裂缝性碳酸盐岩微细观组构及力学性能研究

针对川西马鞍塘—雷口坡组碳酸盐岩地层多口井发生不同程度井壁坍塌和掉块的情况,开展了该地区碳酸盐岩被钻井液浸泡前后微细观组构及力学性能变化研究,分析了井壁失稳原因。结果表明：马鞍塘—雷口坡组碳酸盐岩主要以方解石和白云石为主,膨胀性能较弱;该地区碳酸盐岩微裂缝、孔洞十分发育,相对于基岩,渗透率沿裂缝发育方向有显著提高,但裂缝的存在和走向对孔隙度无明显影响;钻井液浸泡效应对碳酸盐岩的力学性能影响不是特别明显,裂缝走向及力学弱面效应对碳酸盐岩力学性能影响最为显著,当裂缝倾角为60°时力学强度降至最低。该研究为评价裂缝性碳酸盐岩井壁稳定性提供了理论依据。     

A study on wellbore stability in fractured rock masses with impact of mud infiltration

Maintaining a stable borehole is one of the major tasks encountered in the oil and gas industry due to the fact that wellbore instability-related problems will result in additional high drilling costs and have a severe impact on drilling schedule. Wellbore stability analysis has therefore been included at the well planning stage of many operating companies. However, majority of such wellbore stability analyses are based on continuum mechanics. Factors contributing to wellbore instability, such as trajectory of the wellbore, orientation and magnitude of the in situ stress field, rock properties, in situ and induced pore pressures and mud pressure, are normally considered in the analyses. In addition to failure in intact rock, wellbore instability can also be initiated along natural discontinuities, such as bedding planes and fractures, in rock masses. Furthermore, the rock masses will become more prone to wellbore instability along fractures penetrated by mud filtrate due to reduction in the fracture friction angle and loosening of blocks. In this paper, coupled numerical analyses are presented to investigate the influence of fractures in rock masses, in particular mud infiltration into the fractures, on wellbore stability under both isotropic and anisotropic stress states. Two regular fracture geometries are considered in the analyses. A simplified approach is developed to take into account the friction angle reduction of the fractures that occurs when infiltrated with mud. The results show that the presence of natural fractures and friction angle reduction of fractures due to mud infiltration significantly affect wellbore stability during drilling.     

损伤力学在粗面岩水力压裂裂缝延伸机理研究中的应用

水力压裂裂缝的研究一般应用断裂力学进行分析,但由于岩石不仅会发生弹性应变而且会发生损伤变化,应该把损伤力学和断裂力学结合起来研究岩石真实的破裂过程。在运用断裂力学分析裂纹的同时,介绍了裂缝的3种基本模式;在结合断裂力学的基础上引入损伤力学对粗面岩的裂缝延伸机理进行了研究,并分析了井壁上的应力、应变和损伤场的分布;结合辽河油田小龙湾地区现场情况,对相关的岩心进行应力试验,得到了粗面岩的抗压强度、弹性模量、泊松比和抗拉强度等相关的岩石力学参数;结合损伤力学,得到了粗面岩损伤的应力应变关系式和井壁上应力、应变及损伤场分布;得到的研究结果对粗面岩油藏的开发有重要的指导意义。     

钻井液对煤层气井壁稳定性影响实验研究

在对煤层气储层岩石组成及理化性能分析的基础上,通过测定煤岩与钻井液接触后的力学性质,研究各种理化性能对煤层井壁稳定性的影响。研究结果表明,钻井液侵入煤层后容易引起黏土膨胀,煤层气储层存在由于水敏引起的煤层井壁失稳的危险;在岩石强度大、无水敏的煤储层可采用清水钻进,在水敏性强的储层应在钻井液中加入黏土抑制剂,并采用欠平衡等钻井方式,控制钻井液侵入。     

Stresses around horizontal boreholes drilled in sedimentary rocks

The main purpose of this paper is to present the anisotropic stress equations that can used to analyze fracturing or collapse problems for horizontal boreholes, and to show how anisotropy in the elastic properties of the rock affects the stress field around a horizontal borehole.This paper gives the complete equations to determine stresses around boreholes in transverse isotropic, but linear elastic sedimentary rocks. The equations to determine the effective stresses are also given. It is shown that the stress fields and their direction is affected by the anisotropic material properties of the rock. Also shown is the deformation pattern of the borehole, which for anisotropic rocks is elliptical rather than cylindrical. Finally, a hydraulic fracturing example is given to show the effect of elastic anisotropy on the fracturing pressure.     

