Time-dependent Behaviour of Deep Level Tabular Excavations in Hard Rock

Summary Although hard rock is not usually associated with large creep deformation, significant time-dependent behaviour is observed in the tabular excavations of the South African gold mines. Time-dependent closure data was collected in stopes of the Ventersdorp Contact Reef and Vaal Reef. This data typically consists of a primary closure phase after blasting, followed by a steady-state closure phase. This closure behaviour is the result of the rheology of the fracture zone around these excavations and the time-dependent extension of this zone following a mining increment. An elasto-viscoplastic approach was developed to simulate the time-dependent nature of the fracture zone. This model proved successful in simulating the experimental closure profiles. It appears that the closure data may provide useful diagnostic information of the stress conditions in the fracture zone ahead of the face. This may possibly be used to identify hazardous conditions such as areas prone to face bursting. The effect of preconditioning on the time-dependent closure behaviour is also illustrated.     

Determination of Rock Fracture Toughness <Emphasis Type="Italic">K</Emphasis><Subscript>IIC</Subscript> and its Relationship with Tensile Strength

Hydraulic fracture propagation is greatly influenced by mode-II fracture toughness since this is one of the factors which determine whether a fracture diverts. Direct measurement of rock fracture toughness is constrained by high cost, limited number of available cores and long turn around time. Therefore, to overcome these constrains, it is necessary to develop an effective mode-II fracture toughness test which can be used in a prediction analysis for deep rock formations. Consequently, a mode-II fracture toughness test system was designed for rocks using the straight-notched Brazilian disc (SNBD) test methodology. In the experiment, this system was used to test 20 rock samples from the WG oilfield. This enabled a fracture toughness prediction model to be established, based on an analysis of the test data. H341 acoustic, density and gamma-ray logging data were used to predict horizontal stresses and rock tensile strength. When combined with the mode-II fracture toughness prediction model, continuous values were predicted, which were successfully confirmed by field fracturing practices. It was confirmed, therefore, that this successful method met the need of providing continuous fracture toughness data during field fracturing operations.     

Determination of a spatial fracture network

Hydraulic and mechanical properties of rock masses are largely dependent upon the distribution and variation of fracture areas. Consequently, it is essential to develop good models for analysing the spatial geometric characteristics of fracture fields. The latter, however, may be ascertained only by measuring open cuts or boreholes. The most important factor from the analysis of areas is to be able to determine spatial networks. Without an exhaustive and expensive inventory, the classical methods involving measurements of dip and strike do not provide either a correct estimation of fracture orientations in space, or a good estimation of their distribution. At present, automatic analysis of linear traces on large plane areas is used by several authors^1,2 and the field strike and dip measurements are applied only for verification purposes. This method of computing spatial networks is especially suitable because of the nature of data so obtained, since it permits various simulations on computed networks. It is based upon classical methods of vector geometry that involve a measurements data matrix and provide a means of solving a double system of equations. Results are the directional spatial fracture densities. The object of this paper is to define the geometric parameters of this problem and the computation method, to present an example and, finally, to conclude with critical comments supported by certain proposals for a variety of applications.     

利用纵波AVA进行裂缝储层和流体识别

裂缝储层的地震响应表现为各向异性,描述裂缝物理特征参数正柔量和剪切柔量是裂缝密度函数。含气裂缝和含油、水裂缝的剪切柔量相等;而含气时,正柔量不为零,含油、水时,正柔量为零。横波分裂能反演裂缝密度和方位,但是不能指示裂缝中流体类型。近年来,纵波速度和振幅的方位特征检测裂缝得到了广泛应用,这里根据线性滑动模型建立了描述裂缝物理特性参数和各向异性参数之间的关系,分析了流体类型识别的可行性,并利用多方位角纵波AVO反演各向异性参数和柔量参数,以此来预测裂缝中流体类型。     

鄂尔多斯盆地西南部镇泾油田延长组致密砂岩储层裂缝测井识别

低孔低渗致密砂岩储层的测井评价和开发难度较大,裂缝发育情况对这类储层的产能影响十分重要。为了利用常规测井资料实现致密砂岩储层裂缝发育情况的评价,基于井径、电阻率、密度、声波等常规测井曲线计算出8个裂缝指示参数,然后利用这8个参数构建了一个裂缝综合识别参数。该裂缝综合识别参数对裂缝的识别正确率更高。将这一套方法应用于鄂尔多斯盆地西南部镇泾油田延长组致密砂岩储层裂缝识别,对于所有样本,裂缝综合指标法识别正确率为81.4%以上。本研究给出的方法对于利用常规测井资料评价致密砂岩储层的裂缝发育情况和有效性提供了一种途径,对致密砂岩储层的勘探开发有一定的帮助。     

