页岩气井体积压裂井筒温度计算及套管强度变化分析

页岩气井体积压裂排量大、时间长,压裂过程中井筒温度变化剧烈,引起的温度应力对套管强度具有较大影响。在考虑压裂液摩擦生热以及排量与壁面换热系数关系的基础上,建立了页岩气井套管压裂过程中井筒温度场模型,对压裂过程中温度瞬态变化进行了计算。结果表明:页岩气井套管压裂过程中井筒降温幅度较大,储层温度为78.5℃时最大降幅达到64.7℃;跟端和趾端温度差最大值出现在压裂初期60~210 s。依据推导出的套管抗外挤强度计算公式进行强度校核分析,结果表明,温度应力对套管抗外挤强度产生较大影响,且随着压裂排量的不断增大,抗外挤强度降幅也不断增大,冬季施工压裂液排量为20 m~3/min时,套管抗外挤强度降低16.4%。

Calculation of wellbore temperature and analysis of its effect on casing strength during volume fracturing in shale gas well

With the characteristics of large displacement,long time and violent wellbore temperature changes during shale gas well volume fracturing,temperature stress has a significant effect on casing strength.In order to calculate the temperature changes of wellbore during fracturing,wellbore temperature field model was established considering the relationship of displacement and convection heat transfer coefficient and friction heat.The calculated results show that there is a significant reduction in wellbore temperature during volume fracturing,which could reach 64.7 ℃ when the reservoir temperature is 78.5 ℃.The maximum temperature difference between the heel and toe end of the horizontal segment appears at the beginning of fracturing about 60-210 s.Based on the calculation formula of casing collapse resistance,the calculated results show that the thermal stress caused by volume fracturing reduces the collapse resistance of casing about 16.4％ in winter when the fracturing fluid displacement was 20 m3/min.Furthermore,the larger the displacement is,the more the reduction is.