脆性地层井壁稳定性评价准则

脆性地层在钻井过程中的井壁稳定问题一直是困扰石油工业界的一个十分复杂的问题,其评价准则的选取是预测井壁稳定性的关键。目前,讨论最多的针对脆性地层井壁稳定性评价准则有:Mohr-Coulomb准则,Hoek-Brown准则,Mogi-Coulomb准则,三维Hoek-Brown准则。在调研国内外评价准则的基础上,介绍了几种方法的基本原理、误差来源分析及适用范围:Mohr-Coulomb准则公式简单,参数获取方便,但只能反映岩块的线性破坏特征;Hoek-Brown准则虽能更好的反映岩块的非线性破坏特征,但由于没有考虑中间主应力的影响,结果是一个偏低的估计;Mogi-Coulomb准则引入了中间主应力的影响,且拟合结果居中,但进行三轴试验要求较高;三维Hoek-Brown准则既考虑了中间主应力的影响,又继承了Hoek-Brown准则的优点,且拟合偏差最小。建议深入研究中主应力对岩石强度的影响效应,推广脆性地层井壁稳定性评价准则适用范围。     

基于弹性应变能强度准则评价井壁稳定性

岩石强度准则是石油工程的重要基础理论,对于井壁稳定性研究更是不可或缺。然而,以经典弹塑性理论的岩石强度准则作为井壁失稳判断依据时所得结果,往往与现场实际情况存在较大误差。开展三轴力学试验,测试岩石不同围压条件下的极限储能,从能量角度建立岩石能量强度准则,并与其他强度准则对比分析了井壁的稳定性。计算结果表明:使用不同强度准则所得坍塌压力相差较大,其中Hoek-Brown准则的坍塌压力过低且存在井壁坍塌风险,Mohr-Coulomb和Drucker-Prager准则的结果过于保守且存在压破地层风险,而根据能量强度准则所得坍塌压力则能保证井壁不塌不漏。该准则是井壁稳定评价的一种新方法,为钻井液密度设计提供了可靠的理论依据。     

井壁稳定性研究中考虑中间主应力强度准则的对比分析

中间主应力σ_2对岩石强度具有显著影响,在井壁稳定性研究中需考虑σ_2效应。常用中间主应力强度准则有统一强度理论(UST)、Mogi-Coulomb(MG-C)准则、Drucker-Prager(D-P)准则、修正Lade准则、三维Hoek-Brown(H-B)准则和修正Wiebols-Cook(W-C)准则等6种。文中对比分析了上述6种强度准则在σ_1-σ_2平面内的曲线,并进行了井壁稳定分析。结果表明:在σ_1-σ_2平面内,除UST准则为双折线形式外,其他准则均为非线性形式;D-P准则所预测强度最高,其他准则预测结果相近。在当量坍塌压力计算中,UST准则(b≠0)、MG-C准则、修正Lade准则、三维H-B准则和修正W-C准则计算结果相接近,D-P准则所计算当量钻井液密度最低。对于安全钻井方位,UST准则下安全井壁稳定分布区域随着参数b值升高而逐渐减小;D-P准则下井壁稳定分布区域最高;MG-C准则、修正Lade准则、三维H-B准则和修正W-C准则所给出的井壁稳定分布区域相近。对比分析后,在井壁稳定性研究中推荐使用MG-C准则。该准则不仅反映了σ_2对岩石强度的影响,且形式简单,便于在实际工程中应用。     

一种准确预测钻井液安全密度窗口的新方法

准确预测钻井液安全密度窗口是井身结构设计和钻井液密度合理选择的前提。常用的钻井液安全密度窗口预测方法是基于Mohr-Coulomb破坏准则和拉伸破坏准则而建立的。但是,由于Mohr-Coulomb破坏准则未考虑中间主应力的影响,预测的钻井液密度偏高,不利于钻井提速和储层保护。为此,引入Mogi-Coulomb破坏准则,建立了3种地应力状态下预测直井钻井液安全密度窗口的新方法,克服了传统方法计算坍塌压力过于保守和计算破裂压力未考虑剪性破裂的不足。将其应用于川西地区H1井砂泥岩地层的钻井液安全密度窗口计算中,结果表明,新方法的预测结果较传统方法的计算结果更符合实际情况,且精度高、实用性强,对解决钻井中的井壁失稳问题有重要的指导意义。     

