Constraining the stress tensor in the Visund field, Norwegian North Sea: Application to wellbore stability and sand production

In this study we examine drilling-induced tensile wellbore failures in five exploration wells in the Visund oil field in the northern North Sea. We use observations of drilling-induced wellbore failures as well as density, pore pressure, and leak-off test measurements to estimate the magnitudes and orientations of all three principal stresses. Each well yields a very consistent azimuth of the maximum horizontal stress (100°±10°), both with depth and laterally across the field. Stress orientations are constrained at depths as shallow as 2500 m and as deep as 5300 m in these wells. We show that the magnitudes of the three principal stresses (S_v, S_hmin, and S_Hmax) are also consistent with depth and reflect a strike-slip to reverse faulting stress regime. The magnitude of the maximum horizontal stress is shown to be significantly higher than the vertical and minimum horizontal stresses (e.g. S_v=55 MPa, S_hmin=53 MPa, and S_Hmax=71.5 MPa at 2.8 km depth). Data from earthquake focal plane mechanisms (Lindholm et al., 1995, Proceedings of the Workshop on Rock Stresses in the North Sea, Trondheim, Norway [1]) show similar stress orientations and relative magnitudes and thus indicate a stress field that is relatively consistent throughout the thickness of the brittle crust.We illustrate how knowledge of the full stress tensor allows one to place bounds on in situ rock strength and determine optimally stable trajectories for wellbore stability and sand production during drilling, after the completion of drilling, and as pore pressure is reduced during oil and gas production.     

Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude

Detailed investigation of failure of the borehole wall in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forêts, France, has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling. Comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress S_H. A model for the initiation of the fractures is presented which points out the important influences of (a) the tectonic stress state, (b) increased mud pressures during drilling operation and (c) thermal stresses induced by circulation of relatively cold drilling mud. Analysis of drilling-induced fractures in the GPK1 borehole at Soultz-sous-Forêts (where the magnitude of S_H is known from hydraulic fracturing experiments) demonstrates the validity of this model for the initiation of the fractures. Further, a new method is proposed to estimate the magnitude of S_H from the occurrence of drilling-induced fractures and knowledge of thermally induced stress and pumping pressure during drilling. The method is successfully applied to both KTB boreholes. An independent method to estimate the magnitude of S_H based on the analytical calculation of the stress intensity factor for drilling-induced fractures taking into consideration both, increased mud pressure and thermal stress, is also presented. Application of this method confirms the results derived with the analysis described above. Additionally, the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress.     

基于岩芯非弹性应变恢复量测定的深孔三维地应力测试方法

 虽然存在各种地应力测试方法,但并没有一个比较完善的方法测量较大深度条件下主应力的大小和方向。所以,在以科学研究为目的的钻井工程中,最好能够结合钻孔法和岩芯法得到三维地应力的大小和方向。叙述一个新颖而且有效的岩芯地应力测试法——非弹性应变恢复量测定法的基本原理、实验技术和基本操作程序;并给出其在科学钻探工程中的一个具体应用实例。测试结果表明,非弹性应变恢复量测定法对于测试三维地应力的主方向和估测主应力的大小有较大的实用价值。特别是在某些较大深度钻井的条件下,如在应力释放法及水压致裂法都无法实施,或者需要一些其他测试方法来补充一些不足或来增添其测试结果的可信度的时候,此方法的应用价值更高。     

