pressure decrease with permeability but increase with borehole pressurization rate, and the upper and lower limit of the initiation pressure are seen to be given by the impermeable (Hubbert–Willis) and permeable (Haimson–Fairhurst) borehole wall solutions, respectively. The numerical results are shown to be in good agreement with the experimental observations and theoretical results. This coupled damage and flow modeling approach provides an alternative way to solve a variety of complicated hydro-mechanical problems in practical rock engineering with the process coupling strictly enforced.     

三轴加载下煤岩脉冲水力压裂扩缝机制研究

为了研究煤岩脉冲水力压裂扩缝机制,利用三轴加载脉冲水力压裂试验系统,分别从改变脉冲频率和压裂液黏度两个方面对煤岩进行了水力压裂室内试验。试验结果表明:脉冲压力和声发射对压裂过程的响应情况可将煤岩脉冲水力压裂扩缝过程分为零散萌生、均匀扩展、突变贯通和破裂终止4个阶段,其中声发射定位点空间展布的平均速率在均匀扩展和突变贯通阶段较零散萌生阶段分别提高了4.6倍和9.6倍;声发射b值曲线的趋势显示,均匀扩展阶段煤岩内部以小尺度的微裂纹破裂为主,突变扩展阶段煤岩内部出现了较大尺度主裂缝扩展,并且部分b值曲线呈现出山脊线状与阶梯状相结合的特点;在达到最佳压裂脉冲频率前,高频脉冲压力作用的煤岩,扩缝过程的均匀扩展阶段和突变贯穿阶段的延续时间比低频对应的延续时间短;随着压裂液黏度增高,扩缝过程均匀扩展阶段的延续时间呈缩短趋势,但突变贯穿阶段的延续时间却有增长的趋势。     

煤岩水力压裂裂缝扩展形态试验研究

通过室内水力压裂物理模拟试验系统,对大尺寸原煤进行了水力压裂模拟试验,根据水力裂缝的空间展布形态分析了煤岩储层水力裂缝的延伸规律,揭示了网状裂缝的形成机制。结果表明：水力裂缝易在弱层理处分叉和转向,发育的层理和裂缝系统等结构面为压裂形成裂缝网络提供了前提条件。泵压曲线呈现出的频繁波动是煤岩内产生网状裂缝的一个显著特征。水力裂缝的起裂与延伸有4种基本模式,裂缝网络的形成多为这4种基本模式的组合。地应力差异系数和泵注排量对煤层水力裂缝形态有较大影响。较小的地应力差异系数更利于网状裂缝的形成;较高的压裂液排量易形成相对简单的裂缝形态,导致压裂改造效果较差。该试验方法和试验结果可为现场水力压裂参数设计和优化提供参考和依据。     

水力劈裂对岩体中自然裂纹的影响研究

建立了求解自然裂纹和水力裂纹扩展的扩展有限元法,对裂纹附近区域的节点采用广义形函数,并采用线增函数消除混合单元,以提高裂纹附近的精度。引入水力劈裂的非耦合模型,即假设裂纹中的水压力为均布力;用砂浆法（线段-线段接触法）结合增广型拉格朗日乘子法处理受压裂纹段的接触条件。并通过算例分析了以下内容：计算了受压裂纹和裂纹面分布均布水压力的水力裂纹的应力强度因子,并与解析解进行了比较,结果表明,提出的方法具有很高的精度;模拟了水力裂纹对自然裂纹面的影响,并分析了自然裂纹面上的接触力和接触状态。     

Numerical modeling of fracturing in permeable rocks via a micromechanical continuum model

The paper presents a micromechanical approach to describe the failure of low-permeability brittle rocks as a multiscale fracturing process based on a poroelastic microcrack-damage model. Failure is formulated deep down at the fine pore scale as a material degradation phenomenon driven by microcrack growth that also impacts upon hydromechanical properties. A set of damage tensors describes the effect of dual-scale porosities (nanopores and microcracks) on both the hydraulic and poroelastic rock properties. Essentially, the multiscale model reconstructs the coupling effect of hydromechanical forces at the continuum level from the ground up through the upscaling of multiphase interactions at the fundamental structural level of the material. As a result, many macroscopic characteristics emerge naturally such as friction angle, fracture properties, and most importantly, Biot's coefficient taking on a tensorial form that is generally anisotropic. The model is validated within the framework of finite elements to illustrate various baseline constitutive features such as the effect of microcrack growth on the nonlinear stress-strain response and the induced anisotropy in the context of lab experimental tests and boundary value problems. Heterogeneities of the rock samples were incorporated by choosing material properties to be stochastic following Weibull and lognormal distributions. Numerical results appropriately replicated typical experimental observations where fracture localization and propagation are shown to be a multiscale phenomenon emerging from microcrack growth and coalescence at the microscale, with concomitant enhancement in fluid conductivity.     

