基于水泥环完整性分析的许用套管内压力解析计算方法

现有的油气井套管—水泥环—地层系统的应力分析模型可用于求解井筒系统应力分布及水泥环破坏条件,虽然对探讨水泥环完整性受套管内压力变化的影响起到了重要作用,但均未能给出许用套管内压力解析计算公式。为此,基于均匀水平地应力条件下的套管—水泥环—地层系统的应力分析解析模型,针对多种水泥环破坏形式,推导出了受其作用下的水泥环径向开裂、剪切破坏、胶结面剥离等破坏条件的许用套管内压力解析计算公式。研究结果表明:(1)按水泥环径向开裂和剪切破坏条件分别求出最大允许套管内压力,推荐采用二者中的较小值作为最大许用套管内压力;(2)按水泥环胶结面剥离条件求出最小允许套管内压力,推荐将其作为许用套管内压力。结论认为,套管内压力升高可能导致水泥环径向开裂和剪切破坏,套管内压力及温度降低可能导致水泥环胶结面剥离,应综合考虑水泥环破坏条件合理确定出最大和最小许用套管内压力。

An analytical method for the calculation of allowable internal casing pressure based on the cement sheath integrity analysis

The existing stress analysis models for casing–cement sheath–formation system of oil and gas wells can be used to calculate the stress distribution of wellbore system and the failure conditions of cement sheath. Besides, they play an important role in analyzing the effect of internal casing pressure on cement sheath integrity. However, none of them provides analytical equations for calculating the allowable internal casing pressure. Based on the analytical models for calculating the stress of casing–cement sheath–formation system under uniform horizontal crustal stress, the analytical formula for calculating allowable internal casing pressure were derived for various failure conditions of cement sheath, e.g. radial cracking, shear damage and cementation interface disbonding. It is indicated that the maximum allowable internal casing pressure in the failure conditions of radial cracking and shear damage of cement sheath is calculated respectively, and the lower value is proposed to be the maximum allowable internal casing pressure. It is recommended to take the minimum allowable internal casing pressure which is calculated in the failure condition of cementation interface disbonding of cement sheath as the allowable internal casing pressure. It is concluded that pressure increase inside the casing may result in radial cracking and shear damage of cement sheath, and pressure and temperature decrease inside the casing may lead to disbonding of cement sheath bonding interface. In order to determine the maximum and minimum allowable internal casing pressure reasonably, it is necessary to consider the failure conditions of cement sheath comprehensively.