Double Porosity Finite Element Method for Borehole Modeling

Summary. This paper considers the mechanical and hydraulic response around an arbitrary oriented borehole drilled in a naturally fractured formation. The formation is treated as a double porosity medium consisting of the primary rock matrix as well as the fractured systems, which are each distinctly different in porosity and permeability. The poro-mechanical formulations that couple matrix and fracture deformations as well as fluid flow aspects are presented. A double porosity and double permeability finite element solution for any directional borehole drilled in the fractured porous medium is given. Compared with conventional single-porosity analyses, the proposed double-porosity solution has a larger pore pressure in the matrix and a smaller tensile stress in the near-wellbore region. The effects of time, fracture, mud weight, and borehole inclination in the double-porosity solution are parametrically studied to develop a better understanding of physical characteristics governing borehole problems.     

Analysis of the External Pressure on Casings Induced by Salt-Gypsum Creep in Build-up Sections for Horizontal Wells

To alleviate the problems of casing collapse induced by the coupling effect of salt creep movement, as it relates to the curved sections of horizontal wells, an experimental approach was taken to determine the creep (visco-elastic) property of the salt-gypsum formation, using finite-difference software to establish a creep model with curved casing, wellbore and salt formation. With this model, the effects of borehole curvature, drilling direction and casing thickness on external pressure on casing were analyzed, and casing deformation in the non-uniform in situ stress field was simulated. For horizontal wells drilled through salt-gypsum formations, this analysis led to the following conclusions: (1) the casing tends to be much safer when drilling is undertaken along the direction of the minimum horizontal stress; (2) casing deformation occurs and stress increases as the borehole curvature increases; (3) for the same curvature and drilling direction, thicker casings are safer; (4) as the creep time increases, the external pressure on the casing rapidly increases until it reaches its maximum value, whereupon it stays at that value; (5) under the effect of non-uniform in situ stress, a larger non-uniform coefficient would result in greater external pressure on the casing. This model has been successfully applied to analyze the external pressure on curved casings in a build-up section for horizontal wells drilled through salt-gypsum formations in an oilfield in Northwest China. The model has helped to improve the casing design and reduce drilling downtime with greater wellbore stability in salt-gypsum formations.     

利用定向岩心进行AE法原地应力测量

在北京市房山区迎风坡花岗闪长岩300m深钻孔中,采用以水泥端帽法为主的岩心定向技术,并在钻进和取心过程中采取特殊措施,在钻孔中距地表25m~301m整个深度段内取得了直径86mm的定向岩心。对深度294m的定向岩心进行了声发射Kaiser效应试验。岩石试件为圆柱形,直径30mm,高度75mm.在垂直方向,利用声发射Kaiser效应估计的垂直主应力为7.7MPa;在294m岩体静岩压力为7.9MPa;两者基本相同。在水平面内,对4个方向的试件进行声发射Kaiser效应试验,得到相应的各个方向的压应力,由此估算的水平最大和最小主应力分别为21.2MPa和12.1MPa.水平面内最大主应力的方向基本为SN方向。将声发射Kaiser效应测量结果与水压致裂法的测量结果进行对比,二者具有很好的一致性。      

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.     

Borehole instability in high-porosity Berea sandstone and factors affecting dimensions and shape of fracture-like breakouts

Stress-induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far-field stress. Wellbores are sometimes drilled into aquifers or oil reservoirs that are weak, poorly consolidated, and highly porous sandstone formations, which are often conducive to breakout formation. Breakouts are an expression of borehole instability and a potential source of sand production. On the other hand, the breakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition. The average orientation of breakouts, as identified by borehole geophysical logging, is a reliable indicator of in situ stress directions. It has also been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes. The research reported here has concentrated on the unique type of breakouts observed for the first time in high-porosity Berea sandstone. Drilling experiments in rock blocks subjected to critical far-field true triaxial stress regimes, simulating in situ conditions, induced breakouts that were unlike the ‘dog-ear’ ones previously observed in granites, limestones, and low-porosity sandstones. The newly observed breakouts were thin, tabular, and very long, resembling fractures that counterintuitively extended perpendicular to the maximum principal stress. We found that a narrow zone ahead of a fracture-like breakout tip underwent apparent localized grain debonding and compaction. In the field, such zones have been termed ‘compaction bands’, and are a source of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flow of oil or water. In order to determine whether a correlation exists between fracture-like breakouts and in situ stress, we conducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximum horizontal stress (σ_H) was increased from test to test. These tests showed strong dependence of the breakout length on far-field stress, signaling that potentially the ability to assess fracture-like breakout length in the field could be used to estimate in situ stress magnitudes in conjunction with other indicators. Another series of tests revealed that breakout length increased substantially when borehole diameter was enlarged. This result suggested that in the field, where wellbore size is considerably larger, fracture-like breakout could extend to sizable distances, creating a sand production hazard. Two series of tests, one to evaluate the effect of drill-bit penetration rate, and the other to verify the drilling-fluid flow rate effect on breakout formation and dimensions yielded inconsistent results and showed no unique trends. Remarkably, fracture-like breakouts maintained a consistent narrow width of about 5–10 grain diameters, irrespective of the test conditions. This characteristic supports the suggestion that fracture-like breakouts are emptied compaction bands.     

