Fracture-like borehole breakouts in high-porosity sandstone: Are they caused by compaction bands?

We induced borehole breakouts in a 25%-porosity Berea sandstone by drilling 23 mm diameter holes into 152×152×229 mm blocks subjected to constant true triaxial far-field stresses. BSen5 consists of large quartz grains (0.5 mm) cemented mainly by sutured grain contacts. Breakouts in BSen5 are demonstratively different from those observed in granite, limestone, and lower porosity sandstones. Rather than the typically short ‘V’-shaped breakouts, BSen5 displays long fracture-like tabular slots, which counterintuitively, develop orthogonally to σ_H. These breakouts originate at the points of highest compressive stress at the borehole wall, along the σ_h spring line. Micrographs of BSen5 breakouts show an apparent compaction band created just ahead of the breakout tip in the form of a narrow layer of grains that are compacted normal to σ_H. The compaction band characteristics are nearly identical to those observed in the field. The mechanism leading to fracture-like breakouts is seen as anti-dilatant, and related directly to grain debonding and porosity reduction accompanying the formation of the compaction band. Some compacted grains at the borehole wall are expelled as a result of the line of tangential loading and the radial expansion of adjacent grains. The circulating drilling fluid flushes out the remaining compacted loose grains at the borehole-rock interface. As the breakout tip advances, the stress concentration ahead of it persists, extending the compaction band, which in turn leads to additional grain removal and breakout lengthening. By extrapolation, this process may continue for considerable distance (at least several times the wellbore diameter) in field situations, leading potentially to substantial sand production.     

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.     

中国大陆科学钻主孔现今地应力状态

用钻孔崩落法确定了中国大陆科学钻探主钻孔5 047 m深度以上的现今地应力状态.由钻孔声波成像测井资料发现, 科学钻主钻孔在1 200 m深度以下出现了钻孔崩落现象.我们从1 216~5 047 m的深度范围内采集了143个钻孔成像测井图象资料, 对钻孔崩落椭圆长轴方位进行了统计, 结果表明崩落椭圆长轴平均方位为319.5°±3.5°, 最大水平主应力方位平均为49.5°±3.5°.利用崩落形状要素(崩落深度和崩落宽度) 以及岩石的内聚力和内摩擦角, 估算了1 269 m至5 047 m范围内52个深度上的最大和最小水平主应力的大小.结果表明, 在浅处1 216 m深度, 最大水平主应力为42 MPa, 最小水平主应力为30.3 MPa; 在深处5 000 mm深度, 最大水平主应力为160.5 MPa, 最小水平主应力为120 MPa; 地应力随深度近于线性增加.据岩石密度测井资料计算了各个深度上静负载应力.3个主应力的大小和方向反映出科学钻主孔位置的应力场处于走滑应力状态, 与临近地区地震震源机制解和其他方法得到的应力场一致.利用声发射法对岩心试件进行了声发射测量, 得到了最大水平主应力幅值, 并与崩落法测量结果进行了对比, 两者十分一致.      

Compaction bands induced by borehole drilling

Drilling experiments in rock blocks subjected to pre-existing true triaxial far-field stresses simulating real in situ conditions often result in localized failure around the created borehole, which brings about the formation of borehole breakouts. In weakly bonded quartz-rich porous sandstones breakouts take the form of narrow tabular (slot-like) openings extending along a plane perpendicular to the maximum applied-stress direction. Scanning electron microscopes images of failed boreholes strongly suggest that these breakouts are compaction bands that have been emptied to different extents. The bands form as a result of the stress concentration accompanying the creation of the borehole. The evacuation of the compaction bands is brought about by the circulating drilling fluid flushing out debonded and often fragmented grains from within these bands (Haimson and co-workers, 2003–2007). The objective of this paper is to predict the conditions under which compaction bands are formed around boreholes. To this end, a new analytical model is formulated that enables prediction of the stress field around emptied and filled compaction bands, the various factors affecting the breakouts lengths, and their final length. Good agreement of the developed analytical model with experimental results obtained by Haimson and co-workers (Haimson and Klaetsch in Rock physics and geomechanics in the study of reservoirs and repositories, vol 284, pp 89–105, 2007; Haimson and Kovachich in Eng Geol 69:219–231, 2003; Klaetsch and Haimson in Mining and tunneling innovation and opportunity, University of Toronto press, pp 1365–1371, 2002; Sheets and Haimson in Proceedings, paper ARMA/NARMS 04-484, 2004) is demonstrated. The presented study is of practical relevance: boreholes are often drilled deep into weak porous sandstone formations for the purpose of extracting oil and gas, and the question of borehole stability is crucial. In addition, borehole breakouts are often used to estimate the state of stress in the Earth’s crust, and our new formulation will help improve these estimates.     

