地应力变化与油井套管挤毁规律研究

Research on Collapse of Oil Well Casing under the Variation of Ground Stress

油井生产过程中,近套管地层地应力大小和不均匀性逐渐增加,并趋于原始不均匀地应力。生产中后期,套管腐蚀加剧,壁厚减小,管壁应力明显增加,套管易发生挤毁。为了探讨地应力变化与套管挤毁之间的关系,针对因腐蚀壁厚减小的套管,采用强度和稳定性理论进行挤毁规律研究。结合胜利油田某井场油井套管腐蚀情况,选取典型油井建立了套管计算模型。结合生产过程地应力的变化进行套管强度计算,得到生产初期和中后期不同壁厚套管的应力大小和剩余强度。对比不同壁厚套管的应力和失稳临界载荷,得知套管在壁厚腐蚀减小过程中首先发生失稳损坏,且生产中后期套管周围地应力大小和不均匀性明显增加,更易发生套管挤毁现象。所得结论可为初步研究套管在地应力变化时的挤毁机理提供理论依据。

As oil well production progresses, the magnitude and non-uniformity of stresses in the vicinity of the casing are increasing and approaching the original uneven ground stress. In the middle and later stages of pro-duction, as the casing corrosion increases and wall thickness reduces, the stress of the casing increase obviously, thus, the casing is prone to collapse. The theory of strength and stability are used to study behaviors of the casing with reduced wall thickness under condition of corrosion. Taking the casing corrosion condition in a well site in Shengli Oilfield as case study, the casing calculation model has been established for a typical well. The casing strength has been calculated according to ground stress variation during production so as to attain the stress intensity and residual strength of the casing with different wall thickness in the initial and middle stage of the production. Compared with the stress and buckling critical load of casing with different wall thickness, it is found that casing occurs buckling first in the process of its wall thickness reduction. The ground stress magnitude and unevenness a- round the casing are significantly increased in middle and later period, which make it easier to result in casing col-lapse. The study could provide theoretical bases for the preliminary study of the casing collapse mechanism in the ground stress changing.