三维地应力计算模型研究

简要回顾了现有的三维地应力实测方法在实际工程应用中存在的局限性。对钻孔局部壁面应力全解除法进行了系统的研究,从理论上说明了由它确定三维地壳应力张量的可行性。采用线弹性有限元法对钻孔局部壁面应力解除过程做了数值模拟研究,分析了应力解除曲线的变化特征,给出了达到完全应力解除时解除深度和被解除岩芯直径之间的定量关系,还进一步分析了环形解除槽引起的应力集中对其附近局部孔壁应力分布的影响,从而为在钻孔孔壁的局部壁面上如何合理的布置应变计（花）提供了参考依据。最后对本课题的研究前景做了展望。     

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.     

松软煤层瓦斯钻孔失稳分析及动态密封技术

松软煤层钻孔在钻进及抽采瓦斯过程中,容易发生钻孔形变、缩径、坍塌甚至堵孔等工程问题,造成瓦斯钻孔成孔率低、密封性差、服务时间短及瓦斯抽采阻力大等抽采问题。针对上述技术难题,基于松软煤层的构造演化过程,分析了自重应力、构造应力、采动应力及瓦斯应力等因素对松软煤层瓦斯钻孔稳定性的影响,得出了松软煤层钻孔的多应力耦合作用失稳机制。同时,针对松软煤层瓦斯钻孔失稳规律,提出以护孔为基础,自适应动态密封为关键的"护-封"一体化松软煤层瓦斯钻孔密封技术。工程试验结果表明,该技术可使单孔瓦斯抽采体积分数增加至90%以上,单孔瓦斯体积分数提高2~3倍,且抽采浓度稳定。      

井底应力场对气体钻井井斜的影响

建立了油气井钻井过程中的井底力学模型,该力学模型考虑了井眼内压力、岩石节理、井眼深度、地应力、岩石材料特性、井眼直径等影响因素,并讨论了各个因素对井底应力场的影响,阐明了气体钻井时井底应力场分布变化对井眼井斜的影响。气体钻井时井眼内的压力远小于井眼周围的围压,井底会出现很大的拉应力,岩石容易破坏,因此,气体钻井时钻井速度远远大于钻井液钻井,但拉应力越大,井眼与岩石节理面交接处产生的应力集中越大,应力集中处岩石更容易破坏,也更易产生井斜。气体钻井时井底的应力状态对地层倾角、材料特性、井眼直径、井眼深度及初始地应力的变化较钻井液钻井时更敏感。     

A finite element porothermoelastic model for dual-porosity media

An existing dual-porosity finite element model has been extended to include thermo-hydro-mechanical coupling in both media. The model relies on overlapping distinct continua for the fluid and solid domains. In addition, conductive and convective heat transfers are incorporated using a single representative thermodynamics continuum. The model is applied to the problem of an inclined borehole drilled in a fractured formation subjected to a three-dimensional state of stress and, a temperature gradient between the drilling fluid and the formation. A sensitivity analysis has been carried out to study the impact of thermal loading, effect of heat transport by pore fluid flow and, the effect of parameters of the secondary medium used to represent the fractures. Copyright © 2004 John Wiley & Sons, Ltd.     

Pressurized grout applications in fractured tuff for containment of radioactive wastes

Summary Currently under study by the Department of Energy are the geologic and hydrologic characteristics of the ash-flow deposits under Yucca mountain at the Nevada test site. Of interest at this site is the potential for disposal of high-level radioactive wastes in the unsaturated zone of the densely welded portions of the tuffs. These studies include the preformance-assessment of barriers and seals for boreholes, ramps, drifts and shafts at the Yucca mountain site. In-situ tests on standard Type II Portland cement and microfine cement as grout materials have been performed on a similar rock type to Yucca Mountain's near Superior Arizona. The tests were performed in a vertical borehole drilled in highly fractured and densely welded tuff (brown unit of Apache Leap) through a series of pressurized grout applications. Packer flow tests prior to and after each grout application measure the effectiveness of the grout application in reducing the permeability of the rock surrounding the borehole. Overall the grout applications have reduced the permeability of the test hole by three orders of magnitude.     

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.     

