基于井壁崩落的定向井坍塌压力计算模型

发布时间：2018-07-18 13:44

【摘要】：实际钻井过程中,井眼通常会存在一定程度扩径而不会出现与井眼失稳相关的井下复杂。现有坍塌压力计算模型不允许任何程度的井壁破坏,计算得到维持井壁稳定所需的最小井内压力过于保守。井壁稳定并非井眼完整,维持钻井过程中井眼完整需要的井内压力过高,甚至超过地层破裂压力,会造成钻井液漏失。由于不同井斜角的井眼能承受的破坏程度不同,定向井井眼需要承受上覆地层压力,进而井壁稳定问题较直井更加突出。因此需要建立新的定向井坍塌压力计算模型,允许井眼存在一定程度的井壁崩落,以降低钻井液密度的同时维持井壁稳定。本文利用多孔线弹性岩石力学理论,分别结合Mohr-Coulomb准则、Mogi-Coulomb准则、修正Lade准则、Drucker-Prager外接圆及内切圆准则,建立了基于井壁崩落的定向井坍塌压力的计算模型。该模型可计算定向井井眼在不同破坏程度时所需的井内压力,即坍塌压力,并根据坍塌压力优化定向井井眼轨迹。地应力是钻井液密度及井眼轨迹设计的重要参数,利用井壁崩落确定最大水平地应力大小的方法已得到广泛的运用,然而相应的模型仅适用于直井。尽管贝克休斯GMI软件可以利用任意定向井的井壁崩落确定最大水平地应力的大小,但并未有文献提出其方法。在建立定向井坍塌压力计算模型的过程中,本文找出了利用定向井井壁崩落宽度确定最大水平地应力的方法。根据已建立的定向井坍塌压力计算模型,利用Visual C#.NET开发了基于井壁崩落的定向井坍塌压力计算软件。该软件可以利用定向井的井壁崩落宽度计算最大水平地应力的大小,根据Mohr-Coulomb准则和修正Lade准则计算的结果与GMI软件基本一致。结合Mohr-Coulomb准则、修正Lade准则、Drucker-Prager外接圆及内切圆准则,利用该软件计算了允许井壁崩落和不允许井壁崩落的定向井坍塌压力,计算结果与GMI软件基本一致甚至完全一致。基于井壁崩落的定向井坍塌压力计算软件与GMI软件的计算结果对比表明,二者相当接近甚至完全一致,允许井壁崩落可以显著地降低定向井坍塌压力。因此,可以得出结论：本文提出的利用定向井井壁崩落宽度来确定最大水平地应力的方法正确可靠,开发的软件可用于计算最大水平地应力和定向井坍塌压力。
[Abstract]:In the actual drilling process, the borehole will usually have a certain degree of diameter expansion without any downhole complexity associated with borehole instability. The existing collapse pressure calculation model does not allow any damage to the wellbore, and the minimum in-well pressure needed to maintain the stability of the borehole is too conservative. The wellbore stability is not complete. The drilling fluid leakage will be caused by the high pressure in the borehole which is needed to maintain the borehole integrity or even exceed the formation fracture pressure. The directional borehole needs to bear the overlying formation pressure because of the different damage degree of the borehole with different inclined angle, and the problem of wellbore stability is more prominent than that of the straight well. Therefore, it is necessary to establish a new model for calculating the collapse pressure of directional wells to allow a certain degree of wellbore caving, so as to reduce the density of drilling fluid and maintain the wellbore stability at the same time. Based on the theory of porous linear elastic rock mechanics and Mohr-Coulomb criterion Mogi-Coulomb criterion and modified Lade criterion Drucker-Prager outer circle and internal tangent circle criterion, a model for calculating collapse pressure of directional well based on wellbore caving is established in this paper. The model can be used to calculate the in-well pressure, i.e. the collapse pressure, which is required by the directional wellbore under different failure degrees, and to optimize the directional wellbore trajectory according to the collapse pressure. The in-situ stress is an important parameter of drilling fluid density and borehole trajectory design. The method of determining the maximum horizontal stress by wellbore caving has been widely used, but the corresponding model is only suitable for vertical wells. Although the baker Hughes GMI software can be used to determine the maximum horizontal stress by using the sidewall caving of an arbitrary directional well, no method has been proposed in the literature. In the course of establishing a model for calculating the collapse pressure of directional wells, a method for determining the maximum horizontal ground stress by using the sidewall caving width of directional wells is found in this paper. Based on the established collapse pressure calculation model of directional well, the collapse pressure calculation software of directional well based on wellbore caving is developed by using Visual C#.NET. The software can be used to calculate the maximum horizontal stress by using the wellbore caving width of the directional well. The results calculated by the Mohr-Coulomb criterion and the modified Lade criterion are in good agreement with the GMI software. Combined with Mohr-Coulomb criterion and modified Lade criterion Drucker-Prager circumscribed circle and internal tangent circle criterion, the collapse pressure of directional well with and without wall caving is calculated by using this software. The calculated results are consistent with or even completely consistent with GMI software. The calculation results of directional well collapse pressure based on wellbore caving and GMI software show that both of them are quite close to or even consistent with each other, and allowing sidewall caving can significantly reduce the collapse pressure of directional well. Therefore, it can be concluded that the method proposed in this paper to determine the maximum horizontal ground stress by using the sidewall caving width of the directional well is correct and reliable, and the developed software can be used to calculate the maximum horizontal ground stress and the collapse pressure of the directional well.
【学位授予单位】：西南石油大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TE21

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