水泥环弹性模量和厚度对套管外挤载荷影响的理论研究

针对水泥环弹性模量变化对套管强度是否有帮助的讨论,采用弹性力学分析方法研究了水泥环弹性模量对不同性质地层内套管外挤荷载的影响规律。研究认为,在岩性较软的地层,应选用刚度较大的水泥环,可帮助套管抵卸一部分载荷;在地层岩性较硬时,选用刚度小的水泥环有助于提高套管抵抗外挤载荷能力。理想的水泥环特性应为高强度、低刚度。其机理是利用高强度抵御地层载荷、低刚度降低载荷传递系数,从而达到保护套管目的;单纯增加水泥环的厚度不一定能够改善套管的受力情况,只有当水泥环弹性模量大于一定值后,加大其厚度可改善套管的受力状况。因此,在实际现场中,应根据具体的地层情况,调整水泥浆特性,达到最好的固井效果。

Theoretical Study on Effects of Cement Sheath Elastic Modulus and Thickness on Casing Load

Considering the change of cement sheath elastic modulus on casing strength, whether is helpful or not, by the elastic mechanics mathematical method, the effect of cement sheath elastic modulus on casing external collapse load was researched on the conditions of uniform in-situ stress on different properties in formation. The study shows that in a formation with soft lithology, the cement sheath shall have higher stiffness to help casing withstand a part of load; while in a formation with hard lithology, the stiffness of the cement sheath shall be reduced in order to protect casing. The ideal property of concrete sheath should be of high strength and low stiffness. The high strength of concrete sheath could resist higher loads of formation stress. At the same time, the low stiffness of that could reduce the transfer coefficient of stress greatly. Increasing the thickness of the cement sheath is not always reducing the stress of the casing. When the elastic modulus of cement sheath achieves some value, increasing the thickness of cement sheath could decrease the casing stress. From this point, the property of cement slurry shall be adjusted according to formation conditions in the process of cementing to reduce casing load and achieve the best cementation.