套管试压对水泥环密封性的影响

Effect of casing pressure testing on the sealing performance of cement sheath

套管试压后产生的微环隙是导致水泥环密封性下降的主要原因之一.根据套管-水泥环受力模型,推导了套管试压后产生的微环隙的定量计算公式.通过计算对影响微环隙尺寸的因素进行了分析.结果表明:试压时微环隙的尺寸与井口压力成正比;与围岩压力、水泥环的厚度呈负线性相关;随套管壁厚的增加而减小;随水泥环弹性模量的增加呈先增大后减小趋势.在水泥环弹性模量为25GPa时,微环隙的尺寸最大,但在其弹性模量小于25GPa时,弹性模量对减小微环隙尺寸的作用更明显,故对于需要高压套管试压的油气井,宜选用低弹性模量的水泥石.

The micro-gap between casing and cement sheath generated from casing pressure testing is one of the main reasons leading to the decline of the sealing performance of the cement sheath.A quantitative formula for calculating of the size of the micro-gap is deduced according to the force model of casing--cement sheath,and the factors of influencing the size of micro-gas are analyzed.The results show that during pressure testing,the size of the micro-gap is directly proportional to wellhead pressure,it is negative linear correlation with the pressure of the surrounding rock and the thickness of cement sheath,it is decreases with the increase of the wall thickness of the casing.The size of micro-gap firstly increases and then decreases with the increase of Young's modulus of the cement sheath.When the Young's modulus is 25 GPa,the size of micro-gas reaches to the greatest value.When the Young's modulus is less than 25 GPa,the Young's modulus plays a more important role in reducing the size of the micro-gap.It is suggested that the cement stone of the lower Young's modulus should be selected when the high-pressure testing of casing is needed.