Orientation and magnitude of in situ stress to 6.5 km depth in the Baltic Shield

Understanding the state of stress in the earth is important for a broad range of engineering and geological problems. To obtain the state of stress in boreholes where conditions are such that conventional stress measurement techniques are impossible, we have used recent developments in the analysis of compressive and tensile wellbore failure in an integrated stress measurement strategy, involving also direct measurement of the least principal stress. The analysis is carried out in the two deep boreholes in the Siljan Ring area of the Baltic Shield. The Gravberg-1 borehole reached 6779 m true vertical depth (TVD) in the Siljan region, central Sweden, and the Stenberg-1 borehole, drilled 10 km to the south of Gravberg-1, was completed at 6529 m TVD. Analysis of vertical, drilling-induced tensile fractures in the nondeviating part of the Gravberg-1 well indicated that one principal stress is vertical and thus could be calculated from density estimates. Borehole breakouts and tensile fractures indicated that the average direction of the maximum horizontal stress, S_H, is N72°W±7° in Gravberg-1 and N53°W±9° in the Stenberg-1 well. The direction of S_H is on average very stable in both wells. Lower bound limits on the magnitude of the minimum horizontal stress, S_h, in the Gravberg-1 well were obtained from controlled and uncontrolled hydraulic fracturing and formation integrity tests. At 5 km depth in the Gravberg-1 borehole the minimum horizontal stress is approximately two-thirds of the vertical stress. We estimated the magnitude of the maximum horizontal stress in Gravberg-1 on the basis of drilling-induced tensile fractures identified in the borehole. S_H was estimated by calculating the stress at the borehole wall necessary to cause tensile failure of the formation, incorporating our lower bound S_h estimates, corrections for the cooling of the wellbore by drilling fluids and differential fluid pressures. Our results indicate a strike-slip faulting regime in the Siljan area and that the state of stress is in frictional equilibrium with a coefficient of friction in the range 0.5 to 0.6.     

A combined Bayesian–wavelet–data fusion approach for borehole enlargement identification in carbonates

Petrophysical logs are usually acquired in most of the drilled wells and some of them have good correlations with mechanical properties of the rock. In this paper, a new multi-variable workflow is proposed in order to identify the location of borehole enlargements along the wellbore in correlation with some of the petrophysical logs acquired using wireline or logging while drilling tools in addition to mud weight and in-situ vertical stress data. This approach employs number of data processing techniques including Bayesian classification, wavelet de-noising, and data fusion to determine borehole intervals with maximum likelihood of enlargement. The application of the proposed method is to identify enlargement zones and does not provide information about stresses orientations and magnitudes. This paper explains the methodology and presents its results in five study wells in a carbonate field. The study confirms the applicability and the generalization capability of the method in carbonate formations with a significant accuracy.     

Determination of the stress profile in a deep borehole in a naturally fractured reservoir

In this paper we attempt to determine the stress profile in a naturally fractured reservoir. Having calculated the vertical stress using the conveniently available density data, in dealing with the minimum horizontal stress component, a contribution of tectonic effect has been added to the conventional approach taking the advantages of step rate injection tests which have been performed in the wellbore. This extra term has been taken into account regarding the layered nature of the reservoir and two different producing formations present in the region. Eventually the maximum horizontal stress component has been determined using the frictional faulting theory concepts, and has been validated by taking drilling induced tensile fractures and wellbore breakouts into consideration. In a few depths where both types of wellbore failures were present, consistent results have been reached. A discussion of the results of the maximum horizontal stress determination is provided.     

南水北调西线工程区地应力测量及地应力场特征分析

基于对南水北调西线一期工程场区现场水压致裂地应力测试工作,得出如下初步结论:(1)各线路孔中最大水平主应力值最高约为25 MPa,最大水平主应力方向均为NNE或NEE;(2)各坝址孔中最大水平主应力值最高约为17 MPa,最大水平主应力方向受局部地形地势影响变化较大,但大多数孔方向仍为NNE或NEE;(3)在试验深度范围内,线路孔侧压系数随着钻孔深度的增加逐渐减小;当测试深度在200 m以下时,侧压系数值基本维持在2左右;(4)各测孔侧压系数均大于1,表明工程场区地应力以构造应力为主导;(5)在测试深度范围内,各测孔的最大、最小水平主应力随岩层深度的增加均有增大趋势;(6)回归分析结果表明线路孔的最大水平主应力值随深度呈现良好的线性关系;(7)由于隧洞洞室埋深较深,且穿越高应力区,存在中等强度岩爆或流变的可能性。     

