井下复杂条件下固井水泥环的失效方式及其预防措施

Modes and prevention of cement sheath failures under complex downhole conditions

固井水泥环是封固系统的薄弱环节,而了解井下复杂条件下水泥环的失效方式对于预防其失效具有重要的指导意义。为此,在考虑封固系统初始作用力的基础上,利用数值模拟和室内实验等方法研究了蠕变地应力、井下压力和温度变化等因素对封固系统应力状态及界面胶结强度的影响,进而分析了对应的水泥环失效方式,并提出了相应的预防措施:①蠕变地应力作用下水泥环的失效方式为屈服破坏,建议使用高杨氏模量和高抗压强度的水泥环来预防失效;②套管内压力升高时水泥环的失效方式为切向拉伸破坏,套管内压力降低时水泥环的失效方式为界面挤压应力降低或界面剥离,建议使用低杨氏模量的水泥环或使用膨胀水泥来预防失效;③井下温度升高时水泥环的失效方式为屈服破坏,建议使用低杨氏模量的水泥环来预防失效;④井下温度降低时水泥环的失效方式为界面挤压应力降低或界面剥离,建议使用低杨氏模量的水泥环或使用膨胀水泥来预防失效。

In order to prevent the {allure of cement sheath, a weak part of the isolation system, it is of great importance to study its failure modes under complex downhole conditions. By conducting numerical simulation and lab experiments considering the initial stress of the isolation system, we first studied the effects of the changing creep stress, pressure and temperature on the stress state and the interracial bonding strength of the isolation system. On this basis, we revealed the failure modes of cement sheath and pro- posed the corresponding preventive measures as follows, a. The cement sheath is subject to the yielding failure when creep stress acts on the isolation system; the cement sheath with high Young＇s modulus and high compressive strength is recommended for preventing this failure, b. The cement sheath is prone to the tangential tensile failure when the pressure in the casing increases, and it fails in a manner of interracial extrusion stress dropping or interface debonding when the pressure in casing drops; the cement sheath with low Young＇s modulus or expansive cement are recommended for preventing these failures, c. The cement sheath is apt to yielding failure when downhole temperature rises, and the cement sheath with low Young＇s modulus is recommended for preventing this failure, d. The cement sheath fails in a manner of extrusion stress dropping or interface debonding when temperature drops, and the cement sheath with low Young＇s modulus or expansive cement is recommended for preventing this failure.