基于三维莫尔圆的裂缝有效性评价方法及应用

复杂储层裂缝评价是储层研究的重要内容。针对复杂储层现今裂缝有效性定量评价的难题,通过阵列声波测井、电成像测井提取的地应力及裂缝参数信息,采用基于临界应力断层假说的三维莫尔圆方法定量分析裂缝有效性的方法,通过井眼周围应力场分布特征,选取合理的地应力模型计算地层三维应力并根据成像测井拾取的裂缝倾角、倾向等属性。在此基础上根据岩石强度理论通过三维莫尔圆计算裂缝面上岩石正应力与切应力,求取岩石裂缝面上的摩擦系数,采用临界应力断层假说定量分析裂缝有效性特征。渤海地区JZ油田应用结果表明,通过三维莫尔圆计算的裂缝摩擦系数能够反映裂缝有效性的差异,与产能匹配较好,可以用于产能分析,是裂缝定量评价的一种有效手段。     

利用井眼坍塌信息求取最大水平主应力的方法

准确获取深部地层地应力的方向和大小,对于井眼轨道设计、井壁稳定、压裂增产等具有重要意义。重点研究了利用井眼坍塌信息求取最大水平主应力的方法,提出结合测井资料分析采用数值模拟方法反演最大水平主应力大小的方法。研究表明:利用井眼坍塌信息求取最大水平主应力大小的数值模拟方法可行,能够真实反映深部地层的应力环境、地层岩石的力学特性,比较准确地确定最大水平主应力的大小,且方法简单易行。      

岩石脆性评价方法进展

脆性是岩石（尤其是深部岩石）的一种非常重要的性质,脆性研究对深部岩体工程建设和资源开发利用等具有重要意义。在页岩油气和致密油气储层“七性”评价中,脆性是地层可钻性分析、压裂选层及施工参数优选的重要指标。国内外学者针对岩石脆性开展了大量研究工作,但是关于岩石脆性的定义和评价方法仍存在分歧,岩石脆性的定量描述还没有一个统一的标准。本文总结了各种现有岩石脆性评价方法的基本原理和研究、应用现状,探讨了岩石脆性评价存在的问题及今后的发展趋势,以期为岩石脆性评价新方法的提出以及评价标准的建立提供参考。     

岩石力学参数的计算及应用

岩石力学性质及地层应力在油田勘探开发中有特别重要的指导作用.实践表明,岩石的力学性质及地层应力与地层的声学性质之间有密切的关系.利用从各种声波测井资料提取出的纵、横波速度(或时差)计算岩石的力学参数,并对岩石力学性质在井眼稳定性、砂岩强度分析、水力压裂作业等方面的应用进行了初步探讨.     

层理页岩水平井井周剪切失稳区域预测方法

为了揭示层理页岩强度各向异性对水平井井周剪切失稳的影响规律,基于直剪试验研究了川南龙马溪组页岩的强度各向异性特征,并与国外典型层理页岩进行了对比;建立了层理页岩井周坐标转换关系模型,结合井周应力弹性解、Mohr-Coulomb弱面强度理论、Mohr-Coulomb准则,建立了层理页岩地层井周剪切失稳区域预测方法;分析了钻进方位对井周剪切失稳区域分布规律的影响。结果表明:弱面与本体强度参数基本呈线性关系,本体内聚力约为弱面内聚力的2.10倍,本体内摩擦角约为弱面内摩擦角的1.20倍,其中川南龙马溪组页岩强度各向异性特征更加显著;考虑层理等弱面影响计算出的失稳区域的形状不是传统的"狗耳朵"形状,而是近似于长方形,这种形状与典型的物模试验结果基本吻合;若页岩地层中存在显著的层理,则井周地层更容易沿弱面剪切滑动,导致水平井坍塌失稳的风险显著增大。利用该方法可以分别计算出井周地层沿弱面剪切、本体剪切和全部剪切的失稳区域分布,为层理发育地层井眼稳定分析、成像测井反演地应力方位等研究提供基础理论支撑。      

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地层破裂压力是钻井工程设计的重要基础数据〔1～2〕，对于开发性地层，利用测井资料或液压实验即可求得，但对于勘探性地层，它则是一个复杂性问题。由于现有预测模型的参数求取多采用地震层速度法，尽管这种方法简单可行，也得到广泛使用，但它的预测精度不高，往往只能作为钻井工程设计的参考。文章基于井眼受力状态分析，给出了一种预测地层破裂压力的力学模型，它涵盖了泊松比、构造应力、多孔介质弹性系数、上覆岩层压力、孔隙压力和岩石拉伸强度等影响因素。鉴于勘探性岩层求参数的固有难度，介绍了一种预测地层破裂压力的求参数新方法，即用空间源直接探矿仪（简称CYT）岩性探测资料求取法——CYT 法。验证实例表明，该方法获得的地层破裂压力预测精度高。     

