Maintaining the stability of deviated and horizontal wells: Effects of mechanical,chemical and thermal phenomena on well designs

Wellbore instability, particularly in shale formations, is regarded as a major challenge in drilling operations. Many factors, such as rock properties, in-situ stresses, chemical interactions between shale and drilling fluids, and thermal effects, should be considered in well trajectory designs and drilling fluid formulations in order to mitigate wellbore instability-related problems. A comprehensive study of wellbore stability in shale formations that takes into account the three-dimensional earth stresses around the wellbore as well as chemical and thermal effects is presented in this work. The effects of borehole configuration (e.g. inclination and azimuth), rock properties (e.g. strength, Young's modulus, membrane efficiency and permeability), temperature and drilling fluid properties (e.g. mud density and chemical concentrations) on wellbore stability in shale formations have been investigated. Results from this study indicate that for low-permeability shales, chemical interactions between the shale and water-based fluids play an important role. Not only is the activity of the water important but the diffusion of ions is also a significant factor for saline fluids. The cooling of drilling fluids is found to be beneficial in preventing compressive failure. However, decreasing the mud temperature can be detrimental since it reduces the fracturing pressure of the formation, which can result in lost-circulation problems. The magnitude of thermal effects depends on shale properties, earth stresses and wellbore orientation and deviation. Conditions are identified when chemical and thermal effects play a significant role in determining the mud-weight window when designing drilling programmes for horizontal and deviated wells. The results presented in this paper will help in reducing the risks associated with wellbore instability and thereby lowering the overall non-productive times and drilling costs.     

Borehole Stability Analysis of Horizontal Drilling in Shale Gas Reservoirs

Serious wellbore instability occurs frequently during horizontal drilling in shale gas reservoirs. The conventional forecast model of in situ stresses is not suitable for wellbore stability analysis in laminated shale gas formations because of the inhomogeneous mechanical properties of shale. In this study, a new prediction method is developed to calculate the in situ stresses in shale formations. The pore pressure near the borehole is heterogeneous along both the radial and tangential directions due to the inhomogeneity in the mechanical properties and permeability. Therefore, the stress state around the wellbore will vary with time after the formation is drained. Besides, based on the experimental results, a failure criterion is verified and applied to determine the strength of Silurian shale in the Sichuan Basin, including the long-term strength of gas shale. Based on this work, horizontal well borehole stability is analyzed by the new in situ stress prediction model. Finally, the results show that the collapse pressure will be underestimated if the conventional model is used in shale gas reservoirs improperly. The collapse pressure of a horizontal well is maximum at dip angle of 45°. The critical mud weight should be increased constantly to prevent borehole collapse if the borehole is exposed for some time.     

A Fully Coupled Chemo-Poroelastic Analysis of Pore Pressure and Stress Distribution around a Wellbore in Water Active Rocks

Water active rocks consist of minerals that hold water in their crystalline structure and in pore spaces. Free water from drilling fluid can be attracted by the formation depending on the potential differences between pore space and drilling fluid. The fluid movement into the formation or out of the formation can lead to a change in effective stress, thus causing wellbore failures. In all previous studies it is found that the solute transport from or to the formation is primarily controlled by diffusion process and the effect of advection on solute transfer is negligible for a range of very low permeable shale formations (>10⁻⁵ mD). In this study a range of permeable shale formations (10⁻⁵ to 10⁻³ mD) commonly encountered in drilling oil and gas wells are considered to investigate the solute transfer between drilling fluid and formation due to advection. For this purpose a finite element model of fully coupled chemo-hydro-mechanical processes was developed. Results of this study revealed that the solute transfer between the drilling fluid and the shale formation is controlled primarily by permeability of the shale formations. For the range of shale formations studied here, there exists a threshold permeability below which the solute transfer is dominated by diffusion process and above which by fluid in motion (fluid flow). Results from the numerical experiments have shown that when the permeability of shales is greater than this threshold permeability, the chemical potential gradient between the pore fluid and drilling fluid reaches equilibrium faster than that when the permeability of shales is below this threshold value. Also it has been found that when advection is taken into account, effective radial and tangential stresses decrease around the wellbore, particularly near the wellbore wall where the solute concentration has reached near equilibrium.     

