Study of Deformation Law of Casing Local Lateral Collapse Based on the Principle of Virtual Work
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Abstract:
In view of the water swelling of mudstone and the creep induction function of formations in the process of oilfield water injection, the casing incurs collapse deformation under local lateral load. In this study, according to the actual collapse deformation characteristics of the casing in the second section of the Qing formation of the Songliao Basin in China, the yield surfaces of the casing collapse deformation are considered as plane plastic areas (half rhombus) with symmetric parabola shaped boundaries, and a mechanical model for the local lateral collapse deformation of casing is presented based on the principle of virtual work. Four types of casing, 4½″J55, 5½″J55, 4½″N80 and 5½″N80, are selected as examples. The relation of the casing intensity, the absolute reduction of intensity and the relative reduction ratio of intensity change with casing wall thickness, yield stress, radial maximum deformation, and deformation length are calculated and analyzed. The results show that the casing intensity of the casing is reduced under local lateral load, which is lower than the design standard value of the American Petroleum Association specification (API SPEC) 5CT. The relative reduction ratio declines linearly with the wall thickness of the casing wall as the yield stress increases, and increases linearly with increasing maximum deformation. In addition, the local lateral bearing capacity of the casing reaches the minimum value when the plastic deformation length reaches the critical value or the deformation quantity is less than the critical value. The conclusions provide scientific guidance for preventing casing failure accidents caused by deformation.Keywords:
Intensity
A 3D simulating model for calculating casing in fracturing wells is established based on the casing force-summing analysis. The laws of casing stress changes caused by inner casing pressure changes are calculated by simulation. According to the calculation the factors of fracturing influencing on casing failure are analyzed. It is considered that the casing failure is the result of combined action caused by many factors. Through analyzing the data of casing failure wells, the causes of casing failure in the fracturing wells are compared analytically, by which theoretical bases are provided for preventing casing failure under special operation condition.
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Oil well casing damage is a kind of catastrophic accidents in oilfield engineering site which may make the whole oil well abandoned. When the production casing is inserted into the intermediate casing, interaction occurs between them and possibly induces failure of the production casing. Because the casing is very long, mostly longer than 1 000 m, the calculation of the interaction is difficult. Some simplified methods are used in this paper to increase the calculation speed, including simplifying the intermediate casing as ana- lytical rigid body, reversing the production casing inserting process, simplifying the production casing as shell elements, cutting the inserting process into several steps, using the explicit method, increasing loading speed, artificially enlarging the mass, adding point mass on rigid casing. The calculation on the 1 000 m casing was done. And it was found that at the end of the production casing, the stress varied greatly; the stress along the diametrical direction varied greatly too. Fatigue damage could occur on the production casing.
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Decisions by storage operators to repair or replace well casing with metal loss require an accurate assessment of the degree of metal loss and the ability to determine the effect of that loss on the remaining strength of the casing. A new generation of logging tools (developed with PRCI cofunding) has increased the ability to accurately size metal loss defects in casing. Prior work funded by PRCI has also included testing the remaining strength of casing with metal loss defects. This testing was performed on casing samples that were not subject to the tensile loads that typical well casing in service may experience. This is the user manual for an associated Excel spreadsheet to calculate the remaining strength of corroded casing under combined pressure and axial loads.
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Casing completion is the most common and popular completion way but casings in a well can be inevitably damaged to different extent and by different measures. The defect always concludes both mechanical and chemical damage, which has large influence and damage on well safety and oil-and gas-bearing layers. So it is very important to detect casing damage. The MID-K logging instrument made in Russia to detect casing damage can find erosion and defect of different casings in combination casing tubes and provide reference for casing maintenance. Based on the introduction of logging theory of the MID-K instrument, in combination with logging interpretation examples, the applying effect of casing defect detection logging technology is explained.
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To improve casing utilization and casing uniformity,a two-step burley tobacco casing process was adopted,at the first casing step,burley strips were cased after conditioning,heating and loosening;at the second casing step,the cut strips were cased.The preferable casing proportion at each casing step was investigated as well.The results showed that comparing with traditional twice strip casing,the two-step casing raised the casing uniformity coefficient from 53.52% to 90.14% and the effective utilization rate of casing from 61.19% to 88.25% when the ratio of strip casing to cut strip casing was 3∶1.
