海上油气井采油树升高机理及其计算方法

Mechanism of Christmas tree rise in offshore oil & gas wells and the related calculation method

中国南海某气田投产采气后发生了采油树升高现象,影响了平台结构布局并引起生产管线弯曲变形,严重时还可能会出现生产管线断裂,采油树升高机理及其计算方法研究成为保障海上采气平台安全的重要内容。为此,通过建立采油树升高力学分析模型来确定采油树升高机理;建立多层套管耦合系统轴向刚度计算模型,综合考虑井筒温度场、压力场和压力端末效应等影响因素,提出了一种采油树升高计算方法,使计算模型更接近于工程实际。最后,以我国南海某气田为例,进行了采油树升高计算并与现场实测值进行对比分析。结果表明:投产采气后未固井段套管温度场和压力场变化是引起采油树升高的主要原因,采油树升高计算值与现场实测值吻合良好;随着水泥返高距泥线距离的增大采油树升高值增大,其中技术套管的水泥返高对采油树升高影响较为明显;随着产气量的增大采油树升高值增大,但增大的幅度逐渐减小。

Christmas tree rise occurred in a gas field in South China Sea after the field was put into production, which influenced the platform layout and caused bending deformation or even fractures of production lines. Therefore, it is quite necessary to research the mechanism of Christmas tree rise and the associated calculation method for the ensured safety of offshore gas production platforms. A mechanical analysis model was built for Christmas tree rise to determine such mechanism. An axial stiffness calculation model was developed for multi-casing coupled system. Finally, a calculation method for Christmas tree rise was put forward based on the comprehensive consideration of wellbore temperature field, pressure field and pressure end effect, etc. , making the value obtained from the calculation model closer to the real value. In a case study of a gas field in South China Sea, a comparative analysis was made be tween the calculated value from this model and on-site measurements. According to the results, in this field after being put into pro duction, the Christmas tree rise was mainly caused by the variation in the temperature and pressure fields of non-cemented casing sections; the rise value calculated agreed well with the actual measurements. Moreover, the rise value increased with the increase of the distance from the top of cement ring to the mud line. Christmas tree rise was greatly influenced by the top height of cement ring in the intermediate casing. The rise increased as the gas output went up, but the margin gradually decreased.