基于水泥石实验数据的水泥环力学完整性分析

气井环空带压是高温、高压、高酸性气井开发中面临的一个重要难题。为此，针对从固井到生产作业中影响固井水泥环力学完整性的问题，采用弹塑性力学理论，建立套管-水泥环-地层系统耦合力学模型，求解水泥环Tresca应力和径向位移。结合龙岗气田X井尾管固井水泥石三轴应力实验获得的弹性模量、泊松比、抗压强度、屈服应变等岩石力学参数，进行了尾管固井段水泥环力学完整性评价：①固井初期，同一井深处，井筒内压力越大，水泥环受到Tresca应力和水泥环径向位移越大；相同内压下，随着井深的增大，水泥环受到的Tresca应力变化很小，但水泥环径向位移增加较为明显。②生产阶段，由于井筒内压力逐渐降低，水泥环受拉应力作用，水泥环移动方向与固井初期移动方向相反，随着压力的降低，在同一井深处，水泥环受到的Tresca应力和水泥环径向位移都变大；随着井深的增加，Tresca应力和径向位移逐渐增大。评价结论认为该井测试及生产期间不会引起尾管段水泥环力学完整性失效。

Analysis of cement-sheath mechanical integrity based on the experiment data of cement paste

Circular abnormal pressure is a major challenge in the development especially of high temperature, high pressure and high rate sour gas wells. In view of the problems affecting the cement sheath mechanical integrity in the cementing and production operations, the following procedures were adopted in this study. First, based on the plastic elastic theories, a coupling mechanical model of casing cement sheath formation system was established to figure out Tresca stress and radial displacement of cement sheath. Then, rock mechanical parameters including elasticity modulus, Poisson ratio, compressive strength and yield strain were collected from the triaxial stress experiment on cement paste in tail pipe cementing of Well X, Longgang Gas Field, Sichuan Basin. According to these parameters, a cement sheath mechanical integrity analysis was finally conducted of the cemented section on tail pipe. In conclusion, at the early stage of cementing, both the Tresca stress and the radial displacement of the cement sheath increase with the rising of internal pressure in the shaft at one certain well depth, while at the deeper locations, the Tresca stress varies slightly but the radial displacement rises significantly if the internal pressure remains unchanged. At the production stage, the internal pressure of shaft reduces gradually, making the cement sheath bear tension stress and the displacement direction of cement sheath reverse compared with that at the early stage of cementing, both the Tresca stress and the radial displacement of cement sheath increase with the drop in pressure at one certain well depth, and both values are becoming gradually higher at deeper locations. The analysis results demonstrate that mechanical integrity failure of cement sheath will never occur on the tail pipe in the test and production period of the well in this case study.