Investigation of hydraulic fracturing operation and fracture propagation simulation in reservoir rock

For many decades most oil wells in Iran have produced using their natural flow potential and haven’t needed to be fractured. As time goes by, the reservoir pressure depletes and the need for hydraulic fracturing as a stimulation practice arises. Nonetheless there is no record of successful hydraulic fracturing in Iran.

The Bangestan reservoir with a suitable amount of oil in place and good rock reservoirs, has been selected for the present research work. In this work, the in situ stress profile was calculated by using the available petrophysical data. This is achieved by using poroelastic theory for the stresses, and the Mohr–Coulomb criterion to predict failure. The model leads to easily computed expressions for calculating the pressure required to maintain hydraulic fracturing. Then the appropriate depth for treatment was determined. The results indicate that Ilam and Sarvak formations could be good candidates for hydraulic fracturing. Then, for two layers, a hydraulic fracture was designed and the production was predicted and the Net Present Value (NPV) resulting from the fracture of both layers was investigated.     

岩溶地区隧道裂隙水突出力学机制研究

岩溶地下水是诱发隧道发生突（涌）水地质灾害的主导因素之一,岩溶裂隙水对隧道围岩的危害越来越成为岩溶地区隧道建设中的热点研究问题之一。基于目前岩溶裂隙水突出机制研究现状,运用岩溶地质学、工程水力学和断裂力学相关理论分析岩溶地区隧道水岩相互作用机制,探讨了水岩相互作用对岩溶地区隧道施工发生突（涌）水的影响,揭示岩溶地区隧道裂隙水突出前后过程的力学机制。研究表明：岩溶隧道裂隙水突出是裂隙岩体在岩溶水及水压的持续作用下受施工外力干扰发生劈裂的结果,岩溶水和水压在裂隙岩体突水破坏过程中的力学作用主要体现在4个方面,即突水蓄势期岩溶水对裂隙岩体的软化溶蚀作用、水压对裂隙岩体的劈裂作用,突水失稳期水流的冲刷扩径作用、水压对突水量的动力控制作用。基于上述分析,以断裂力学、弹塑性力学理论为基础,提出岩溶隧道岩溶裂隙水突出的最小岩石防突厚度概念,推导了其半解析解表达式,并为工程实例所验证,其结果可为高风险岩溶地区隧道突水理论与防治措施提供参考。     

天然矿物材料的多孔结构、结构组装和光催化性能

自然界存在丰富的多孔矿物资源,其孔隙结构使其具有良好的吸附性能,能够吸附聚集水体中的污染物(特别是有机污染物)。在分析天然多孔矿物的孔结构基础上,文中提出了结构组装的概念,即在多孔矿物中进行功能粒子组装,对多孔矿物进行表面修饰、微结构组装(如层状结构矿物的插层)。通过结构组装,在多孔矿物中引进功能粒子(如有光催化性能的TiO2 颗粒)。组装在天然矿物中的功能粒子能够降解聚集在其中的污染物,大幅度提高功能粒子的效果,提高光催化降解的功效,获得性能优异、成本低廉的天然矿物型有机污染物降解材料。此外,在同一矿物材料中复合小孔、中孔、大孔不同大小的孔结构,实现不同大小的孔径组装,也能够扩大光催化剂TiO2 的承载面积,提高吸附性能。经过结构组装的天然多孔矿物材料复合了吸附、光催化等性能,在环境治理特别是有机物污染降解方面有很好的应用价值。     

Graphical representation of mineral equilibria and materials balances in igneous rocks

The Thompson projection traditionally used by metamorphic petrologists is modified and used to study mineral equilibrium and mass balance relations of igneous rocks. Proportions of minerals in rocks and equilibrium minerals assemblages are predictable from bulk rock compositional data, consequently the projection simplifies chemical studies of plutonic and volcanic rock suites, and mixed plutonic-volcanic suites particularly, because bulk rock compositions can be directly compared with mineral compositions. As an example, changes to bulk magma compositions resulting from differentiation by crystal fractionation (Thingmuli Volcano; Red Hill Dyke) are immediately discernible and tholeiitic calc-alkaline and alkaline differentiation trends are quite distinct on the diagram. As well, minerals which have been removed from a magma during crystal fractionation generally can be identified and their compositions estimated. Magmas the compositions of which result from the mixing of two components (Kilauea Volcano) are easily identified as are the end-member mixing components of the mixed magmas.The diagram is applicable to both igneous and metamorphic rock suites, consequently it should be of particular use to those studying anataxis and granite genesis.     