基于Mohr-Coulomb准则的岩石损伤统计本构模型

通过损伤变量来建立合适的岩石本构模型可以更真实地反映岩石的破坏过程。基于岩石微元强度概率分布理论,以Mohr-Coulomb破坏准则作为分布参量,从理论上建立了岩石损伤变量演化方程和三轴条件下损伤统计本构模型,可采用曲线拟合法与极值法求解所建立模型的参数。用试验结果对所建模型进行了验证,较好的吻合性说明所建模型是合理的。研究表明,用Mohr-Coulomb准则作为统计分布变量建立的损伤模型能够较好地反映岩石内部缺陷分布和变形特征,真实反映岩石强度受应力状态影响的现象,在岩石破碎和井壁稳定等方面具有一定的应用价值。     

基于井壁稳定分析的井眼轨迹优化方法

井壁稳定问题是钻井工程关注的焦点之一,但目前基于井壁稳定分析优化井眼轨迹方面开展的工作还很少。为此,基于斜井井壁应力分布模型和井壁失稳破坏模式,分别采用Mohr-Coulomb准则、Drucker Prager准则和拉张破坏准则导出了维持井壁稳定的钻井液安全窗口上下限计算模型,利用数值计算方法建立了安全密度窗口、安全和危险井眼轨迹的求解方法,计算和分析了6种典型地应力状态下的井壁稳定性,并分析了相应地应力状态下安全和危险的井眼轨迹。结果表明:①Mohr-Coulomb准则计算结果高于Drucker-Prager准则,主要由于Mohr-Coulomb准则忽略了中间主应力的影响,低估了地层岩石的强度;②在σ_v≥σ_Hσ_h情况下沿最小水平地应力钻进稳定性最好,σ_Hσ_vσ_h情况下钻进稳定性最好的方向为偏离最大水平地应力30°~45°的方向上,σ_(11)σ_h≥σ_v情况下沿最大水平地应力钻进稳定性最好,σ_H=σ_vσ_h情况下直井稳定性最好;③数值计算方法得出的安全井眼轨迹与AZ模型吻合较好,验证了该方法的正确性和准确性。数值计算方法可以直观给出不同轨迹下的安全密度窗口,对于窄安全密度窗口地层,可采用该方法优化井眼轨迹改善井眼稳定性。     

油气井出砂预测研究

用塑性应变准则建立了油井出砂预测的有限元力学模型，岩石的屈服应力准则用Drucker-Prager准则，在国内是油井出砂预测的一种新方法。在某油田地层数下，用该模型得到了不同井筒压差所对应井壁附近的塑性应力、应变变化结果，用这些结果可预测油井出砂情况。根据临界塑性应变，确定了井壁不出砂的临界生产压差，为油气井正常生产提供了理论数据。     

钻井液密度与井壁围岩破坏的关系

钻井液密度对井壁围岩的稳定性具有重要的作用。通过有限元分析,根据Drucker-Prager破坏准则,模拟了井壁剪切破坏的连续过程,并且给出了井壁围岩破坏尺寸与钻井液密度的关系,对于分析钻井液密度对井壁围岩破坏区域的影响提供了定量依据。计算出的模拟规律与现场观测和实验室内试验结果基本一致。     

