水平井造斜段盐膏层套管等效应力分析

ANALYSIS OF THE EQUIVALENT STRESS ON CASINGS INDUCED BY SALT-GYPSUM CREEP IN BUILD-UP SECTIONS OF HORIZONTAL WELLS

针对盐膏层蠕变和水平井造斜段耦合作用下套管变形破坏的关键技术难题,采用FLAC3D有限差分软件建立了弯曲套管-水泥环-盐层三维地质模型,分析了非均匀地应力场条件下井眼曲率、钻井方位以及非均匀地应力对套管等效应力的影响。研究表明,在盐膏层钻水平井时,沿水平最小地应力方位钻进套管更安全;其他条件一定,井眼曲率越大,套管等效应力和位移越大,越容易发生屈服;随着蠕变时间的增加,套管等效应力迅速达到一最大值,最后趋于稳定,1.5年左右达到平衡;当地层受均匀地应力作用时,套管等效应力较小;当套管受非均匀地应力时,套管等效应力增加,且非均匀系数越大,套管所受的等效应力越大。该方法应用于中国西北部某油田盐膏层水平井造斜段弯曲套管损坏分析,完善了盐膏层套管设计理论,为盐膏层井筒安全提供了有力的保障。

To alleviate the problems of casing collapse induced by the coupling effect of salt-gypsum creep movement, as it relates to the curved sections of horizontal wells,a 3D geological model with curved casings,cement and salt formation was established by using finite-difference software FLAC3D . With this model,the effects of borehole curvature,drilling direction and non-uniform in-situ stress on equivalent stress of casing were analyzed. It indicates that: The casing tends to be much safer when drilling is undertaken along the direction of the minimum horizontal stress; Casing deformation occurs and stress increases as the borehole curvature increases; The equivalent stress on casings increases gradually with creep time increasing and will get to balance in 1.5 years or so; Under the effect of non-uniform in-situ stress,a larger non-uniform coefficient would result in greater equivalent stress on the casing. This model has been applied to analyze the equivalent stress of curved casings in a build-up section for horizontal wells drilled through salt-gypsum formations in an oilfield in Northwest China. The model has helped to improve the casing design and reduce drilling downtime with greater wellbore stability in salt-gypsum formations.