Effects of poisson's ratio and core stub length on bottomhole stress concentrations

Core disk fractures are produced by the concentration of in situ stresses at the wellbore bottomhole cavity. In order to better understand core disking, these stress concentrations were calculated using detailed three-dimensional finite element modeling of a bottomhole geometry with a variety of core stub lengths. Biaxial compression applied perpendicular to the wellbore axis induces high tension at the root of the core stub. This tension is reduced when a uniaxial compression directed parallel to the wellbore axis is applied. A state of incipient core disking consequently depends on the relative magnitudes of these in situ stresses. Hypothetical incipient failure curves derived from the modeling are in good agreement with early experimental results, and indicate that the core disks produced under a combined state of vertical and uniform horizontal farfield stresses result from tensile fracture. A Mohr-Coulomb shear mechanism cannot explain the experimental observations. The magnitude of the stress concentrations depend strongly on Poisson's ratio and the stress concentrations are higher in materials with small Poisson's ratios. The length of the core stub influences the magnitudes of the concentrated stresses with tensions increasing to a maximum for normalized core stub lengths of 0.25. Additional hypothetical failure curves for differing core stub lengths suggest that core disk thickness can aid in the estimation of in situ stress magnitudes.     

射孔对地层破裂压力的影响研究

射孔孔眼是沟通井筒和地层的通道，压裂施工中射孔参数的选择影响到施工效果，使用三维有限元模型结合岩石的抗拉破坏准则，系统研究了垂直井中射孔对于地层破裂压力的影响。考虑的射孔参数包括射孔密度、射孔方位角、射孔孔眼直径和射孔孔眼长度。通过计算分析得到了一些新的认识。主要结论包括射孔密度和射孔方位角是影响地层破裂压力的主要因素。射孔孔眼长度和射孔孔眼直径的影响不大。结论对于实际的射孔参数优化设计和压裂施工具有参考意义。     

Determination of three dimensional in situ stress from core discing based on analysis of principal tensile stress

To determine all of the components of in situ stress from core discing, both the directions and magnitudes of the principal in situ stresses must be determined for a disc of a given thickness. In this study, we analyzed the direction and magnitude of tensile stress below an HQ core stub for 11 core lengths using stress conditions under which core discing is likely to occur. First, based on an analysis of the direction of tensile stress below the core stub, we propose a method for determining directions of in situ stress from the height distribution at the periphery of the end surface of a disc. This method can be used with a disc of any thickness. Next, based on an analysis of the magnitude of tensile stress in the central part of a core, we propose a linear criterion for core discing, which can be applied to a core of any length. This criterion was in good agreement with an empirical formula obtained previously in laboratory experiments. By combining information on the direction of in situ stress and the linear criterion for core discing, we propose a method for determining all of the components of in situ stress from core discing under the assumption that vertical stress is given by the overburden stress. Finally, these methods were applied to discs obtained from a field where hydraulic fracturing was performed to measure horizontal stresses. The results showed that the azimuths of the principal stresses estimated from core discing were consistent with those of the principal horizontal stresses determined by hydraulic fracturing and that while the magnitudes of the principal horizontal stresses estimated from core discing showed a large scatter, they were similar to those determined by hydraulic fracturing.     

深孔水压致裂地应力测量及应力场反演分析

 万福煤矿是一座拟建特大型矿井,上覆冲积层厚度高达700 m,主煤层平均埋深达1 004.12 m。选择7个勘探钻孔、37个试验段进行深孔水压致裂地应力测量。为保证超千米深孔地应力测量的成功,对传统水压致裂法的测试技术和设备进行改进,提高系统的耐压能力和测试精度。测孔最大水平主应力与垂直主应力的比值较大,平均为1.97,说明勘探区现今应力场以水平应力为主导。测得的21个印模中有18个点位于NEE-SWW向,3个点位于NW-SE向,符合本区震源机制解得出的挤压应力场方向,且与新构造活动所反映的区域构造应力场方向一致。结合实际的地质条件和地层参数,在实测地应力结果的基础上,提出基于有限元三维地应力拟合分析法反演勘探区现今地应力场,给出煤层顶板主应力分布曲线并分析井下可能出现的各类不良地质现象。     

