Generalized poroelastic wellbore problem

This paper presents a novel analytical solution to the transient, z‐dependent, and asymmetric problem of an infinite wellbore drilled into a fluid‐saturated porous medium. The formulations are based on Biot's linear theory of poroelasticity, in which the dependency of poroelastic field variables to spatial coordinates as well as time domain is considered in the most general form. This gives flexibility to the solution in cases that cannot be analyzed using the conventional plane strain or symmetric models. One such case is when calculating the stress variations around an inclined wellbore where the far‐field stresses are acting over a finite vertical section. The results of our solution to this case with a three‐dimensional state of far‐field stress are used to analyze the stability of inclined wellbores passing through abnormally stressed formations. The presented solution is capable of finding expressions for fundamental solutions with stress or flow boundary conditions at the wellbore. These solutions are here adopted to analyze the pressure disturbances generated by multiprobe formation tester, a standard wireline device that is designed for downhole fluid sampling as well as estimating the directional permeabilities of subsurface earth formations. A comparison with the conventional solution for the relevant pressure diffusion equation indicates that the poroelastic effect is relatively significant in relation to the transient response of the pore pressure. Further, it is shown that the finite dimensions of sink probe would, to a great extent, contribute to the formation's pore pressure variations at its immediate proximity. Copyright © 2013 John Wiley & Sons, Ltd.     

A semianalytic time-resolved poro-elasto-plastic model for wellbore stability and stimulation

Wellbore stability problems and stimulation operations call for models helping in understanding the subsurface behaviour and optimizing engineering performance. We present a fast, iteratively coupled model for the flow and mechanical behaviour that employs a time-sequential approach. Updates of pore pressure are calculated in a timestepping approach and propagated analytically to updates of the mechanical response. This way, the spatial and temporal evolution of pressure and mechanical response around a wellbore can be evaluated. The sequential approach facilitates the incorporation of pressure diffusion and of time-dependent plasticity. Also, it facilitates the implementation of permeability evolving with time, due to plasticity or stimulation. The model has been validated by means of a coupled numerical simulator. Its capabilities are demonstrated with a selection of sensitivity runs for typical parameters. Ongoing investigations target geothermal energy operations through the incorporation of thermo-elastic stresses and more advanced plasticity models.     

Hydro‐mechanical erosion models for sand production

Sand production is a complex physical process that depends on the external stress and flow rate conditions as well as on the state of the material. Models developed for the prediction of sand production are usually solved numerically because of the complexity of the governing equations. Testing of new sand production models can very well be performed through calibration with laboratory experiments, which by construction possess geometric symmetry facilitating explicit mathematical analysis. We introduce an erosion model that is built upon the physics (poro‐mechanical coupling of the fluid‐solid system) usually incorporated in erosion models for the prediction of sand production. Around this model, we set up a mathematical framework in which sand production models because of erosion can be tested and calibrated without having to resort to complex numerical work or specialised software. The model is validated by data of volumetric sand production from a hollow cylinder test on synthetic sandstone. Generalisations of the model, which are naturally incorporated in the same framework and have useful phenomenological features, are discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

维持页岩井壁稳定的物理封堵模拟和化学抑制实验研究

页岩气作为一种储量巨大的非常规能源,是实现现代多元能源体系的重要一环。本文采用HKY-3压力传递实验仪研究不同盐溶液对页岩井壁稳定的影响规律。此外,基于离散元颗粒流模型,重构纳米颗粒拖拽力,模拟流体粘度及颗粒粒径、浓度、形状对页岩孔隙封堵效率。结果表明：（1）物理封堵方面,颗粒大小和浓度明显影响封堵效率,当颗粒最大值小于孔隙出口时,颗粒粒径由出口尺寸的1/5增加到出口尺寸的1/3和1/2时,孔隙封堵效率分别增加13%和23%。（2）流体物性对页岩纳米孔隙的封堵效果表明,粘度5 mPa·s时纳米颗粒封堵效率比1 mPa·s时高16.26%。（3）化学抑制方面,阻滞页岩孔隙压力传递的最佳的盐溶液及其浓度为20%HCOONa。研究成果可为适用于龙马溪组页岩钻进的水基钻井液体系遴选提供理论和技术基础。     

An anisotropic strength criterion for jointed rock masses and its application in wellbore stability analyses

