固井界面微环隙产生机制及计算方法

An assessment method for occurrence of micro-annular fractures on cementing interfaces of oil and gas well casing

针对油气井生产过程中固井界面微环隙的产生和发展规律进行仿真试验研究,并对套管-水泥环界面的黏结力进行测量,基于以上研究建立固井界面微环隙的理论计算方法。结果表明:套管内压过大导致水泥环产生塑性变形,套管内压减小时固井界面将受拉,实测固井界面存在数量级为0.1 MPa的黏结力,当界面拉力足以克服界面黏结力时,固井界面将会脱离产生微环隙;套管内压值越大,压力卸载后产生微环隙的机率越大,且微环隙尺寸越大;相同胶结质量下固井一界面比二界面更容易产生微环隙;理论计算结果与仿真试验结果具有较好的一致性。

The generation and development of micro annular fractures on cementing interface of oil and gas well casing during long term production were investigated via simulation experiments,and the bonding strength of cementing interface was measured in the experiments.Based on the experimental results,a theoretical assessment method of the micro annular fracture was developed,and a good agreement between the experimental results and modeling was observed.The results show that an excessive inner casing pressure can lead to plastic deformation of cement sheath,while a tensile stress can be subjected on the cementing interface when the inner pressure is reduced during the casing pressure unloading process.The magnitude of the interfacial bonding strength is of01MPa as it was measured in the experiments,and micro annular fractures can appear at the cementing interfaces when the tensile stress is sufficient high to overcome the bonding strength.The larger the inner casing pressure,the higher probability for the micro annular fractures to occur during pressure surge,and the larger the fractures.Micro annular fractures are more likely to appear at the first casing cement interface due to a higher tensile stress even if the bonding strength at two interfaces is the same.