页岩气水平井压裂过程水泥环力学特性评价研究

Assessment on the Cement Mechanical Properties of Shale Gas Horizontal Well in Fracturing Process

针对页岩气水平井大规模体积压裂过程中,套管-水泥环组合系统在外部载荷作用下产生破坏而引发的井筒完整性问题,采用弹性力学和复变函数方法,建立了该组合系统的力学特性分析模型,分别以Von-Mises屈服准则和Drucker-Prager岩土屈服条件作为套管和水泥环的屈服判据,对页岩气井压裂过程组合系统的薄弱环节进行了分析;在此基础上,对井筒完整性进行了参数评价和敏感性讨论。计算结果表明:页岩气井压裂过程中,水泥环比套管更易屈服,从而引发井筒完整性问题;套管壁厚和水泥环泊松比越大,井筒越安全;低弹性模量的水泥环,对井筒完整性有利;水泥环厚度存在最优值使井筒最安全;需要根据井筒完整性和压裂施工效果,综合确定压裂施工压力。

Wellbore integrity faliure,which is a common problem in horizontal well volume fracturing process for shale gas,is easily induced by damage of casing-cement sheath under external loads.The analysis model is established through elastic mechanics and complex variables functions,and the weak part of the system is obtained based on the Von-Mises yield criterion and the Drucker-Pragercriterion for geotechnical materials.On this basis,the wellbore integrity of shale gas horizontal well is assessed,and the parameter sensitivity analysis is presented.Analysis results show that the cement sheath is more likely to yield than casing during fracturing process,and is the main precipitating factor for wellbore integrity.Bigger casing wall thickness,Poisson′s ratio of cement sheath,and lower elastic modulus of cement sheath are advantageous for wellbore integrity.There is an optimal value of cement sheath thickness from the point of view of wellbore integrity.The fracturing pressure should be determined by wellbore integrity and the effect of hydraulic fracturing operation.