Effect of initial damage on rock pulverization along faults

Pulverized rocks have been found in the damage zone around the San Andreas Fault, at distances greater than 100 m from the fault core. This damage is atypical in that it is pervasive and strain is not localized along main fractures as expected at these distances from the fault core. With high strain rate experiments, the authors have previously shown that above a strain rate threshold, the localization of strain along a few fractures is inhibited. Pulverized rocks may be generated by seismic waves at high frequency. Here we generalize these conclusions by discussing the effect of the initial fracture network in the sample on the transition from strain localization along a few fractures to diffuse damage throughout the sample. Experimental data are compared with statistical theory for fracture propagation. This analysis shows that the threshold in strain rate is a power law of initial fracture density and that a pre-damaged rock is easier to pulverize. This implies that pulverized rocks observed on the field may result from successive loadings.     

鄂尔多斯盆地西缘冲断带平凉段地质—地球物理特征与地热勘查

本文对近期平凉地热勘查中引用以往推断的近SN向和NEE向隐伏断裂的推断依据进行了核实,并研究该区石油地质和地球物理场特征对区域断裂构造的反映,结合石油地质构造探讨了重力场特征在该区识别断裂和褶皱的标志,认为平凉城区地热勘查推断的两条近SN向隐伏断裂向深部延伸依据不足,NEE向断裂解释有误,同时根据重力场特征反映的平凉颉泾河断陷和基底断裂的构造格局,论述平凉市区内颉泾河断陷不具备南北向深断裂及其导水导热的构造条件。     

井斜角对裂缝特征参数统计的影响及其意义

井轨迹对裂缝钻遇率有重要的影响。以某裂缝性碳酸盐岩油藏为例,利用100余口井的FMI资料按照小斜度井（包括直井）和大斜度井（包括水平井）分组分别统计裂缝的平均倾角、裂缝密度参数,并与不考虑井斜角利用所有FMI资料统计的结果相互对比,以分析井斜角对裂缝倾角、裂缝密度参数统计的影响。结果表明,按小斜度井钻遇裂缝统计的裂缝平均倾角和裂缝密度均明显低于按大斜度井和所有井未分组钻遇裂缝的统计结果;因此,小斜度井钻遇高角度裂缝的几率明显低于大斜度井。这一结论具有三方面的意义：对于本文中某裂缝性碳酸盐岩油藏,目前主要为直井和小斜度井钻遇的G-H层,应部署水平井进一步评价其油气勘探潜力;为更准确地统计高角度裂缝性储层的裂缝倾角并客观地评价裂缝密度,应尽量利用井轨迹垂直于裂缝走向和主要裂缝面的水平井钻遇的裂缝来分析统计;对于同类高角度裂缝性油藏的勘探评价和开发,应尽量采用大斜度井和水平井并确保井轨迹垂直于主要裂缝面。     

致密碎屑岩储层裂缝和产能预测的单井建模—以XZ气田沙溪庙组为例

近年来，利用三维地震资料进行致密碎屑岩储层裂缝和产能的识别预测取得了一些进展，而这之中最重要也是基础的工作是单井裂缝和产能的建模工作，本文以XZ气田沙溪庙组致密储层为例，详细介绍了综合运用岩芯资料、测井资料和录井资料进行裂缝和产能单井建模的方法，为利用三维地震资料建立裂缝和产能的灰色遗传过程识别预测模式打了良好的基础。     

扩展有限元法在岩石断裂力学中的应用

岩石裂纹的扩展是一个经典的不连续问题,常规有限元方法难以实现裂纹扩展过程的仿真模拟。扩展有限元法(XFEM)实现了计算网格与不连续面相互独立,因此模拟移动的不连续面时无需对网格进行重新剖分。本文介绍了XFEM基本原理和岩石断裂力学常用判据,尝试对岩石类材料单缝Ⅰ型三点弯曲、单缝剪切和双缝平板实验进行模拟。分析结果表明:扩展有限元模拟岩石类材料断裂问题不受网格划分限制,裂纹以实际应力场分布随机扩展;直观地给出岩样的微裂纹产生、演化,直至完全破坏的全过程,并与实验结果吻合。该方法能够应用到岩石断裂力学方面的研究,模拟岩石类材料的宏细观破坏过程,为解决复杂问题提供了方便的途径。     