冻土钻孔内的温度分布——冻土钻探专题之一

分析了冻土钻孔中温度分布的意义，提出了孔内温度的数学模型，计算了钻头破碎岩石产生的温度增量，提出了防止钻孔坍塌的条件。     

高位瓦斯抽放钻孔在降低综采工作面瓦斯浓度中的应用

林南仓属于低瓦斯矿井,但存在高瓦斯区域。煤层和采空区是瓦斯的主要来源,尤以采空涌出量大,给煤矿生产和安全带来了极大隐患。通过在1129综采工作面风道施工高位瓦斯孔,把钻孔打到采空区一侧煤层顶板以上冒落裂隙带内,用钻孔进行瓦斯抽放,使采空内的瓦斯通过裂隙带沿钻孔抽出,有效降低综采工作面瓦斯浓度,保证综采工作面正常回采和安全生产。     

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.     

Extrapolation of O-cell drilled shaft tests for load and resistance factor design (LRFD) calibration

Load tests of drilled shafts are often performed using Osterberg cell (O-cell) testing, a popular load test method for drilled shafts, which measures both side and tip resistance. However, it is common that only one of the resistance components can be fully mobilized. Therefore, extrapolation of the partially mobilized resistance is often required to determine the total resistance or the equivalent top-down curve. The extrapolation tends to introduce errors to the constructed total resistance values, which subsequently affect the calibrated resistance factors required for the LRFD design of drilled shafts. In this study, eight O-cell tests of drilled shafts with total measured resistances close to the failure criteria defined by FHWA, 5% of the shaft diameter (B), were collected among 64 drilled shaft load tests from Louisiana and Mississippi. For each of the eight cases, extrapolation was performed on both tip and side movement curves for the construction of the equivalent top-down load-settlement (ELT) curves. Data points from the measured side or tip movement curve were removed systematically to create a total of 80 cases with partially mobilized movement curves, and extrapolation exercises were performed on each fabricated case to obtain its equivalent top-down curve. The error of bias for each fabricated case was determined for statistical analyses. Multiple linear regression analysis was performed on the bias errors to model the bias errors caused by extrapolation. Calibrated resistance factors were determined and compared between the original database and fabricated database needing extrapolation. A correction method is proposed, based on a linear regression relationship, to estimate and minimize the extrapolation error of bias for less mobilized databases.

     

Borehole failures in crystalline rocks of South-Western Nigeria

Boreholes drilled in the crystalline rocks of Nigeria have a reputation of failures and low yields. The performance of 256 boreholes drilled in different parts of the crystalline rocks of SW Nigeria were reviewed to determine their failure characteristics. 100% success was claimed for 105 boreholes constructed by one driller. Out of 111 other borehole records supplied by three other drillers 35 were abortive, representing a failure rate of 32%. In contrast, 24 owners of 40 boreholes reported 24 failures representing a failure rate of 60%.The common causes of borehole failure in the crystalline rocks of SW Nigeria are seasonal variations in water level, improper casing of the overburden, damage to pumps and other system failures such as blocked pipelines and malfunctioning tanks. Non-penetration into the water bearing horizon also features as a cause of borehole failure in this study.It is suggested that proper construction, operation and maintenance of boreholes will reduce the incidence of borehole failures in crystalline rocks of SW Nigeria.     

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.     

Interpretation of in situ stress measurements in anisotropic rocks with the doorstopper method

Summary A correct interpretation of in situ stress measurements by means of the doorstopper technique in cross anisotropic rocks requires that 10 stress concentration factors at the bottom of the borehole be known instead of the 3 which are needed for isotropic rocks. In order to analyse the data obtained in a site investigation in a gneiss of the Alpine region, these factors were determined by a three-dimensional high-order finite-element model. The results are presented and compared with those obtained by means of first approximation methods. The feasibility of carrying out measurements in anisotropic rock by means of only 2 boreholes is discussed and it is shown that in practice 3 properly oriented boreholes are needed in this case too.     

基于钻孔形态分析的地应力测量方法研究

地应力作用下的钻孔孔壁会发生一定程度的变形,导致钻孔横截面形状会发生变化,变形后的钻孔几何形态也反映了钻孔受力状态。在经典岩石力学理论的基础上,研究钻孔在平面二维应力作用下的几何形态,建立应力与变形后圆孔几何参数的关系,实现基于钻孔椭圆参数的应力解算,并提出钻孔椭圆参数的测量方法和测量技术,形成一种新的基于钻孔形态分析的地应力测量方法。结果表明,（1）在平面二维应力作用下圆孔变形后的几何形态为椭圆,推导了椭圆参数与应力的关系表达式;（2）利用3个不同方向的孔径值可以实现椭圆参数的解算,研发触头式显微光学孔径测量技术,实现了钻孔形态的测量,从原理上分析了技术实现的可行性,并就该技术进行地应力测量过程中可能存在的问题进行了探讨;（3）通过室内模拟试验,验证了技术原理的可行性,并通过误差分析和实例计算对该方法测量结果的准确性进行了讨论。