裸眼测试井壁失稳概率及参数敏感性分析

为保证南海高温高压区块裸眼测试期间的井壁稳定性，基于裸眼层段高温和渗流特点，结合温度应力理论和多孔渗流效应，建立了裸眼井壁渗流-温度耦合应力模型。对比分析Mohr-Coulomb准则、Drucker-Prager准则、Hoek-Brown经验准则和能量破坏准则下的井壁失稳临界测试压差，并利用蒙特卡罗模拟，用正态分布和三角分布模拟岩石力学参数计算井壁失稳概率，采用Sobol方法对临界测试压差参数进行全局敏感性分析。研究结果表明，Drucker-Prager准则更适合作为裸眼测试井壁失稳临界测试压差计算准则，小于临界测试压差3～5 MPa时已经存在井壁失稳的可能，最大水平主应力对临界测试压差起到决定性作用。裸眼测试过程中，临界测试压差与实际测试压差的差值应保持在5 MPa以上，对于最大水平主应力变化较大的地层减小测试压差，以防止发生井壁失稳。     

页岩水平井井眼轨迹方位与层理面产状的关系

准确掌握层理面产状对页岩水平井井眼轨迹方位选取的影响规律,能有效预防井壁坍塌,节约钻井成本。对长宁区块H1井龙马溪组岩心进行三轴压缩实验,依据页岩岩石强度单一弱面剪切破坏准则和页岩水平井井壁应力分布模型,建立了页岩水平井井壁坍塌压力计算模型。利用页岩水平井测井资料和岩石力学实验数据,采用数值模拟方法,得到层理面产状对页岩水平井井壁坍塌压力的影响规律。当页岩层理面倾向一定时,随层理面倾角的逐渐增大,页岩水平井井壁坍塌压力当量密度范围逐渐变宽。当层理面倾角小于45°时,井壁岩石发生层理面的剪切破坏;当层理面倾角大于45°时,井壁岩石发生本体破坏,且当井壁岩石发生本体破坏时,页岩水平井沿层理面倾向的相反方向钻进有利于井壁稳定。     

A Mud Weight Window Investigation Based on the Mogi-Coulomb Failure Criterion and Elasto-Plastic Constitutive Model

Abstract

A successful stable borehole scheme results in saving time and money. To reach this aim many parameters such as bottom hole pressure, and stress distribution around the borehole must be considered in the design of a well. During drilling there are two main wellbore stability problems, namely, borehole collapse and fracture. These stability problems can lead to the need for fishing, stuck pipes, sidetracking, and lost circulation. These drilling problems can be mitigated or often eliminated by proper determination of the critical mud pressure.

The most common approach for these studies is a linear elastic and isotropic constitutive model in conjunction with linear failure criteria like Mohr-Coulomb. Numerous researchers have found situations in which the Mohr-Coulomb criterion was deficient because mid-principle geotechnical stress is not incorporated. In addition, the conventional mechanical model is based on the linear elasticity condition, while in reality, it is believed that the fluid barrier and a part of the bore hole wall behave plastically, which provides higher fracturing pressure than Kirsch's equation.

In this article, the mud window is estimated using Mogi-Coulomb failure criterion and the elasto-plastic model. This is based on a hypothesis that wellbore behavior indeed is not strictly elastic and intermediate principle stress plays an important role in rock strength. Therefore, an elasto-plastic model and the Mogi-Coulomb failure criterion should be incorporated. This hypothesis is verified and used in this article for the South Pars gas field (phases 6, 7, and 8) in the Persian Gulf. The results indicate that the newly developed model is more accurate than the conventional approach.     