NON‐DESTRUCTIVE INVESTIGATIONS OF THE THERMAL MATURITY AND MECHANICAL PROPERTIES OF SOURCE ROCKS

Source rocks consist of diverse lithofacies whose composition can vary according to the depositional environment and the provenance of the sediment composing the rock matrix. In general, the rock fabric is made up of very fine grained, laminated and non‐laminated intervals composed of authigenic, detrital and biogenic minerals, together with organic matter consisting of kerogen, bitumen and pyrobitumen. During burial, exposure to elevated temperatures and pressures transforms the kerogen in a source rock into hydrocarbons. This transformation is tracked according to different stages of maturity, and results in increases in both the porosity and aromaticity of the kerogen within the rock and the formation of pyrobitumen. Little is understood about how these changes affect the mechanical properties of the organic matter contained in the rock, or to what degree this also influences bulk rock mechanical moduli. Understanding this relationship is essential to determine the production potential of a “source rock reservoir”, i.e. an unconventional self‐sourced reservoir from which oil and gas can be recovered through hydraulic fracturing of the rock matrix. Direct measurement of the mechanical properties of organic matter in relation to its thermogenic transformation, however, is a challenge due to the small size of the materials composing the rock fabric, especially with source rocks that are finely laminated. To explore this relationship, the results of a non‐destructive, dual‐mode examination of source rock maturity and kerogen mechanical properties is presented using Raman spectroscopy and Atomic Force Microscopy (AFM), respectively. The analysed samples (n = 5) are a Silurian source rock from the Middle East. The results demonstrate the ability to measure both mechanical moduli and thermal maturity of organic components in intact source rock samples which range in maturity from the catagenesis through the metagenesis stages i.e %R_o from 0.6 to 1.6. These techniques provide an opportunity to examine kerogen, bitumen and pyrobitumen properties at the micro‐ and nano‐scale using intact rock samples without disruption of the rock fabric, which readily occurs with conventional bulk core analysis techniques.     

富县探区中生界油藏储层压裂裂缝形态研究

水力压裂裂缝形态是油层压裂设计首先要考虑的问题针对中原油田富县探区中生界油藏储层埋藏深度浅,跨度大(300～1100 m),储层三向地应力变化大,压裂裂缝形态复杂的情况,通过对区域构造特征、岩芯地应力、岩石抗张强度、瞬时停泵压力、压裂时缝内净压力、岩石横向非均质性等综合分析,得出该区人工裂缝以水平缝为主,垂直缝及高角度斜交缝也有可能存在,而且人工裂缝在延伸过程中还可能发生转向。     

A Mechanical Model of Borehole Stability for Weak Plane Formation Under Porous Flow

Abstract

Based on influence of porous flow on weak plane model, the authors established a mechanical model of borehole stability for weak plane formation under porous flow and analyzed effect of weak plane on borehole stability under porous flow. The results indicated that porous flow decreased strength of weak plane, enlarged the affecting domains of weak plane for rock mass strength, and worsened borehole instability in weak plane formations. With porous flow increasing, water content of weak plane increases. For the weak plane with DIP < 10°, the borehole is stable; for 10° < DIP < 30°, the borehole is likely unstable when azimuth is close to the direction of maximum stress, and the larger the dip, the worse the borehole stability; for DIP > 30°, it is opposite to the situation of 10° < DIP < 30°. While weak plane formation is next to be saturated, the minimum drilling fluid density for borehole stability does not change with weak plane azimuth and borehole stability is the worst. The mechanical model of borehole stability for weak plane formation under porous flow is applied to a well in Tarim Basin piedmont.     

Numerical Wellbore Stability Analysis Using Discrete Element Models

Wellbore instability, which is considered as failure of borehole wall, is a complex and time-consuming analysis. Numerous factors such as well path, rock mechanical properties, in situ stress regimes, pore pressure, mud weight, bedding planes, and fractures play a role in borehole instability. Many wellbore stability analyses methods consider only a few of mentioned parameters and majority of them are based on continuum mechanics. However, most of the methods presented for wellbore stability analyses do not consider the impact of fractures. In the present work discrete element models (DEM) are used for numerical wellbore stability analysis of horizontal wells, while the presence of fractures is considered. In this study the effect of mud cyclic loading, hole length, stress regimes, and fluid pressures in fracture plane are examined. The results of analyses showed that direction of minimum horizontal stress is the optimum drilling direction. To evaluate the effects of mud cyclic loadings and open hole lengths, two other models are also considered. These models reveal that mud cyclic loading reduces the stability of borehole. Moreover, it was concluded that mud weight must be increased as open hole length increases and it should not exceed the upper mud weight limit.     