STRIPS
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在 build 节的中间的 casings 服从于严重在扩大活动范围的钻穿。这篇论文为在中间的 casing 上预言一条 wear 沟的深度论述一个新方法。根据精力，原则和 casing 的动态累积由工具关节穿，一个模型被建立在 casing 的内部墙上计算 wear 区域。在 wear 沟深度和区域之间的关系功能基于在 drillstring 和 wear 节和假设之间的几何学关系被获得 casing 穿沟是新月形的。casing 的变化在不同尺寸的 drillstrings 下面穿沟深度对钻的英尺数也被讨论。一个机械模型为预言 casing 被建议穿地点，它基于井轨道和 drillstring 运动。casing 穿一口计划的井的沟深度与 casing 的倒置被预言从钻的井穿因素并且必然修订了为钻的井和计划的井之间的差别改进预言精确性。为预言 casing 的方法在扩大活动范围的钻穿通过实际案例研究被验证。casing 上的 drillstring 尺寸的效果穿应该在 casing 被考虑穿预言。
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Degradation of the casing strength in relation with thermal cycles of steam injection process is still less explored in literature. In this paper, three-dimensional finite element (FE) analysis of casing strength degradation in thermal environment of steam injection wells is presented. 3D FE models consisting of casing-cement-formation system are developed in this study. Grade N80 casing is employed with the casing length of 3.048 m. In the analysis, cyclic thermal stresses induced on the casing in thermal environment of steam injection wells from 25 °C to 360 °C are first examined to verify the feasibility of the 3D FE models. Degradation of the casing strength in the thermal environment is subsequently investigated by applying an external pressure that represents formation pressure to the casing-cement system. The results show that the casing capability to resist the pressure is lowering as the number of thermal cycles extends, thus causing casing strength degradation in the thermal application. It is also shown that the casing may fail under external pressure below its specified collapse strength i.e. 10 % lower than the reference casing strength obtained at 360 °C.
Steam injection
Degradation
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In this study, effect of casing imperfection on the casing strength in thermal environment of steam injection wells was investigated. The imperfections were modelled in the form of casing pits which may represent local cavitation, wear or corrosion attacks. Several pits with various depths (C) relative to the casing thickness (t) and different casing diameter to thickness (D/t) ratios were considered in this study. For the purpose of investigation, 3D FE models consisting of defective casing-cement-formation system were developed. N80 casing with the casing length of 3.048 m is analyzed in the numerical study. In the analysis, the combined effects of cyclic thermal stress (from 25 °C to 360 °C), collapse pressure (from formation) and defects on casing were taken into account. From the numerical study, it is found that the casing imperfection greatly affected the casing strength in steam injection wells. The presence of pits markedly reduces the casing collapse strength in comparison to that of non-defective casing. For instance, the collapse strength of casing having 2 pits along its circumferential direction with depth C = 0.5t reduced about 49% and 43% for D/t = 22.09 and D/t = 17.26, respectively. In addition, the combination of high temperature, imperfection factor and formation pressure induces severe deformation on casing, even at lower formation pressure.
Steam injection
Internal pressure
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The total number of casing deformation wells in Shengli oil field has reached 2539 at the end of 2002 since Shengli oil field entered into comprehensive production in 1970, which greatly restricted its development. The casing deformation types mainly include casing deformation and casing breakage. The casing deformation mechanism includes bed glide and mudstone creepage which cause casing deformation or breakage under uneven ground stress. Aiming at the cause of casing deformation and breakage, the correspondingly preventive action was made. One is to prevent extraneous force from exceeding casing yield strength,the other is to augment casing strength. Obvious anti-deformation effect is obtained with the application of actions.
Breakage
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Casing strength under the complex geological conditions is one of the major problems urgently to be solved at home and abroad. Catching the main characteristics and trends of casing collapse under the complex geological conditions, and based on which, carrying out the method study of casing strength analysis under the inhomogeneous load, as well as consideration of the law about how the non-uniformity load and the casing eccentricity influence on casing strength are of great significance. Setting a suit of method and theory about casing strength under the complex geological conditions has certain reference value. This paper affords a new sight for the casing strength design and safety assessment under the complex geological conditions by research on the relevance between the casing strength and the complex geological conditions. Meanwhile, consideration gives the non-uniformity load and the law about how the casing eccentricity influences on the casing strength. The results show that the increasing off-center distance will aggravate the non-uniformity of the loads when the casing skews along the relative off-center direction of 90° and 270°, thus the casing collapse risk goes up evidently.
Eccentricity (behavior)
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