中牟页岩气区块泥页岩井壁稳定影响因素分析及技术对策

井壁失稳是钻井过程中常常遇见的难题之一,尤其在钻遇泥页岩井段。针对河南中牟页岩气区块在山西组-太原组泥页岩钻进过程中发生井壁坍塌掉块、卡钻、井径扩大率大等复杂情况,利用MY1井钻井岩心从泥页岩类型划分、孔渗特征、微观结构、岩石力学以及地应力等方面开展井壁稳定性影响因素实验分析。通过分析可知,中牟区块泥页岩属硬脆性泥页岩,造成井壁失稳的主要因素为地层非均质性强,岩石脆性程度较高,微裂缝发育,易导致井壁机械剥落、坍塌掉块。通过合理控制钻井液密度、优化井身结构及钻井参数、研发低自由水强封堵钻井液提高井壁稳定性,在区块钻井实践中取得了较好的应用效果,基本解决了井壁失稳难题。     

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.     

叶舞凹陷ZKX井深部盐层钻井液技术研究

叶舞凹陷ZKX井位于叶县盐田范围内,该钻探的目的是河南叶舞凹陷盐矿的普查,该井设计井深2630 m,在2000～2400 m连续取心,主要钻遇以泥页岩、砂岩、含盐膏层及盐岩为主的沉积岩地层,钻进过程中存在防塌、护心、井壁稳定等方面的问题和难点。针对ZKX井的钻井难点,在钻井液体系优化的基础上,根据不同地层分别采用了防塌钻井液体系、欠饱和盐水钻井液体系和饱和盐水钻井液体系,解决了水敏性泥页岩钻进的防塌及盐岩地层连续取心的钻井液技术难点,探索出了一套适用于盐膏层取心钻进的低成本钻井液技术思路。     

Borehole Stability in High-Temperature Formations

In oil and gas drilling or geothermal well drilling, the temperature difference between the drilling fluid and formation will lead to an apparent temperature change around the borehole, which will influence the stress state around the borehole and tend to cause borehole instability in high geothermal gradient formations. The thermal effect is usually not considered as a factor in most of the conventional borehole stability models. In this research, in order to solve the borehole instability in high-temperature formations, a calculation model of the temperature field around the borehole during drilling is established. The effects of drilling fluid circulation, drilling fluid density, and mud displacement on the temperature field are analyzed. Besides these effects, the effect of temperature change on the stress around the borehole is analyzed based on thermoelasticity theory. In addition, the relationships between temperature and strength of four types of rocks are respectively established based on experimental results, and thermal expansion coefficients are also tested. On this basis, a borehole stability model is established considering thermal effects and the effect of temperature change on borehole stability is also analyzed. The results show that the fracture pressure and collapse pressure will both increase as the temperature of borehole rises, and vice versa. The fracture pressure is more sensitive to temperature. Temperature has different effects on collapse pressures due to different lithological characters; however, the variation of fracture pressure is unrelated to lithology. The research results can provide a reference for the design of drilling fluid density in high-temperature wells.     

涠西南地区岩石特性及钻井液技术研究

北部湾盆地涠西南油田群是南海西部重要的原油产区，同时油田群紧邻众多国家级自然保护区，属于环境敏感区域。针对北部湾盆地涠西南地区环保要求，以及涠洲组和流沙港组钻井过程经常出现井壁失稳导致的憋卡、起下钻阻卡等问题，开展了涠西南地区地层泥页岩特性及环保钻井液技术研究。通过地层岩石黏土矿物分析、孔喉结构分析以及理化性能评价，明确了涠西南地区泥页岩井壁失稳的机理；提出以类油基的水溶性复合基基液为核心，构建了一套具有油基钻井液工程特性和水基钻井液环保特性的新型环保防塌钻井液技术，并进行了相关的现场应用。现场应用显示：应用井与邻井相比，12-1/4″井段阻卡划眼时间减少，井径扩大率大幅度降低，并且在128 h的长时间浸泡过程中没有复杂情况产生，有效地解决了涠洲组、流沙港组易失稳地层的井壁稳定问题；同时，钻井液环保性能达到一级海域环保要求，可以替代目前应用的油基钻井液体系，解决了海上使用油基钻井液存在配制成本高、含油钻屑需全回收及环境危害隐患大等技术难题。该研究对于涠西南油田环保、井壁失稳和钻井液技术发展具有较高的理论价值和借鉴意义。     