Inference of in situ stress from thermoporoelastic borehole breakouts based on artificial neural network

The determination of in situ stresses is very important in petroleum engineering. Hydraulic fracturing is a widely accepted technique for the determination of in situ stresses nowadays. Unfortunately, the hydraulic fracturing test is time-consuming and expensive. Taking advantage of the shape of borehole breakouts measured from widely available caliper and image logs to determine in situ stress in petroleum engineering is highly attractive. By finite element modeling of borehole breakouts considering thermoporoelasticity, the authors simulate the process of borehole breakouts in terms of initiation, development, and stabilization under Mogi-Coulomb criterion and end up with the shape of borehole breakouts. Artificial neural network provides such a tool to establish the relationship between in situ stress and shape of borehole breakouts, which can be used to determine in situ stress based on different shape of borehole breakouts by inverse analysis. In this paper, two steps are taken to determine in situ stress by inverse analysis. First, sets of finite element modeling provide sets of data on in situ stress and borehole breakout measures considering the influence of drilling fluid temperature and pore pressure, which will be used to train an artificial neural network that can eventually represent the relationship between the in situ stress and borehole breakout measures. Second, for a given measure of borehole breakouts in a certain drilling fluid temperature, the trained artificial neural network will be used to predict the corresponding in situ stress. Results of numerical experiments show that the inverse analysis based on finite element modeling of borehole breakouts and artificial neural network is a promising method to determine in situ stress.     

Mesoscale numerical study on the evolution of borehole breakout in heterogeneous rocks

Inherent heterogeneity of a rock strongly affects its mechanical behavior. We numerically study the mechanisms governing the initiation, propagation, and ultimate pattern of borehole breakouts in heterogeneous rocks. A two-dimensional finite element model incorporating material heterogeneity is established to systematically examine the effects of several key factors on borehole failure, including borehole diameter, far-field stress, and rock heterogeneity. The inherent heterogeneity of a rock is explicitly characterized by prescribing the rock mechanical properties of mesoscale elements statistically obeying the Weibull distribution. Elastic damage mechanics is used to represent the constitutive law of the mesoscale element. We find that borehole diameter reduction remarkably changes the crack failure from tensile to shear and elevates the critical hydrostatic pressure. Far-field stress anisotropy strongly affects the shape of the borehole breakout. Rock heterogeneity dictates the location of the preferred crack under the hydrostatic stress, which leads to local stress concentration, and determines the types of breakouts around the borehole. Our findings facilitate in-depth understanding of the classic borehole stability problems in heterogeneous rocks.     

Wellbore breakouts of the main borehole of Chinese Continental Scientific Drilling (CCSD) and determination of the present tectonic stress state

The Sulu-Dabie high-pressure (HP)-ultrahigh-pressure (UHP) metamorphic belt as the product of subduction-collision between the northern China plate and Yangtze plate underwent a process of formation and evolution from deep subduction→exhumation→extension→slow uplift. The study of its modern tectonic stress field has great significance for a complete understanding of the process of formation and evolution of the HP-UHP metamorphic belt, especially the exhumation and uplift of the belt. Wellbore breakouts are the most visual tectonic phenomenon which can characterize the modern stress action in the main borehole of Chinese Continental Scientific Drilling (CCSD). Ultrasonic borehole televiewer reflection wave data show that wellbore breakouts began to occur at 1216 m depth of the main borehole. A total of 143 borehole televiewer images were collected from 1216 to 5118 m depth (hole completion depth). After data processing and statistics, the average azimuth of the long dimension of the wellbore breakout obtained was 319.5° ± 3.5°, indicating that the average azimuth of the maximum horizontal principal stress causing wellbore breakout initiation was 49.5° ± 3.5°. The maximum and minimum horizontal principal stress values at 52 depths in the interval of 1269 to 5047 m were estimated using the elements of wellbore shapes (wellbore depth and width), combined with the cohesive strength and internal frictional angle of the rock obtained by rock mechanical tests on samples, and the static load stresses at corresponding depths were calculated according to the rock density logging data. The results indicate that: the maximum and minimum horizontal principal stresses are 41.4 and 25.3 MPa at 1269 m depth respectively and 164.7 and 122 MPa at 5047 m depth respectively; the maximum vertical stress is 141.3 MPa at 5047 m depth with a density of 2.8 g/cm³; the in-situ stresses increase nearly linearly with depth. The magnitudes and directions of the three principal stresses reflect that the regional stress field around the CCSD main borehole is mainly in a strike-slip state, which is consistent with the basic features of the regional stress field determined using other methods.     