各向异性地层中斜井井壁失稳机理

深部层理性地层井壁稳定性必须考虑地层物理力学性质的各向异性,然而目前井壁稳定设计方面仍粗略地将其近似为各向同性体,使得预测的维持井壁稳定的坍塌压力和破裂压力不能满足安全钻井的需要。笔者在研究层理地层岩石力学特性的基础上,获得了一种考虑地层方向特性的井壁围岩应力分布公式,分析了横观各向同性地层的井壁应力分布特征,建立了新的斜井地层井壁稳定分析模型。研究结果表明,岩石的弹性模量、地应力和地层倾斜度的变化对井壁围岩的应力分布和坍塌、破裂压力均会产生影响,克服了常规各向同性井壁稳定分析模型不能准确预测层理性地层斜井井壁稳定的难题。因此在钻井施工设计当中,须根据地层特性选择合适的分析模型,以更好地指导现场实践。     

利用地震属性钻前预测井壁稳定性

钻前准确预测井壁稳定性是防止钻进过程中井壁失稳的有效手段。声波时差和地层密度是井壁稳定分析的两个关键参数。根据地震记录与地层声波时差及密度之间存在的非线性关系，提出了利用地震属性钻前预测井壁稳定性的方法。从井旁地震记录中提取地震属性，通过RBF神经网络在已钻井段的地震属性与声波时差及密度之间建立起映射模型，以此为基础预测待钻地层的声波时差和密度。运用预测结果结合井壁稳定力学模型，确定岩石力学参数和地应力状态，计算井壁坍塌压力和破裂压力，确定安全钻井液密度窗口，实现钻前井壁稳定预测。该方法在塔里木油田的应用中取得了良好的预测效果。     

渗流对欠平衡钻井井壁稳定性的影响

欠平衡钻井过程中井壁稳定是保证欠平衡钻井成功的关键。在常规钻井时，井壁不稳来自力学和化学作用，而欠平衡加大了这些作用机理。在欠平衡钻井过程中，地层流体不断流入井内，井眼形成后井筒周围的应力将伴随地层流体的渗流而重新分布，进而影响井壁的稳定性。文章将由原地应力产生的应力与地层流体向井眼径向渗流产生的应力叠加而求得了欠平衡钻井井周应力的解析解，运用Mohr-Coulomb强度准则建立了欠平衡钻井坍塌压力的计算式。计算结果表明:在欠平衡钻井过程中，考虑地层流体在岩石中的渗流作用后、地层坍塌压力更大即井壁更易失稳，应用于塔中722井的井壁稳定性分析表明，理论结果与工程实际吻合，说明了模型的正确性，可进一步推广应用。     

非均匀地应力下井壁Von Mises应力及稳定性分析

为分析非均匀地应力作用下井壁Von Mises(米塞斯)应力及其对井壁稳定性的影响,建立了非均匀地应力作用下井壁附近岩石的力学分析模型。根据弹性力学中应力与应力函数之间的关系,应用叠加原理,导出了非均匀地应力下井壁径向应力、环向应力和剪切应力计算公式,分析了非均匀地应力对井壁稳定性的影响,确定了以钻井液密度为自变量的井壁Von Mises应力的二次拟合公式。采用拟合公式计算的井壁Von Mises应力误差小于2.5%,利用该公式可以确定井壁岩石不会破裂的钻井液密度。     

煤系地层坍塌压力预测与井眼轨迹优化

由于煤岩割理交错发育,采用常规井眼坍塌压力预测模型不能准确指导钻井作业,甚至有可能引发井壁失稳事故。通过分析煤岩理化性能和微观结构,确定井壁失稳类型以力学失稳为主;结合弹性力学、岩石力学、单一弱面破坏准则等理论,建立了煤岩地层水平井坍塌压力计算模型;以DB-X01井、DB-X04井与红河油田煤层段为例,根据所建模型进行井眼轨迹优化,得出最佳钻进井斜角与方位角;结合DB-X01井J₂kz煤层,分析了割理产状、地应力机制以及井眼轨迹对坍塌压力的影响,为煤系地层水平井井眼轨迹优化研究提供参考。