温压耦合影响下科学钻探深井结晶岩层井壁稳定性分析

科学钻探深井结晶岩处在高地应力、高地层压力和高地温的“三高”环境中,自身的结构特征和物化特性会发生根本性变化,因温度变化引起的应力变化极易导致井壁失稳。本文依据温压耦合影响下坍塌压力和破裂压力计算模式,确定不同井壁温度下的安全钻井液密度窗口,通过FLAC3D开展温压耦合影响下井壁稳定数值模拟,分析在不同钻井液密度下井眼打开后不同时间的井壁周边温度和井壁稳定变化情况。结果表明：地层弹性参数对地层破裂压力的影响较大,对于花岗岩此类弹性模量较大的结晶岩,温度变化对破裂压力影响更大;井壁处附加应力受温度变化的影响程度：附加周向应力>附加径向应力>附加垂向应力;在最小主应力方向的井壁周边地层等效应力与钻井液密度呈正比关系,在最大主应力方向呈反比关系。     

煤层气钻井井壁失稳机理及防塌钻井液技术

针对沁水盆地煤层气钻井中存在的井壁稳定技术难题,采用X射线衍射、扫描电镜、泥页岩膨胀实验、页岩分散实验测试了泥页岩和煤岩体的组构及理化性能,分析了井壁失稳机理。依据"多元协同"防塌原理,研选出了高效包被抑制剂、封堵防塌剂和活度平衡剂,构建了泥页岩地层防塌钻井液体系QSFT和煤层钻开液QSMZ。评价结果表明,QSFT体系的粘度、切力适中,滤失量低、泥饼薄,对200 μm裂缝的封堵承压能力达3.0 MPa;QSFT和QSMZ具有较强的抑制水化能力,并能显著增加岩样单向抗压强度,对煤岩体岩心湿测渗透率的损害均低于30%,干测渗透率恢复值达到95%以上,可作为沁水盆地煤层气井的钻井液和储层钻开液。      

3D numerical stability analysis of multi-lateral well junctions

The most important factors in multi-lateral well stability analysis are the magnitude of in situ stresses, the relation between the amount of in situ stresses and orientation of lateral wellbore. In this research, the stability analysis of multi-lateral junction is carried out using FLAC3D numerical code by considering seven varied stress regimes and different lateral wellbore orientations. The Normalized Yielded Zone Area (NYZA, ratio of surrounding yielded cross-sectional area to initial area of well) is determined for different junction mud pressures as well as diverse orientations of lateral wellbore. Then, the junction optimum mud pressure of each lateral wellbore orientation is calculated; hence, the optimum trajectory of lateral wellbore, in which the junction has got the lowest optimum mud pressure, is selected in each stress regime. The stability analysis of multi-lateral wells by means of finite difference method shows that in each stress regime the required mud pressure for the stability of junction is much more than that of the lateral branch and the main wellbore.     

硅酸钾钻井液在页岩气水平井中的可行性研究

页岩气开发以水平井为主,由于页岩地层裂缝与层理发育,在长水平段钻井过程中极易发生垮塌等井眼稳定问题,同时还有携岩、摩阻等一系列问题;大量的研究工作表明,硅酸盐钻井液的抑制性是较为接近油基钻井液体系的,但其能否用于页岩气水平井的钻井过程中是值得探讨的。本文在对硅酸盐钻井液体系及页岩稳定分析的基础上,建立了一套硅酸钾钻井液体系,该钻井液体系除了具有良好的抑制性能以外,更具有较低的滤失量(高温高压滤失量＜6 mL),并且流变性易于控制,润滑性好,分析认为该体系能够满足页岩气水平井钻井的要求。     

低油水比油基钻井液在保页3井的应用

为了解决油基钻井液成本高、钻屑含油量高、后期环保处理难度大等技术难题,加快页岩气的开发进度,缓解油气资源短缺局面,通过室内实验,成功研制出了一套低油水比油基钻井液体系。该体系具有良好的流变性、电稳定性和抑制性,滤失量低,能有效地保护井壁稳定,降低摩阻和扭矩,降低钻井液使用成本。经过在保页3井的应用,满足了现场的施工要求,保证了井下安全。     

Geomechanical study of gas reservoir rock using vertical seismic profile and petrophysical data (continental shelf in southern Iran)