The tectonic regime in Italy inferred from borehole breakout data

In Italy, the horizontal stress directions are well constrained in many regions, but the tectonic regime is not well known because the stress magnitudes are unknown. Our intention is to improve the knowledge of crustal stress in Italy, both at shallow depth and in low seismicity areas. Therefore, we inferred the tectonic regime from the comparison between the depth of breakout occurrence and the physical properties of the rocks in 20 boreholes. The critical value of the maximum horizontal stress, for which the effective tangential stress at the borehole wall overcomes the rock strength to form breakouts, could be computed from rock strength and density. Comparing the theoretical stress distributions for different tectonic regimes with the depth distribution of breakout occurrence, it is possible to infer the tectonic regime that fits best to the breakout depth distribution. We investigated boreholes up to 6 km deep located in different tectonic environments over the Italian peninsula: the Po Plain, the Apenninic chain, the Adriatic foredeep and the Tyrrhenian Quaternary volcanic region. These wells are characterised by breakout data of good quality (A, B and C, according to World Stress Map quality ranking system). The results are in general agreement with the style of faulting derived from earthquake focal mechanisms and other stress indicators. Our results show a predominance of a normal faulting (NF) regime in the inner Apennines and both normal faulting and strike–slip faulting (SS) style in the surrounding regions, possibly also associated with changes in the tectonic regime with depth.     

Crustal Stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms

The regional stress field in the northern North Sea (offshore western Norway) has been studied through the acquisition and analysis of directions of maximum horizontal compression (_H) as extracted from borehole breakouts and from earthquake focal mechanism solutions.
The results indicate that the regional stress field is dominated by NW-SE compression, with good consistency between shallow borehole breakouts (2–5 km depth) and deeper earthquakes (10–25 km depth). The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the results are in good agreement with the western Europe trend found in earlier investigations.
The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW-SE and NE-SW. From Tampen Spur in the west to the Sogn graben in the east an anomalous stress field is indicated, with NE-SW oriented maximum horizontal compressions. This anomaly is clearly seen both in the borehole breakout data and in the earthquake data. Possible sources for this anomaly are discussed, and include postglacial uplift and/or lateral variations in the physical properties of the crust.     

Stress Concentration at the Bottom of a Borehole and its Effect on Borehole Breakout Formation

Summary Stress concentration at the bottom of a borehole due to the corners with small radius of curvature in an axial section and its effect on the azimuth of breakout was studied. To this end, a 3-D finite element analysis was conducted and the stress around the borehole was examined for boreholes arbitrarily oriented to three principal axes of remote stress. Results show that, in the case of high strength rock, compressive failure resulting in spalling of a borehole may occur only at the bottom of the borehole. The spalling can occur continuously with drilling, and results in continuous spalling with depth, i.e., a breakout. This type of breakout tends to form on one side of the borehole and its orientation is approximately perpendicular to the orientation of standard breakouts, inferred from the stress concentration due to the cylindrical shape of the borehole.     

Neotectonic state of stress in the Jura Mountains

Abstract

For the determination of the tectonic stress field in the region of the Jura Mountains, three different relief techniques have been used in 18 test sites. In six test sites measurements have been carried out in horizontal boreholes using the doorstopper method. In the remaining test sites measurements were made in vertical boreholes by the doorstopper method (8), triaxial strain cell method (3) and borehole slotter (1). Additionally, analyses of borehole breakouts from six deep wells are presented.

Orientation of maximum horizontal stress clearly deviates from the uniform NW-SE orientated central and western European stress field. Local stress sources, which are probably related to active decollement tectonics in the Jura Mountains, may explain the deviation in stress orientations in comparison to the foreland. Normal stress magnitudes perpendicular to the trend of compressional structures in the Folded Jura are largest in the southeastern Upper Rhine Graben and its southwards prolongation into the Jura Mountains. Low stress magnitudes are characteristic for the eastern termination, intermediate ones for the southern foreland of the Jura Mountains.     

In Situ Stresses in Borehole Blanche-1/South Australia Derived from Breakouts,Core Discing and Hydraulic Fracturing to 2 km Depth

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of S _v ≤ S _h < S _H and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress S _h derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.     