利用地震P波确定煤层瓦斯富集带的分布

查明煤层瓦斯富集区域,对可能的瓦斯突出点进行预报,是当前煤矿生产中亟待解决的重要课题。利用地震P波对裂缝性地层所表现出的方位各向异性特征,根据地震属性随方位角变化可以预测裂隙发育方向和密度的基本原理,应用多种地震P波方位属性预测裂隙发育带。通过对淮南张集煤矿西三采区三维地震P波资料的处理,获得6个方位地震数据体,从中提取多种与煤层和围岩裂隙相关的地震属性,并计算出裂隙的发育方向和密度,为确定瓦斯富集带的分布提供了一种新的途径。      

四川省青川断裂的特征及形成的物理条件

本文详细地讨论了四川省青川断裂的断裂特征及形成的物理条件。对断裂构造岩的微构造和宏观构造进行了细致的描述分析,将断裂岩分为四个带。结合区域资料,讨论了断裂的形成时代及与邻区的关系。     

重庆走马岭岩溶隧道涌水量初步研究

隧道涌水问题普遍存在,从隧道的施工到竣工运行都受其影响,特别是在南方岩溶地区尤为严重。根据勘察资料,走马岭隧道穿越可溶岩段(含断层破碎带)长1525m,占隧道总长的62%,极易发生隧道涌水事故。本文结合已开挖段隧道涌水的实时监测数据及前期工程勘察资料,对隧道涌水做出初步预测评价,为以后的施工提供依据。     

基于Ruger近似式预测裂缝型储层的可行性研究

叠前纵波方位各向异性检测裂缝是目前应用最为广泛的方法,该方法主要是利用AVAZ地震资料椭圆拟合的长轴与短轴信息来评价裂缝,该方法在具体应用时,认为裂缝型储层AVAZ响应特征的变化仅由各向异性参数决定,而没有考虑非各向异性参数的影响,从而引起裂缝评价的多解性。为提高该技术的裂缝评价精度,从模型分析入手,将模型中各向异性参数及岩性参数设置为概率密度分布函数,然后采用Monte Carlo随机方法进行叠前AVAZ正演模拟。首先,对各向异性参数对椭圆拟合的影响进行简单分析,得出各向异性参数γ对椭圆扁率B/A与各向异性因子B影响最大、δ次之、ε最小。然后,进一步重点分析不同标准差的速度与密度模型的B/A与B响应特征,得出地层的速度与密度的变化在较小范围内时,该技术评价裂缝才具有一定的可行性,其中,地层纵波和横波速度的变化（尤其是纵波速度）对叠前AVAZ响应影响较大,地层密度几乎没有影响,故当横向岩性变化较大时,该方法在评价裂缝型储层时的结果具有一定的不确定性。      

热液流体泵吸上侵管流动力学模拟及其预测意义

地球流体相在各种地质过程中的行为和作用 ,是地球物质科学的核心课题。通过对山西义兴寨脉状金矿热液流体上侵动力学的模拟计算证明 ,构造泵吸驱动的矿液流动具有有压管流的性质 ,并探索引入柏努利方程对其流动状态进行描述。论证了地质流体因其性质和所处地质环境的不同 ,可划分为多孔介质中的渗流和较宽阔裂隙中的管流 (流动 )两种。赋存于地壳中深部或由岩浆冷却分异形成的成矿流体 ,在张剪性断裂活动中 ,主要由构造泵吸机制驱使其上侵成矿。热液在相对封闭 (未喷出地表 )的较宽阔的断裂系统中受应力驱动上侵流动 ,其性质接近于流体力学中的有压管流 ,可以用柏努利方程进行描述。推导出了描述热液管流上侵动力学特征的柏努利方程关系式 ,表明热液的流速与所处构造压力差、流体的性质 (温度和密度等 ) ,以及流动通道的结构特征有关。对义兴寨脉状金矿进行的计算结果证明 ,热液上侵流速场的空间变化与金矿化分布格局相吻合 ,流体的高流速地段指示含矿石英脉密集区 ,因此具有成矿预测意义。针对热液管流上侵过程建立的动力学总方程中 ,每一个变量都有明确的地质意义 ,并可以在矿区实际测量 ;绘制的几组理论关系曲线有助于具体分析各种地质和地球化学条件对成矿的控制作用。     

热液汉体泵吸上浸管流动力学模拟及其预测意义

地球流体相在各种地质过程中的行为和作用,是地球物质科学的核心课题。通过对山西义兴寨脉状金矿热液流体上侵动力学的模拟计算证明,构造泵吸驱动的矿液流动具有有压管流的性质,并探索引入柏努利方程对其流动状态进行描述。论证了地质流体因其性质和所处地质环境的不同,可划分为多孔介质中的渗流和较宽阔裂隙中的管流（流动）两种。赋 于地壳中深部或由岩浆冷却分异形成的成矿流体,在张前性断裂活动中,主要由构造泵吸机制驱使     