In this paper, an anisotropic strength criterion is established for jointed rock masses. An orientation distribution function (ODF) of joint connectivity, is introduced to characterize the anisotropic strength of jointed rock masses related to directional distributed joint sets. Coulomb failure condition is formulated for each plane of jointed rock masses by joint connectivity, where the friction coefficient and cohesion of the jointed rock mass are related to those of the intact rock and joint and become orientation dependent. When approximating joint connectivity by its second‐order fabric tensor, an anisotropic strength criterion is derived through an approximate analytical solution to the critical plane problem. To demonstrate the effects of joint distribution on the anisotropic strength of jointed rock masses, the failure envelopes are worked out for different relative orientations of material anisotropy and principal stress axes. The anisotropic strength criterion is also applied to wellbore stability analyses. It is shown that a borehole drilled in the direction of the maximum principal in situ stress is not always the safest due to the anisotropic strength of the jointed rock mass. Copyright © 2007 John Wiley & Sons, Ltd.     

A field quantification of coastal dune perennial plants as indicators of surface stability, erosion or deposition

Dune plants both modify the wind field around them and are impacted by various stress factors, among them sand erosion and sand deposition. As coastal dunes are being either stabilized or remobilized, in response to the changes in the rates of sand and dune movement, the vegetation cover and composition are expected to vary reflecting the differences in the sedimentary conditions. In this field study 315 quadrats of 100 m², in which the perennial plant species were sampled, were analysed with respect to annual rates of sand erosion and deposition that were measured using erosion pins. A visual exploratory data analysis was introduced, based on selective filtering of samples according to their vegetation cover. This method, combined with established statistical tools, enabled the authors to uncover the inclination and indicative power of nine perennial dune plants to either a stabilized or a mobile environment, and to establish whether they are more prevalent in places undergoing sand erosion or sand burial. Two species were found to be clear indicators of a stabilized environment, Stipagrostis lanata , and Retama raetam . Of the species indicating a mobile environment, only one may be stated as a clear indicator of sand erosion: Silene succulenta , with Cyperus macrorrhizus coming close to being an indicator of a less mobile erosive environment. The best indicator species for sand burial was found to be, as expected, Ammophila arenaria , with Artemisia monosperma also indicating high rates of sand mobility especially when its relative cover is higher than 80%. Such information can be used to monitor natural processes of dune stabilization or reactivation, or to assess the success of a management plan that aims at stabilizing a dune, or at remobilizing it by removing vegetation.     

Effect of using transient polyaxial failure criterion in wellbore instability analysis

ABSTRACT

Wellbore instability is one of the major challenges facing the drilling industry and it shows itself in the form of wellbore collapse and/or fracture. The major reason for this challenge is the inability of the mud density to balance the weight of the excavated materials. While most drilling operations are carried out through shale formations, the complexities of drilling such wells are increased because of the tendency for shale to react with water-based mud. Moreover, the very low permeability of shale means that time-dependent effects may set in. The convention used to incorporate time-dependent effect is to couple the constitutive stress model and chemical diffusion, but in this study, we considered the effect of incorporating time variable into the rock failure criterion. The choice criterion used is the Mogi-Coulomb rock failure criterion, which was modified to incorporate the time effect. The results indicate that in an open-hole the mud density to prevent wellbore collapse increases with time while that to prevent wellbore fracture decreases, thus making the mud window reduce with time.     

水流冲刷过程中河岸崩塌问题研究

采用土工数值分析方法,对水流冲刷过程中的河岸崩塌问题进行了分析。分析了水流冲刷引起的河岸后退距离及后退过程中坡内的渗流情况与坡体的稳定性;探讨了水流冲刷过程中岸坡崩塌的发生机理;指出了坡内的渗流作用与坡前水流的冲刷作用是岸坡发生崩塌的主要动力因素,而岸坡土体的物质组成和分布、岸坡坡度等则是岸坡发生崩塌的主要内在因素;并提出了相应的工程防护措施。     

An adaptive finite element model for wellbore breakout simulation

A finite element formulation is proposed and implemented for analysing the stability of excavated wells using the DiMaggio-Sandler constitutive elastoplastic model with a typical carbonate reservoir configuration. The quality of the finite element approximation is ensured by applying smooth curved elements adapted to the wellbore geometry, and h − p adaptive finite element meshes in the plastic zone. General purpose procedures are defined to transfer the elastoplastic deformation history to newly created integration points. A breakout damage criterion is proposed based on the second invariant of the deviatoric plastic deformation tensor. This damage criterion is used to apply a mesh movement algorithm to represent material collapse. The automatic successive application of the breakout damage criterion results in elliptical realistically looking geometries obtained in experiments reported in the literature.     