Numerical modeling of plastic deformation and failure around a wellbore in compaction and dilation modes

In many wellbore stability analyses, the ability to forecast both the occurrence and extent of plastic deformation and failure hinges upon a fundamental understanding of deformation mode and failure mechanism in the reservoir rock. This study focuses on analyzing plastic zones, localized deformations, and failures around a borehole drilled overbalanced or underbalanced through a highly porous rock formation. Based on several laboratory experiments, porous rocks are prone to deform under both shear-induced dilation and shear-enhanced compaction mechanisms depending on the stress state. The shapes of the deformation and failure patterns around the borehole are shown, depending on the initial stress state and the local stress paths. The inquiry of the local stress paths in the near-wellbore zone facilitates the understanding of the reasons for different types of failure mechanisms, including the mixed-mode and the plastic deformation structures. The modification of the 2D plane strain condition by imitating third stress in the numerical scheme helps us bring the stress paths closer to the real state of loading conditions. Our modeling reveals that the transition from isotropic to anisotropic stress state is accompanied by an increase in the deviatoric part of effective shear tensor that leads to the development of inelastic deformation, degradation, and subsequent rock failure. Particular interest is devoted to the modeling of strain localization especially in compaction mode around a wellbore and computing the amount of stress concentration at the tips of dog-eared breakouts. Stress concentration can result in a change in irreversible deformation mode from dilatancy to compaction, elucidating the formation of the shear-enhanced compaction phenomenon at the failure tips in the direction of the minimum horizontal stress.     

Borehole failure analysis in a sandstone under anisotropic stresses

Borehole failure under anisotropic stresses in a sandstone is analyze numerically for various borehole sizes using a nonlinear elastic–plastic constitutive model for a Cosserat continuum. Borehole failure is identified as macroscopic failure of the borehole through the development of shear bands and breakouts. The results compare well both qualitatively and quantitatively with experimental results from polyaxial tests on Red Wildmoor sandstone. They show that the hole size effect of the borehole failure strength is independent of the far‐field stress anisotropy and follows a ? power law of the hole size. A similar scale effect equation with a ? power law is proposed for the scale effect of the maximum plastic shear strain at failure. This equation can be useful for better predicting hole‐size‐dependent failure with standard codes based on classical continua. The effect of stress anisotropy on the borehole failure stress is found to be independent of the hole size. The failure stress decreases linearly to 40% as the stress anisotropy increases. However, the maximum plastic shear strain at failure is stress anisotropy independent and therefore the critical plastic shear strain for failure is only hole‐size dependent. Copyright © 2009 John Wiley & Sons, Ltd.     

一种研究山前挤压构造区地应力的新方法

通过对稳定沉降地区与山前挤压构造区的地球物理测井信息的深度域特征进行理论分析和对比,提出了一种基于测井信息计算最大主地应力的统计模型法。应用该方法对塔里木盆地库车地区地应力进行了实际计算,其结果与实验结果相符,同时利用椭圆井眼法确定最大主应力方向。最后,对山前挤压构造区的地应力分布与油气藏分布关系进行了探索。     

In situ stresses and natural fractures in the Northern Perth Basin,Australia

Present-day stress orientations in the Northern Perth Basin have been inferred from borehole breakouts and drilling-induced tensile fractures observed on image logs from eight wells. Stress indicators from these wells give an east – west maximum horizontal stress orientation, consistent with stress-field modelling of the Indo-Australian Plate. Previous interpretations using dipmeter logs indicated anomalous north-directed maximum horizontal stress orientations. However, higher-quality image logs indicate a consistent maximum horizontal stress orientation, perpendicular to dominant north – south and northwest – southeast fault trends in the basin. Vertical stress was calculated from density logs at 21.5 MPa at 1 km depth. Minimum horizontal stress values, estimated from leak-off tests, range from 7.4 MPa at 0.4 km to 21.0 MPa at 0.8 km depth: the greatest values are in excess of the vertical stress. The maximum horizontal stress magnitude was constrained using the relationship between the minimum and maximum horizontal stresses; it ranges from 8.7 MPa at 0.4 km to 21.3 MPa at 1 km depth. These stress magnitudes and evidence of neotectonic reverse faulting indicate a transitional reverse fault to strike-slip fault-stress regime. Two natural fracture sets were interpreted from image logs: (i) a north- to northwest-striking set; and (ii) an east-striking set. The first set is parallel to adjacent north- to northwest-striking faults in the Northern Perth Basin. Several east-striking faults are evident in seismic data, and wells adjacent to east-striking faults exhibit the second east-striking set. Hence, natural fractures are subparallel to seismically resolved faults. Fractures optimally oriented to be critically stressed in the present-day stress regime were probably the cause of fluid losses during drilling. Pre-existing north- to northwest -striking faults that dip moderately have potential for reactivation within the present-day stress regime. Faults that strike north to northwest and have subvertical dips will not reactivate. The east-striking faults and fractures are not critically stressed for reactivation in the Northern Perth Basin.     

