Abstract:

Hydraulic fracturing technology is an important method for increasing the production capacity of shale oil wells. The mechanical behavior of well cement sheath during fracturing has an important impact on the safety and efficiency of production. Based on the cement slurry system in Jimsar region，a cement specimen maintenance mold was designed as well as a cement bonding strength test program，maintaining the standard cylindrical specimen of cement and the set cement bond specimen. The uni/triaxial compression experiments were then carried out as well as the cyclic loading experiments and bonding strength experiments under simulated well bottom conditions to investigate the rules governing damage and destruction of cement bodies and cementing interface. The results show that under uniaxial compression，the set cement has significant elastic-brittle characteristics and that macroscopic cracks are formed in the cement body when it is damaged.The average Young's modulus of cement is 3.054 GPa and the average Poisson's ratio is 0.127. Under triaxial conditions，the strength and plasticity of the cement sheath are enhanced. During cyclic loading，there is a bilinear accumulation of cementite plastic deformation，with the amount of plastic deformation being positively correlated with the peak load. The plastic strain after 20 cycles increased from 0.24% to 2.46% when the peak load increased from 22.7 MPa to 53.2 MPa. The cement bonding strength is lower than 1 MPa，which is much lower than the body strength，and the bonding interface is a natural mechanical weak zone，representing a risk point for wellbore seal failure.

Key words:

cement sheath, well cementing, mechanical behavior, bonding strength, failure form, Jimsar area, shale oil

摘要：

水力压裂技术是提高页岩油井产能的重要手段，压裂过程中固井水泥石力学行为对油气井安全和高效生产有重要影响。基于吉木萨尔地区固井水泥浆体系配方，设计了水泥试样养护模具及水泥胶结强度测试方案，养护了水泥石标准圆柱试样及水泥石胶结试样，开展了模拟井底条件下单/三轴压缩实验、循环加载实验以及胶结强度实验，探究了固井水泥石本体、胶结面损伤及破坏规律。研究结果表明：单轴压缩条件下，水泥石具有显著的弹脆特性，破坏时水泥本体形成宏观裂隙，水泥弹性模量均值为3.054 GPa，泊松比均值为0.127；三轴条件下水泥石强度及塑性特征得到增强；循环加载过程中，水泥石塑性变形存在双线性累积现象，塑性变形量与载荷峰值呈正相关，载荷峰值由22.7 MPa增至53.2 MPa时，循环20次后的塑性应变由0.24%增至2.46%；水泥胶结面为天然力学弱层，水泥石胶结强度小于1 MPa，远低于水泥石本体强度，是井筒密封失效的风险点位。

关键词:

水泥石, 固井, 力学行为, 胶结强度, 失效形式, 吉木萨尔地区, 页岩油