Geomechanics is a science dealing with the study of the behaviour of rocks affected by stress. It has various applications in utilisation from oil and gas reservoirs including of the wellbore stability analysis and determination of safe mud window. The main aim of this paper is geomechanical study of Kangan–Dalan reservoir in South Pars gas field in Persian Gulf in south Iran. Seismic waves are affected by physical properties of rocks when passing underground formations; thus, the velocity of these waves is a desired parameter for estimation of geomechanical properties. The velocity of compressional and shear waves has been determined with processing seismic data resulting from vertical seismic profile. In this paper, after calculation of elastic modules of reservoir rock, the imposed stress field was determined and these concepts were used for engineering calculations such as safe mud window, wellbore stability analysis and sand production potential. For well drilling in Kangan-Dalan reservoir, the minimum and maximum mud weights were proposed in average as 1.093 and 2.011 gr/cc and average critical mud weight as 2.48 gr/cc such that if the weight of mud increases, the tensile fractures will be created on the formation and complete loss of mud will happen.     

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.     

空气潜孔锤在松散地层中的钻进试验

空气潜孔锤钻探工艺在基岩地区已是首选的一种技术。传统观念认为空气潜孔锤钻进工艺适用于坚硬岩石（硬、脆、碎地层）,在松散地层中则需要常规泥浆回转钻进或冲击钻进。通过典型松散地层的试验证明：空气潜孔锤在松散覆盖地层中可以实现正常的钻进,并且具有效率高、成本低、劳动强度小等优点,在粘土和潮湿粘土地层中钻进效率最高可达87和72 m/h。为解决页岩气、地热、煤层气、地下水等能源资源勘探时上覆地层的开孔或一开钻进提供了行之有效的方法,同时,也解决了缺水地区或严重漏失地层钻井液用水困难和传统开孔或一开泥浆回转钻探工艺复杂等问题。     

河南中牟页岩气区块地层特征及钻探问题研究

页岩气主要以吸附和游离状态贮藏在海相、陆相和海陆过渡相泥页岩中,这些岩石的沉积过程、时间和类型不同,表现出了地层或岩石的物理化学性质各向异性的特点。页岩气钻探所遇地层主要是不同时代的泥岩、砂岩、泥质砂岩、砂质泥岩等,这些岩石对钻井液中水分子具有极强的敏感性。所以,在钻探过程中常遇到起下钻遇阻、孔内沉渣多、重复破碎、卡钻等问题,由此花费大量的时间处理,从而减少了纯钻时间。这些问题的根本原因则是由于岩石膨胀缩径、孔壁崩塌超径所造成的。本文首先对河南中牟页岩气区块地层特征进行了描述,并就牟页1井钻探问题从钻孔缩径和超径2个方面进行了理论研究和分析,最后从钻井液和钻探工艺选择方面提出了原则性的建议。     

鲁页参1井钻井液技术

鲁页参1井钻遇地层复杂,水敏地层造浆严重,二叠系地层坍塌掉块,出现卡钻事故,导致钻井施工进度慢,风险大。针对钻井施工中存在的技术难题,开展了一系列的技术攻关,确定了一开和二开以及取心段的钻井液体系,形成了一整套钻井液技术和工程技术措施。实践表明,该套钻井液体系解决了鲁页参1井钻井中出现的坍塌、掉块等难题,减少了井下故障的发生,提高了钻进速度,建井周期明显缩短,为以后的钻井施工提供了技术支持。     

大位移井的井壁稳定力学分析

从井壁稳定的力学机理出发,分析大位移井周的应力分布规律,建立大位移井井壁稳定的力学模型,并据此编制了Windows环境下的井壁稳定分析软件,形成一个方便工程问题分析的集成环境,利用该软件分析了影响井壁稳定性的因素,提出了大位移井钻井的最优井斜方位和安全泥浆密度的选择方法。      

焦页非常规页岩气井优快钻井技术

重庆涪陵焦石坝地区上部地层岩性致密、研磨性强、可钻性差,长兴组浅层气活跃且含硫,三开水平段长、摩阻大、泥质页岩层易垮塌。钻井面临的突出问题是机械钻速低、井控难度大、钻井周期长。针对上述气藏钻井难点,重点从钻井工艺、维持井壁稳定以及钻井液方面进行分析,优化钻具结构、优选钻头选型、抑制井壁失稳以加快焦石坝区块钻井速度。“油基螺杆＋高效PDC钻头”配合优质钻井液,经过焦页12平台4口井的实践应用,大大缩短了钻井周期,焦页12-4 HF井创下了中石化集团公司非常规页岩气水平井最长水平段记录,提速效果显著。