Transverse drilling-induced tensile fractures in the West Tuna area, Gippsland Basin, Australia: implications for the in situ stress regime

Drilling-induced tensile fractures (DITFs) have been interpreted on image logs from vertical wells in the Gippsland Basin, offshore southeastern Australia. Interpreted axial (vertical) DITFs have previously been well described worldwide. We also interpret transverse (horizontal) DITFs, which are horizontal fractures that are electrically conductive, non-planar, bimodal and constrained to the tensile region of the wellbore.Elasticity theory predicts formation of both transverse and axial drilling-induced tensile fractures (DITFs) in vertical wells depending on the magnitude of the principal in situ stresses, pore-pressure and mudweight. Drilling-induced tensile fractures initiate in very specific stress environments. Axial DITFs can closely constrain a lower bound to the maximum horizontal stress (S_H max) magnitude where the minimum horizontal (S_h min) stress is known. If transverse DITFs are observed, they can constrain a lower bound to maximum and minimum horizontal stress magnitudes. The observation of transverse DITFs on image logs can constrain the stress field to one on the border of strike-slip and reverse faulting () without requiring knowledge of the S_h min or S_H max magnitude. The observation of transverse DITFs in the West Tuna area combined with wireline log data, leak-off tests and pore pressure data are used to constrain the in situ stress tensor. The interpreted in situ stress tensor lies on the border of a strike-slip and reverse faulting regime (S_H max40.5 MPa/km>S_h min≈S_v21 MPa/km). Interpreted data from leak-off tests in the West Tuna area confirm that S_h minS_v.     

含层理构造的非均质片麻岩巴西劈裂试验及离散单元法数值模拟研究

 为了研究岩石各向异性及非均质性对其破坏形式的影响,对含层理构造的非均质片麻岩进行了多组加载角度的巴西劈裂试验,获得不同层理方向片麻岩试件“抗拉强度”。其计算“抗拉强度”随着层理方向与加载方向夹角减小而迅速减小,当层理方向平行于加载方向时,计算“抗拉强度”可认为是片麻岩软弱层理面间的抗拉强度;当层理方向垂直于加载方向时,计算“抗拉强度”可认为是片麻岩岩石矿物基质的等效抗拉强度;当层理方向与加载方向夹角小于90°时,则属于拉–剪复合破坏形式,是片麻岩各向异性和非均质性共同作用的结果,此时对计算“抗拉强度”的应用需要特别谨慎。用UDEC程序建立离散单元数值模型,利用随机分布的条状块体集合,通过设置层理界面和矿物颗粒之间的接触参数,模拟层理构造对片麻岩破坏的影响。数值模拟与试验结果吻合较好,解释了劈裂破坏形式产生的机制,揭示岩石在荷载状态下的破裂过程是裂纹从萌生初期的无序分布,到受微观构造影响而有序集中的自组织过程。     

Evidence of thermally induced borehole elongation: a case study at Soultz, France

When they occur, borehole breakouts are considered strong markers of principal stress directions at depth. An innovative processing method for automatically identifying breakouts from ultrasonic borehole wall images has been developed. It has been applied to data sets from two deep, sub-vertical wells (GPK1 and GPK2) at the Soultz geothermal site in eastern France. In well GPK1, below 3 km depth, compression breakouts, with a 95°±7° azimuth, increasingly occur with depth. They result from time dependent compression failure at sub-critical stress levels and are indicators of the minimum horizontal principal stress orientation. However, in the uppermost logged section of well GPK2 (1.6–2.9 km depth), continuous borehole elongations share roughly the same azimuth with so called drilling-induced fractures (164°±18° and 175°±17° azimuth, respectively). Both features concomitantly vanish with depth, together with the amplitude of the thermal perturbation induced by drilling. It is proposed that these latter borehole elongations result from a pervasive, cooling-induced, tensile micro-cracking process prior to macroscopic failure localization. They are termed thermal elongations and are indicators of the maximum horizontal principal stress orientation. Had a simple logging caliper tool been used for this work, these thermal elongations might have been confused with classical compression breakouts. A simple criterion for differentiating compression breakouts from thermal elongation is proposed.     

