温度作用影响套管抗挤强度的定量评价方法——以页岩气水平井大型压裂施工为例

A quantitative evaluation method for the effect of temperature on casing collapsing strength:A case study of large-scale hydraulic fracturing in shale gas horizontal wells

目前通常采用大规模水力压裂的方法对页岩气储层进行改造,由于施工排量大、施工时间长,造成井筒温度下降速度快且幅度大,由温度作用而产生的拉应力会降低套管抗挤强度,不利于安全生产。针对页岩气水平井压裂施工工艺及工况条件,建立了水力压裂时的热传导控制方程,并利用有限差分法对其进行求解,分别探讨了不同井口温度下常规水力压裂和大规模水力压裂时井底温度的变化情况;然后再根据API套管挤毁压力计算公式求得了不同钢级套管在不同温度变化时套管抗挤强度的变化值。结果表明:①较之于常规水力压裂,大型水力压裂时,井底温度变化对套管抗挤强度影响较大且对不同钢级的套管抗挤强度的影响程度也有所不同,其中高钢级套管的抗挤强度受影响较小,而低钢级套管抗挤强度受影响则较大;②当在冬季进行施工及水平井垂深更深时,这种由于温度作用造成套管抗挤强度的降低更为明显。结论认为,在对页岩气水平井大规模水力压裂时,必须考虑因为井底温度变化所造成的套管强度降低的问题,并进行合理的套管设计。

At present,a large-scale hydraulic fracturing technology is generally used in the stimulation of shale gas reservoirs.Due to high operation discharge and long operation time,however,the wellbore temperature drops quickly and drastically,and consequently tensile stress is generated,resulting in the reduction of casing collapsing strength,which is unfavorable for safe production.In this paper,a heat conduction equation of hydraulic fracturing was established on the basis of the operation conditions and fracturing technologies for shale gas horizontal wells,and then solution was figured out by the finite differential method.Comparative analysis was performed on the bottomhole temperature changes between conventional hydraulic fracturing and large-scale hydraulic fracturing at different wellhead temperatures.A reasonable casing design was made taking into account the impact of bottomhole temperature change on casing strength in the large-scale fracturing shale-gas well.And finally,the collapsing strength changes of casings with different steel grades at different temperature changes were calculated by using the API casing collapsing stress formula.It is shown that the casing collapsing strength is more affected by bottomhole temperature change in large-scale hydraulic fracturing than in conventional hydraulic fracturing,and the impact varies with the steel grades.The casings of high steel grades are less affected,while the effect on those of low steel grades is more.If the operation is carried out in winter,the casing collapsing strength related with the temperature effect decreases significantly when a horizontal well is vertically deeper.It is concluded that when large-scale hydraulic fracturing is performed in shale gas horizontal wells,it is necessary to take into consideration the reduction of casing collapsing strength caused by the change of bottom hole temperature if a proper casing design is made.