固井水泥石力学性能直接关系到水泥环的长期密封效果.获取固井水泥石的力学性能参数主要通过三轴力学试验进行,有效制备试样对固井水泥石三轴力学试验研究乃至水泥环完整性分析至关重要.基于固井水泥石三轴力学试验试样的特殊性,设计探索了正方体水泥石养护成形后取心的正方体取心制样以及借助于模具一次养护成形制样等2种试样制备方法;研制了用于模具养护制样方法的?25.4 mm×50.8 mm小试样和?50.8 mm×101.6 mm大试样2套圆柱形水泥石养护模具;通过散点分布、均方根误差分析、标准差分析等对比研究了56组不同制样方法获取试样的力学试验数据,结果表明,小试样模具制样养护方法在数据可靠性、制样效率及成本等方面均优于其他方法,推荐作为固井水泥石三轴力学试验试样制备方法.

地层类型制约地应力对固井水泥环的力学作用方式,明确地层类型对水泥环应力和结构完整性的影响特征,是建立井筒完整性保障技术的基础.以蠕变、弹性、刚性地层为研究对象,建立套管—水泥环—地层固结体有限元力学模型,分析地层类型、施工项目等对水泥环界面应力的影响.结果表明:蠕变地层中,地应力增加,界面接触压力增大;弹性、刚性地层中,地应力对界面应力无影响.蠕变、弹性地层中,地层弹性模量对界面应力的影响与地层、应力类型有关;刚性地层中,地层弹性模量对界面应力无影响.弹性地层中,加载时,水泥环易发生周向拉伸破坏,危险部位为窄间隙内界面.套管试压施工时,存在水泥环拉伸破坏的危险,危险井段为井口及深部弹性地层井段;储气库井注气施工时,存在的主要危险为加载过程中弹性地层井段水泥环本体拉裂破坏.

This paper aims to solve the issue of differential pressure in the annulus commonly existed in shale gas and deep oil & gas well drilling.The reasons for cement sheath integrity failures mainly caused by a high differential pressure in the exploration and development process of deep,low-permeability,unconventional oil and gas resources were achieved from theoretical research and experimental simulation.Then,in the case studies of shale gas wells,HTHP wells in the Anyue gasfield,Sichuan Basin,and the Kuche Shanqian block in the Tarim Basin,differential pressures during drilling,fracturing and production process were sorted and quantitative calculation was made to further interpret the above achievement,that is,the cement sheath integrity failure by micro-annulus resulted from tensile stress under gravity-free condition in the solidification period of cementing,plastic deformation in the cement-sheath under loading and unloading working conditions,tensile stress in the interfaces in the process of unloading.Patterns of cement sheath integrity failures under a high differential pressure were analyzed,and the risks of integrity failures during different periods of a wellbore full life cycle were summarized.Preliminary measures and techniques to ensure cement-sheath integrity were proposed as follows:to improve the deformation resistivity of cement stones and to enhance the cementing force of the primary and the second interfaces as well as the force of interface contact.

Whether cement thickness is reasonable or not is related to cementing efficiency and cementing quality,simultaneously,the late cement mechanical reliability will influence long-term zonal effective isolation.So it is necessary to find out the influence law of cement thickness and its mechanical parameters to cement stress,analyze the thickness sensitivity of cement and optimize cement mechanical parameters.For this purpose,an analysis and study of cement stress state has been carried out based on elastic theory.The results show that circumferential stress at innner surface is maximum,therefore,is cement's dangerous stress.Maximum circumferential stress decreases with the increase of cement thickness,but its gradient is larger when cement thickness is less than 25.4 mm,as cement thickness increases,the maximum circumferential stress gradient decreases gradually until hardly changes.For cement of a certain thickness,the greater the elastic modulus,the greater the circumferential stress;the greater the poisson ratio,the smaller the circumferential stress.This study result can be used to guide optimization of cement thickness and cement slurry formulations.