Response of dry- and liquid-filled porous rocks to static and dynamic loading by variously-shaped projectiles

Summary The present investigation was undertaken to study the response of both dry and liquid-filled porous rocks to static and dynamic loading of penetrators with various tip shapes. Eighteen static and forty-eight dynamic tests were conducted involving 6.35 mm diameter indenters featuring flat, hemispherical and 60° conical tip shapes on three different rocks in the energy range from 1.57 to 5.63 J. Force-penetration data were collected by means of strain gages attached to the penetrators. For all three materials, the magnitude of the resistive forces for a given input energy decreased in the following order: (i) dynamic penetration into a dry sample, (ii) dynamic penetration into a fluid-filled specimen, (iii) static loading of the fluid-filled rock, and (iv) static, loading of the dry rock.An unusual observation for the dynamic force-penetration curves was the temporal disparity between the peak force and maximum deformation, which must be attributed to inertial factors. Based on the experimental data, an analytical rigid-body model was constructed that contained both a static and a dynamic component of the resistive force with the objective of quantifying the effect of dynamic loading and the presence of fluid on the response of the rocks. The dynamic component was portrayed as a viscous resistance proportional to the velocity of the penetrator and the contact surface area which is a function of the indentation. It was found that the model provides a good predictive capability for the dynamic force-indentation relations for prescribed materials and tip geometries upon use of but a single constant for the viscosity.This paper is dedicated to the memory of Corwin O. Rogers.     

基性岩流变的复杂性与成分对岩石流变影响的实验研究

在采用天然基性岩样品进行的流变实验中,存在因实验温度低而样品处于半脆性变形域与因实验温度高而样品颗粒边界出现熔体的矛盾。文献中给出的流变参数中,应力指数n的可靠性高,具有良好的重复性,激活能Q在一定范围内具有重复性,而系数A重复性差。采用6种样品进行流变实验。结果表明,矿物成分对岩石流变的影响比根据端员组分确定的双组分和多组分岩石流变物理模型要复杂得多。由斜长石、单斜辉石和角闪石组成的样品中,当斜长石和单斜辉石含量接近,而角闪石含量低于10%,斜长石和单斜辉石控制了岩石的流变;如果样品中有超过10%的角闪石参与流变,角闪石在变形中所起作用非常显著。由斜长石、石英和角闪石组成的样品,当石英含量小于10%而斜长石和角闪石含量接近时,斜长石和角闪石控制了岩石的流变;当斜长石含量达到70%时,样品变形更接近长石特性;如果石英含量超过20%时,石英在变形中起到主要作用。选择与基性麻粒岩中主要矿物成分接近的天然辉长岩(辉绿岩),或者从天然基性麻粒岩中挑选出主要矿物,通过热压合成人工样品,进行高温流变实验是研究基性麻粒岩流变的最有效途径。     

深孔水力采矿碎岩与矿浆举升机具设计与试验

钻孔水力采矿技术作为一种新型非传统采矿方法,在开采易分散、疏松、多孔、弱胶结性等固体矿产时显示出独特的优点。基于钻孔水力采矿原理,主要对碎岩高压水枪工作原理和结构进行了研究,对矿浆举升系统等方面进行了优化设计,通过数值模拟分析和地表、孔内实物模型试验进行了设计验证,并研究了相应的钻孔水力采矿工艺,这些研究为未来深部固体矿产领域开发提供技术支撑。     

Cauldron subsidences,granitic rocks,and crustal fracturing in S. E. Australia

The structure and history of several Devonian cauldron-subsidences and related granitic intrusions showing evidence of sub-surface ring-fracturing in South Eastern Australia are discussed. The Marysville Igneous Complex includes fish and plant bearing beds in the Acheron Cauldron, which has a diameter of 15 miles, and contains 6000 ft. of acid lavas, with subordinate andesites and basalts. Most of the occurrences are consanguineous. The parent magma is intermediate, close to the hypersthene dacites of the region, and is believed to have been injected as a magma wedge from the east.     

A possible mechanism for elastic wave propagation in porous rocks

Doklady Earth Sciences -