Two-dimensional elastoplastic analysis of cylindrical cavity problems in Tresca materials

This paper presents analytical elastic-plastic solutions for static stress loading analysis and quasi-static expansion analysis of a cylindrical cavity in Tresca materials, considering biaxial far-field stresses and shear stresses along the inner cavity wall. The two-dimensional static stress solution is obtained by assuming that the plastic zone is statically determinate and using the complex variable theory in the elastic analysis. A rigorous conformal mapping function is constructed, which predicts that the elastic-plastic boundary is in an elliptic shape under biaxial in situ stresses, and the range of the plastic zone extends with increasing internal shear stresses. The major axis of the elliptical elastic-plastic boundary coincides with the direction of the maximum far-field compression stress. Furthermore, considering the internal shear stresses, an analytical large-strain displacement solution is derived for continuous cavity expansion analysis in a hydrostatic initial stress filed. Based on the derived analytical stress and displacement solutions, the influence of the internal shear stresses on the quasi-static cavity expansion process is studied. It is shown that additional shear stresses could reduce the required normal expansion pressure to a certain degree, which partly explains the great reduction of the axial soil resistance due to rotations in rotating cone penetration tests. In addition, through additionally considering the potential influences of biaxial in situ stresses and shear stresses generated around the borehole during drillings, an improved cavity expansion approach for estimating the maximum allowable mud pressure of horizontal directional drillings (HDDs) in undrained clays is proposed and validated.     

利用钻孔崩落数据再认识白鹤滩右岸地应力场特征

白鹤滩水电站是仅次于三峡水电站的第二大水电站,位于中国西南地区川滇菱形块体内的金沙江上。通常地壳应力状态是影响地下工程安全的重要地质因素,对地下硐室稳定性分析具有重要意义。在水电站右岸厂房建设过程中,为了水电站的长期安全运营,采用超声波井下电视录井测试系统对白鹤滩右岸厂房锚固洞内7处钻孔进行测试,基于钻孔崩落数据计算了现今白鹤滩右岸厂房区域上方工程岩体的主应力方向。研究结果表明:白鹤滩右岸厂房区域最大水平主应力（S_H）方向为北北东—南南西方向,主要受到构造应力、自重应力、河流剥蚀作用以及岸坡卸荷作用的共同影响,属于局部构造应力场。      

Fracture and stress orientation from borehole image logs: A case study from Cambay basin,India

The Deccan trap basalt, laid down by multiple lava flows during upper Cretaceous to Paleocene times forms the basement of current study in Cambay basin. As such, there is great interest and value in fracture detection and evaluation of fractured basement reservoirs in the Cambay basin. The procedure for identification and evaluation of natural as well as induced fractures in basaltic basement of the Cambay basin is presented in this work. In this study formation micro-imager (FMI) and extended range micro-imager (XRMI) log data for fracture identification is used. The Deccan trap basaltic basement of the study area, comprising five wells in the Tarapur-Cambay block, has potential for holding commercial hydrocarbon due to the presence of fractures and weathered basement. Both image logs (FMI, XRMI) identify three types of fracture including open (conductive), partially open and closed (resistive) fractures, of which open and partially open fractures are important for hydrocarbon accumulation. Fracture dip ranges from 10° to 80°. Image logs have also identified washout, breakout and drilling-induced fracture zones. The strike direction of the open natural fractures for four wells varies from N60°E to N30°E whereas the strike direction of most natural fracture in the fifth well is oriented towards N20°W. The orientations of drilling-induced fractures and breakouts may be interpreted for the in-situ stress direction over the logged interval. Drilling-induced tensile fractures, identified over the depth interval of 1969–1972 m, and borehole breakouts over the interval of 1953–1955 m in one well, suggest an orientation of maximum in-situ horizontal compressive stress (S_H) lies in the north-south direction. The azimuths of open natural fractures in the same well vary from north-south to N30°E. It is expected that the direction of fluid flow will be controlled by open natural fractures and therefore would be in a direction parallel to the S_H direction, which is orthogonal to the minimum horizontal stress (S_h) direction. The orientations observed are consistent with the present day S_H direction in the study area of Cambay basin.