Analysis of the Mechanical Stability of Boreholes Drilled in Sedimentary Rocks

This article presents the method and results of wellbore stability analysis for three common reservoir lithologies consisting of a consolidated sandstone, a shaly sandstone, and a limestone formation. The effect of stress anisotropy on the mechanical stability of wellbores is evaluated while varying the inclination angle from 0 to 90°, for both the Mohr-Coulomb and the Drucker-Prager failure criteria. The selected failure criterion, and the in-situ rock stress regime are found to have significant effects on the safe drilling fluid density required to maintain wellbore integrity. According to some field examples, the Drucker-Prager failure criterion appears to systematically mimic rock conditions more realistically than the Mohr-Coulomb failure criterion. The simulated consolidated sandstone formation is found more stable with lesser drilling fluid density, at any inclination angle, than the simulated shaly sandstone formation. The simulated limestone formation is even more stable than the consolidated sandstone at all inclination angles since it requires lighter fluid density to prevent wellbore collapse. For all these rock types, the higher the deviation angle (from vertical), the higher the drilling fluid density needed for maintaining wellbore integrity. For the depth and rock conditions simulated, both consolidated and shaly sands are unstable in a strike-slip stress regime, but stable in an extensional stress regime. The simulated limestone formation was found stable in both stress regimes. However, in an extensional stress regime, the limestone formation required lighter fluid density to maintain wellbore integrity than in a strike-slip stress regime. This article introduces the theory of using a practically-oriented model to assess the mechanical stability of a wellbore in a linearly-elastic stress field. The model can be used to determine the range of mechanically stable well inclinations for a given formation, and to suggest drilling-fluid density programs tailored to efficient and safe drilling.     

3D finite element modelling of multilateral junction wellbore stability

Wellbore failure can occur at different stages of operations. For example, wellbore collapse might happen during drilling and/or during production. The drilling process results in the removal of an already stressed rock material. If the induced stresses near the wellbore exceed the strength of rock, wellbore failure occurs. The production process also changes the effective stresses around the wellbore. Such changes in stresses can be significant for high drawdown pressures and can trigger wellbore failure. In this paper, the Mohr–Coulomb failure criterion with a hyperbolic hardening is used. The model parameters are identified from triaxial compression tests. The numerical simulations of laboratory tests showed that the model can reproduce the mechanical behaviour of sandstone. In addition, the simulations of multilateral junction stability experiments showed that the model was able to reproduce yielding and failure at the multilateral junction for different levels of applied stresses. Finally, a numerical example examining multilateral junction stability in an open borehole during drilling and production is presented. The results illustrate the development of a localized failure zone proximate to the area where two wellbore tracks join, particularly on the side with a sharp approaching angle, which would significantly increase the risk of wellbore collapse at the junction.     

Analysis of the Mechanical Stability of Boreholes Drilled in Sedimentary Rocks

Abstract

This article presents the method and results of wellbore stability analysis for three common reservoir lithologies consisting of a consolidated sandstone, a shaly sandstone, and a limestone formation. The effect of stress anisotropy on the mechanical stability of wellbores is evaluated while varying the inclination angle from 0 to 90°, for both the Mohr–Coulomb and the Drucker–Prager failure criteria. The selected failure criterion, and the in-situ rock stress regime are found to have significant effects on the safe drilling fluid density required to maintain wellbore integrity. According to some field examples, the Drucker–Prager failure criterion appears to systematically mimic rock conditions more realistically than the Mohr–Coulomb failure criterion. The simulated consolidated sandstone formation is found more stable with lesser drilling fluid density, at any inclination angle, than the simulated shaly sandstone formation. The simulated limestone formation is even more stable than the consolidated sandstone at all inclination angles since it requires lighter fluid density to prevent wellbore collapse. For all these rock types, the higher the deviation angle (from vertical), the higher the drilling fluid density needed for maintaining wellbore integrity. For the depth and rock conditions simulated, both consolidated and shaly sands are unstable in a strike-slip stress regime, but stable in an extensional stress regime. The simulated limestone formation was found stable in both stress regimes. However, in an extensional stress regime, the limestone formation required lighter fluid density to maintain wellbore integrity than in a strike-slip stress regime. This article introduces the theory of using a practically-oriented model to assess the mechanical stability of a wellbore in a linearly-elastic stress field. The model can be used to determine the range of mechanically stable well inclinations for a given formation, and to suggest drilling-fluid density programs tailored to efficient and safe drilling.     