深层泥页岩井壁失稳机理与预测模型研究进展

深层油气资源开发已成为国家油气发展的重要战略，泥页岩井段井壁失稳是制约深层油气资源安全高效开采的技术“瓶颈”。本文在分析深层泥页岩井壁失稳的力学机理和化学机理基础上，着重论述考虑泥页岩水化作用、弱结构面、各向异性和多场耦合的井壁稳定性预测模型的研究进展和发展趋势。钻井液密度不合理是井壁力学失稳的主要原因，泥页岩水化作用是井壁化学失稳的主要原因；在井壁流-固-化-热四场耦合作用下，考虑弱结构面、各向异性对泥浆安全密度窗口的影响，是目前研究深层泥页岩井壁稳定性的主要趋势和方法。     

流体黏性对油井砂岩力学响应的影响

随着黏度较大的油藏陆续投入开发,油藏黏性对储层砂岩力学特性的影响研究意义重大。基于柱坐标系建立射孔试验的三维颗粒流数值模型,考虑不同黏性的流体运动对砂岩力学响应的影响,反映油井的出砂过程。砂岩的宏观应力曲线说明流速相同时,随着黏滞系数的增大,切向应力和偏应力均增大,使得砂岩剪切破坏的几率增大,砂岩更容易屈服破坏而出砂。另外,砂岩黏结应力图说明油井附近的应力较大,且随着黏滞系数增大,黏结张拉应力的增大是局部的,而剪应力的增大是全局的,且变化趋势更明显;颗粒的旋转也说明随着流体黏性的增大,颗粒旋转增大,砂岩形成离散颗粒而出砂的几率增大。上述结果与实际开采中的砂岩力学响应吻合,说明了在相同的外界条件下,黏性越大的流体运动对砂岩受力的影响越大,出砂越明显,该成果对不同黏性的油藏开采采用有效的防砂方法提供了重要的科学依据。     

土工格室用于岩石边坡植被侵蚀防护的稳定性分析

采用在土工格室内植草的方法对岩石边坡进行冲蚀控制 。 基于对格室破坏模式的实践认识和理论分析,提出了格室稳定分析的计算方法,分析了边坡坡度 、铆钉间距 、格室深度以及土工格栅加筋对格室稳定性的影响 。 对格室在铺设过程中的主要问题进行探讨 , 得出了对设计和施工有一定指导意义的结论。     

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.     

Micromechanical analysis of sand production

We present a micromechanical approach based on zero-thickness interface elements for modelling advanced localization and cracking states of cemented granular materials, such as reservoir sandstones. The proposed methodology is capable of reproducing the complex behaviour of intergranular and intragranular localization, cracking, and fracturing of rock formation that leads to sanding in hydrocarbon production. The model is calibrated at the macroscale, using only a few physical parameters, by reproducing the typical behaviour of compression element tests. The model exhibits clear transition behaviour from brittle dilatant to ductile compactant behaviour with increasing confining stress. The methodology is implemented for sand production prediction analysis based on the simulation of 2D micromechanical models of hollow cylinder cross sections. The obtained results are compared well with published experimental data from hollow cylinder tests characterized by strong scale effect in the range of small perforations.     

Volumetric sand production model and experiment

A sand production model was developed for volumetric sand production predictions that take into account the effects of the external stresses and fluid flow rate. The model couples the poro‐mechanical behaviour of the solid–fluid system with the erosion behaviour of the solids due to fluid flow. It predicts reasonably experimental volumetric sand production data from a hollow cylinder test on a weak sandstone. The test results show that in weak and compactive sandstones, sand production is associated with decohesioning and plasticification of a zone around the inner hole which can then be mobilized by the hydrodynamic forces of the fluid flow. The sand production rate increases both with external applied stress and fluid flow rate but it is constant with time under constant external stress and fluid flow rate. In both cases a critical lower limit has to be exceeded for sand production initiation. Copyright © 2001 John Wiley & Sons, Ltd.     

