蠕变岩层地层刚度和松弛时间对套管外挤力的影响

蠕变岩层套管外挤力计算要考虑蠕变松弛时间和固井水泥环性能的共同作用,目前的计算模型没有同时考虑两者的影响,使得计算结果偏差较大。在考虑水泥环作用条件下,建立了蠕变地层下套管外挤力黏弹性模型,利用该模型研究了松弛时间、地层刚度和水泥环性能对套管外挤力的影响。研究结果显示,不考虑水泥环作用,蠕变岩层套管外挤力与无蠕变作用的套管外挤力最终值相同,考虑水泥环作用时,蠕变岩层套管外挤力取决于水泥环性能;与以往蠕变岩层套管外挤力主要受地应力与蠕变率影响的认识有所不同,影响蠕变岩层套管外挤力的主要因素还包括地层刚度和松弛时间,它们影响套管外挤力大小和峰值、稳定值变化趋势;考虑内压和水泥环作用时,所得到的蠕变岩层套管外挤力远大于SY/T 5724-2008套管设计标准计算得到蠕变岩层套管外挤力,该标准中蠕变岩层套管外挤力计算方法仍然值得商榷。利用所建模型和实验结果,分别对伊拉克Missan油田和中原文中油田蠕变盐膏层段套管外挤力进行比较分析,在相同地应力和蠕变松弛时间作用下,地层刚度不同,套管外挤力相差很大,所得结论与现场情况相吻合。

Influences of stratum stiffness and relaxation time of creep layers on external squeeze pressure of casing

External squeeze pressure of casing for creep layers should be calculated in consideration of the combined effect between creep relaxation time and cement sheath performance. However, the present calculation model takes no account of both influences, leading to large deviations in calculation results. An external squeeze pressure viscoelasticity model of casing under the creep layers was established as considering the effect of cement sheath, which aims to study the influences of relaxation time, stratum stiffness and cement sheath performance on the external squeeze pressure of casing. The results show that without consideration of cement sheath effect, the external squeeze pressure of creep-layer casing is equal to that of non-creep-layer casing; in consideration of cement sheath effect, the external squeeze pressure of creep-layer casing depends on cement sheath performance. Different from the previous understanding that the external squeeze pressure of creep casing is mainly influenced by geo-stress and creep rate, the main influential factors for external squeeze pressure of creep-layer casing also include relaxation time and stratum stiffness, which have impacts on the size and peak of external squeeze pressure as well as the changing trends of stable value. In consideration of internal pressure and cement sheath effect, the external squeeze pressure of creep-layer casing obtained in this study is rather larger than that calculated in line with the casing design standard SY/T 5724-2008. Further discussions are still required for the calculation method of external squeeze pressure of creep-layer casing in this standard. The established model and experimental results were used for comparative analysis on the external squeeze pressure of casing in creep salt-gypsum beds of Missan oilfield in Iraq and Zhongyuan Wenzhong oilfield in China. The conclusion has been achieved that in case of the same geo-stress and creep relaxation time, stratum stiffness is different and the external squeeze pressure of casing varies greatly, consistent with the onsite situation.