Micro-annulus generation under downhole conditions: Insights from three-dimensional staged finite element analysis of cement hardening and wellbore operations

A micro-annulus (MA) is defined as a high permeability zone or gap initiating/occurring at the casing-cement and cement-formation interfaces during the wellbore life span. An MA can significantly compromise wellbore integrity by establishing enhanced fluid flow pathways. This study uses a staged finite element approach to simulate wellbore integrity during various loading steps of wellbore operations under downhole conditions. Particular emphasis is placed on the processes of cement poro-elastic property evolution, volume variation, and pore pressure variation as part of the cement hardening step. The resulting state of stress during the life cycle of a typical injection well (i.e. hardening, completion, and injection) is analyzed to assess the onset and evolution of micro-annuli at various interfaces of the composite wellbore system under downhole conditions. The results show that cement shear failure is observed at the casing-cement interface during pressure testing (excessive wellbore pressure); and tensile debonding failure initiates at the cement-formation interface due to cement shrinkage during hardening and injection-related cooling (thermal cycling). Sensitivity analyses considering several parameters show that: (1) the degree of poro-elastic bulk shrinkage has significant implications for both shear and tensile failure initiation – the less the cement shrinks, the less likely the failure initiation is; (2) cement integrity increases with increasing depth; (3) cement pore pressure evolution has significant implications for tensile failure – if cement pore pressure decreases more, higher temperature differences can be sustained before an MA occurs; and (4) cement temperature fluctuations during hardening promote initiation of debonding failure. In summary, the results presented indicate that establishing downhole conditions to quantitatively analyze MA generation is necessary. The results are different compared to laboratory studies without considering/simulating downhole conditions. The knowledge from this study can raise the awareness of predicting and evaluating MA under downhole conditions and can be used to supplement and improve future laboratory experiments.     

Practical application of failure criteria in determining safe mud weight windows in drilling operations简

Wellbore instability is reported frequently as one of the most significant incidents during drilling operations. Analysis of wellbore instability includes estimation of formation mechanical properties and the state of in situ stresses. In this analysis, the only controllable parameter during drilling operation is the mud weight. If the mud weight is larger than anticipated, the mud will invade into the formation, causing tensile failure of the formation. On the other hand, a lower mud weight can result in shear failures of rock, which is known as borehole breakouts. To predict the potential for failures around the wellbore during drilling, one should use a failure criterion to compare the rock strength against induced tangential stresses around the wellbore at a given mud pressure. The Mohr–Coulomb failure criterion is one of the commonly accepted criteria for estimation of rock strength at a given state of stress. However, the use of other criteria has been debated in the literature. In this paper, Mohr–Coulomb, Hoek–Brown and Mogi–Coulomb failure criteria were used to estimate the potential rock failure around a wellbore located in an onshore field of Iran. The log based analysis was used to estimate rock mechanical properties of formations and state of stresses. The results indicated that amongst different failure criteria, the Mohr–Coulomb criterion underestimates the highest mud pressure required to avoid breakouts around the wellbore. It also predicts a lower fracture gradient pressure. In addition, it was found that the results obtained from Mogi–Coulomb criterion yield a better comparison with breakouts observed from the caliper logs than that of Hoek–Brown criterion. It was concluded that the Mogi–Coulomb criterion is a better failure criterion as it considers the effect of the intermediate principal stress component in the failure analysis.     