Appraisal of fracture sampling methods and a new workflow to characterise heterogeneous fracture networks at outcrop

Characterising fractures at outcrop for use as analogues to fractured reservoirs can use several methods. Four important fracture data collection methods are linear scanline sampling, areal sampling, window sampling and circular scanline sampling. In regions of homogeneous fracture networks these methods are adequate to characterise fracture patterns for use as outcrop analogues, however where fractures are heterogeneous, it is more difficult to characterise fracture networks and a different approach is needed.We develop a workflow for fracture data collection in a region of heterogeneous fractures in a fold and thrust belt, which we believe has applicability to a wide variety of fracture networks in different tectonic settings. We use an augmented circular scanline method, along with areal sampling to collect a range of fracture attribute data, including orientation, length, aperture, spatial distribution and intensity. This augmented circular scanline method more than halves the time taken for data collection, provides accurate, unbiased data that is representative of local fracture network attributes and involves data collection of a wider range of fracture attributes than other sampling techniques alone.     

准噶尔盆地西北缘二叠系火山岩储层裂缝发育特征及分布预测——以金龙2井区佳木河组为例

准噶尔盆地西北缘金龙2井区二叠系佳木河组裂缝是该区火山岩储层油气主要的渗流通道。综合岩心、岩石薄片及成像测井等资料,识别出该区主要发育的裂缝类型为半充填或未充填高角度缝,其次为半充填低角度斜交缝与网状缝。成像测井解释裂缝方位近东西向,与岩心古地磁解释现今地应力最大主应力方向近似平行,有效性开启较好。火山岩储层裂缝发育主要受构造与岩性两种因素影响。距离断层越近,由于构造曲率增大,裂缝越发育,裂缝多沿断裂呈条带状分布。不同的火山岩类型,裂缝发育程度也不同。通过成像测井资料分析,认为研究区中-酸性火山熔岩及火山碎屑熔岩裂缝较发育,并进一步定量计算出单井裂缝密度、裂缝倾角、裂缝孔隙度等,确定单井裂缝发育特征。结合叠前地震预测方法,即叠前方位各向异性法(AVAZ),优选衰减起始频率属性,预测了佳木河组火山岩储层裂缝分布特征。     

Simulation of drilling‐induced compaction bands using discrete element method

Rock failure is observed around boreholes often with certain types of failure zones, which are called breakouts. Laboratory‐scale drilling tests in some high‐porosity quartz‐rich sandstone have shown breakouts in the form of narrow localized compacted zones in the minimum horizontal stress direction. They are called fracture‐like breakouts. Such compaction bands may affect hydrocarbon extraction by forming barriers that inhibit fluid flow and may also be a source of sand production. This paper presents the results of numerical simulations of borehole breakouts using 3D discrete element method to investigate the mechanism of the fracture‐like breakouts and to identify the role of far‐field stresses on the breakout dimensions. The numerical tool was first verified against analytical solutions. It was then utilized to investigate the failure mechanism and breakout geometry for drilled cubic rock samples of Castlegate sandstone subjected to different pre‐existing far‐field stresses. Results show that failure occurs in the zones of the highest concentration of tangential stress around the borehole. It is concluded that fracture‐like breakout develops as a result of a nondilatant failure mechanism consisting of localized grain debonding and repacking and grain crushing that lead to the formation of a compaction band in the minimum horizontal stress direction. In addition, it is found that the length of fracture‐like breakouts depends on both the mean stress and stress anisotropy. However, the width of the breakout is not significantly changed by the far‐field stresses. Copyright © 2013 John Wiley & Sons, Ltd.     

Scanline Method to Determine the Fractal Nature of 2-D Fracture Networks

A previous method proposed to measure the fractal dimension of pore spaces is adapted and modified for 2-D fracture networks. The method relies on scanning a 2-D fracture network through successive straight lines from top to bottom and measuring the distance between two fractures. The fractal dimension is then obtained using the log–log plot of the feature size and the number of features for this particular size at different magnifications. It is shown in this study that the method proposed to measure the fractal dimension of porous structures can be applicable to 2-D fracture networks with some modifications after testing it on synthetic and natural fracture patterns. The method is simplified to be useful for practical applications in the fractal analysis of fracture networks. The results reveal that, on the basis of the direction of scanning lines, different fractal behavior and dimensions can be obtained indicating that 2-D fracture networks possess multifractal character. This approach takes into account the effect of fracture orientation on the fractal behavior and anisotropic nature of fracture networks as well as the fracture density, length, and spatial distribution.