A new well path optimization model for increased mechanical borehole stability

Borehole collapse is the commonly encountered wellbore stability problem during drilling. The precursor of such borehole failure can be often eliminated by a proper determination of the critical mud pressure. Recently, Al-Ajmi and Zimmerman developed a new 3D analytical model to estimate the mud pressure required to avoid shear failure (i.e., borehole collapse) at the wall of an arbitrarily oriented borehole. This was achieved by using linear elasticity theory to calculate the stresses around the borehole, in conjunction with the fully-polyaxial Mogi–Coulomb criterion to predict failure. This model was used to study the behaviour of the collapse pressure under different in situ stress regimes. We found that the optimal drilling trajectory is directly related to the principal in situ stresses. Accordingly, we now develop a new analytical solution to optimize well path. The stability model has been applied to several field cases, from the North Sea, Indonesia and the Arabian Gulf, and the new model in each case is consistent with the field experience. We conclude that the optimal wellbore profile can be ensured by employing this new analytical stability model.     

Study on Mechanisms of Borehole Instability in Naturally Fractured Reservoir During Production Test for Horizontal Wells

During the production test for horizontal wells, down-hole pressure will drop down significantly to induce the crude oil flowing out of the wellbore, which always leads to wellbore collapse problems, especially in naturally fractured reservoirs. Hence, some special researches on this problem were carried out. Based on the mechanisms of wellbore stability and failure criterion of weak plane, the authors established the stress state around the horizontal wellbore and mathematical model of minimum down-hole pressures required to maintain the wellbore stable. This model is complex nonlinear equations, which involves several variables, including natural fracture occurrences, wellbore orientation, in situ stress, and rock strength. The combination impacts of these variables on the minimum down-hole pressure were quantified by solving this nonlinear model. The study helps deepen the understanding of the mechanisms of wellbore stability in naturally fractured reservoir.     

基于井壁稳定控制建立井眼轨迹优化预测模型

通过线性规划最优解理论及井壁剪切破坏条件,建立了井眼轨迹优化的控制方程及提出相应的数值求取方法。研究表明：当地应力大小关系满足控制方程的临界条件时,正断层作用应力区域钻直井或沿水平最小地应力方位钻水平井时,两者井壁稳定性相同,该情形也发生在逆断层作用应力区域钻直井或沿水平最大地应力方位钻水平井,此外,走滑断层作用应力区域,水平井沿水平最大或最小地应力方位钻进时,其井壁稳定性 也存在相同的可能性;另外,当水平最大地应力与上覆岩层压力比一定时,水平地应力比值（σ_h/σ_H）越大,正断层作用应力区域沿水平最小地应力方位,及逆断层作用应力区域沿水平最大地应力方位钻进的最优井斜角越小,反之,走滑断层作用应力区域,水平井的最优钻进方位与水平最大地应力方位夹角越 大。研究成果有助于指导现场施工高效钻进。     

节理煤层井壁稳定性的评价模型

对节理煤层的井壁失稳机理分析表明,近井壁的裂纹受到地应力和裂纹应力的双重影响,比煤岩块体更容易遭到破坏。用应力强度因子描述了裂纹的受力集中程度,利用强度准则构建了裂纹扩展破坏的判据,据此提出了节理煤层井壁稳定评价模型。该模型可以定量判断节理煤层直井或水平井井壁是否稳定,对其他节理地层的井壁失稳评价也具有借鉴意义。应用该模型并结合实例评价了煤储层水平井的井壁稳定,分析了裂纹倾角、井底压力、裂纹长度和井眼半径对应力强度因子的影响。对于节理煤层水平井,随着井底压力的增加,应力强度因子K_Ⅰ和K_Ⅱ呈线性减小,井壁稳定性变好;随着裂纹长度的增加,应力强度因子K_Ⅰ和K_Ⅱ也显著增加,井壁稳定性变差;裂纹倾角增大,应力强度因子K_Ⅰ减小,而K_Ⅱ增加,裂纹的抗张能力增强,抗剪能力降低,井壁的整体稳定性变差。当井径增加时,K_Ⅱ增加,抗剪能力降低,井壁的整体稳定性变差。节理煤层中井眼存在尺寸效应,煤岩中存在裂纹是其重要原因。裂纹与井径的比值越大,尺寸效应越明显。