融雪侵蚀研究进展

融雪侵蚀在中国及世界上许多地区都有发生,其对表层土壤的破坏作用越来越为人们所认识,已成为土壤侵蚀研究中的重要问题之一。在总结相关研究成果的基础上,分析了影响积雪和融雪的气候、海拔、地形地貌及土地利用等因素。在融雪侵蚀影响因素方面,认为融雪径流、表层土壤解冻深度、解冻期表层土壤可蚀性是影响融雪侵蚀发生的特殊因素。同时指出,融雪侵蚀预报模型中冻融作用对土壤水分迁移的影响、未完全解冻层对表层土壤水分的影响以及坡面融雪过程研究是未来融雪侵蚀研究中应重点解决的问题。     

镇泾地区延安组底界侵蚀谷地震特征及其与煤层和砂体的关系

鄂尔多斯盆地延长组沉积后期的侵蚀谷发育十分广泛。在盆地西南缘镇泾地区三维地震所做的延10底等t₀图上可以识别出沿NE、NW延伸的主谷和沿NW、NE次谷体系。地震剖面上主谷为宽缓的不对称\     

东江下游河段溯源侵蚀特征与机理

1980年以来,东江下游及三角洲发生了高强度的采沙活动,由此导致水文条件发生显著变异,河床形态出现溯源变形,根据不同历史时期的地形与水文资料的对比分析,东江下游段溯源变形特征主要表现为:①大规模采沙直接引起同流量条件下水位大幅下降,尤以枯水期表现最为显著,博罗站水位~流量关系曲线表现为逐年右偏;②采沙引起河床高程大规模降低,即为一级溯源变形,其显著的特征是水面比降大,由此引起河床发生强烈冲刷,遂派生出河床演变的二级溯源变形,二级溯源变形的长期发展将使河床变形趋于平缓;③采沙河段必然发生河床断面形态及尺度的物理变形,按其发生的时间过程分为各具特色的3段:变形完成段(东江网河区)、变形进行段(石龙到博罗段)及变形影响段(博罗以上河段).     

河床采沙挖槽的溯源冲刷影响规律

针对长距离河段大量人工采沙、挖槽发生的河床溯源冲刷影响因素、发展规律及预测方法进行研究。考虑该问题的清水冲刷特征,建立了一维水动力学方程、河床变形和推移质输沙方程组,用TVD（Total Vartation Diminishing）性质的MUSCL-Hancock时空二阶格式和对水流、河床变形方程分别用有限体积和有限差分法进行数值求解,用水槽试验作了验证并研究了溯源冲刷规律。溯源冲刷长度在初期30%的时间内可达平衡时的80%,冲刷速率随来流强度、跌坎高度和跌水水头差明显增大。平衡溯源长度随流量的线性增长率约为0.8,随跌水高差的增长较缓,冲刷最大厚度约为跌坎高度的0.5倍。采沙挖槽河床一旦形成溯源冲刷,则发展迅速,流量和跌水高差是重要影响因素。在合理考虑输沙方程的基础上,该数值模型可用于溯源冲刷的预测,计算速度快且精度较高。     

水库下游河流再造床过程中的河岸侵蚀

根据实测河道地形资料,分析了水库下游河床演变中的河岸侵蚀现象,包括不同横向边界约束条件、不同床沙组成和不同河型河段的河岸侵蚀的特征及规律。结果表明:①水库下游均会有河岸侵蚀发生,但初期一般以下切为主,后期以展宽为主。②对于横向边界约束较强的河段,河岸侵蚀相对较弱;相反,则河岸坍塌严重,河岸侵蚀强烈;③对不同的河床组成和边滩组成的河段:河床较粗特别是形成抗冲粗化层,且边滩组成较细的河段,河岸侵蚀现象非常剧烈;河床组成较细的河段,如果边滩抗冲性较强,则河岸侵蚀现象相对较弱,如果滩地组成较细、容易坍塌,河岸侵蚀也会较强;④不同的河型有着不同的河岸侵蚀现象。分汊河段以主汊为主;游荡段的游荡特性在初期受到抑制;弯曲河段的撇弯切滩现象较普遍。