Kaiser取样偏差对深层油藏地应力测试的 影响分析

 采用Kaiser效应测定深部地层地应力时,由于油藏岩芯不规则及试件制备过程的误差等因素存在,取样时的偏差会影响地应力测量。假定0°方位和90°方位的取样方位准确而45°方位取样存在一定偏差,根据弹性力学和级数展开建立了应力测量误差分析的力学模型。采用深层均质砂岩岩芯,并变化取心角度(偏差)进行一系列Kaiser 效应试验,对比试验和小型压裂解释结果发现 Kaiser岩样偏差对地应力数值确实存在影响,其关系为二次函数;当取样偏差角度在5°以内时,应力测试误差可以忽略不计;大于5°时引起的地应力解释误差不能忽略。对于非定向钻取的全尺寸岩芯,当参考取样方位不同时,取样偏差的影响幅度不同。对于深部地层的不规则岩芯,可根据所建立的模型对Kaiser效应测试结果进行相应的误差分析。     

晋城矿区地应力场研究及应用

 随着矿井开采深度与强度的不断增加,地应力对围岩变形与破坏的影响更加突出,在煤矿矿区进行地应力测量,并分析地应力场分布特征具有重要意义。在晋城矿区,采用小孔径水压致裂地应力测量装置,进行10个煤矿、62个测点的二维与三维地应力测量。实测数据表明：晋城矿区原岩应力以水平应力为主,构造应力占绝对优势,属于典型的构造应力场类型;地应力值属于中等水平;矿区东部与西部水平主应力方向变化较大,主要原因是受晋获褶断带的影响。基于实测数据,绘制晋城矿区地应力分布图;采用回归方法分析地应力随埋藏深度的变化规律;论述水平主应力与垂直主应力的比值同埋藏深度的关系,并与霍克–布朗包线进行比较。选择典型矿井,将地应力测量结果应用于巷道布置与支护设计,根据地应力场分布特征提出合理的巷道轴线布置方向,并在井下应用中得到验证。井下地应力测量为晋城矿区提供了可靠的基础数据,对指导矿区井田开拓、巷道布置与支护设计、采煤方法的选择等工程实践具有重要参考价值。     

云南地区现今应力场特征及成因分析

基于国际开放地震数据库(Global CMT),统计了云南地区1976—2015年期间66组震级(M_w)≥4. 8的震源机制数据。通过TENSOR程序反演震源机制的简化应力张量发现:云南地区应力机制以走滑型(SS)为主,占66. 7%;其次为正断型(NF),占22. 7%,且主要集中分布在云南西北断裂附近。将66组震源机制数据按区域分为4个次级区块分别联合反演,其结果显示:云南西北地区拉张作用显著,最大主应力方向为WNW—ESE和NNW—SSE;云南西南、东北地区以挤压作用为主,最大主应力方向分别为NNE—SSW和NW—SE;云南东南地区板块相对稳定,基本没有中强震发生。以震源机制反演结果为约束,利用有限元数值模拟得到了研究区连续应力场变化特征,即具有顺时针旋转的趋势,其主要原因是喜马拉雅东部构造特殊的下地壳和上地幔物质使得云南地区成为印度板块向北推进的优势通道,进而挤压应力向东南偏转形成旋转的应力场。     

定向射孔对水力裂缝起裂与延伸的影响

 射孔孔眼是沟通井筒和地层的通道,压裂施工中射孔参数影响施工效果。采用大尺寸真三轴水力压裂物理模拟实验系统,研究定向射孔方位角、水平应力差和微环隙对裂缝起裂、延伸、转向、破裂压力及形态的影响,并建立了射孔直井在地应力条件下产生垂直裂缝的破裂压力预测模型。实验结果表明：利用定向射孔压裂技术可以在地层中形成双翼弯曲水力裂缝;随着定向射孔方位角的增大,破裂压力越来越高,转向距离也越来越大;且水平最大主应力与水平最小主应力之间的应力差对裂缝的转向距离有很大影响;微环隙对裂缝形态及破裂压力也有影响,其使得破裂压力较从射孔孔眼起裂所需压力大幅降低,且裂缝形态与裸眼井起裂类似。结论对于实际的射孔参数优化设计和水力压裂施工具有参考意义。