Study Of The Casing Failure Mechanism In Horizontal Shale Gas Wells

In recent years,with the large-scale development of shale gas reservoir,casing failure occurs frequently during multi-stage and multi-cluster fracturing in horizontal wells,which causes a series of problems to the subsequent fracturing operation,even reduces the number of fracturing segments and reduces the productivity of single well.Although many scholars have carried out a series of research and analysis on casing damage in fracturing process and put forward casing failure mechanisms during fracturing process at home and abroad,it has not combined the characteristics of large-scale multi-stage and multi-cluster fracturing in shale gas wells.Therefore,the failure mechanism of casing proposed is limited.Based on extensive investigation of domestic and foreign literature,this paper analyzed the location,timing and shape of casing failure during fracturing.Based on the study of wellbore.temperature distribution and statistical damage constitutive model based on Weibull distribution,a mathematical model of thermal-hydraulic-mechanical coupling during multi-stage and multi-cluster fracturing in horizontal shale gas wells has been established.To solve the problem,the finite element method has been used,and the influence of various factors on the stress of casing-cement sheath-formation assembly have also been analyzed.As a result,the mechanism of casing failure in horizontal shale gas well fracturing and the corresponding technical measures have been proposed.The results indicate that the friction heat generation between fracturing fluid and casing inner wall can not be ignored during fracturing,and the temperature of casing inner wall nearly decreases to the temperature of fracturing fluid after 20 minutes of pumping fluid.The decrease of casing temperature tends to be stable after 20 minutes of pumping fluid.The heel segment of horizontal casing has the largest temperature change and the largest thermal stress during the fracturing process,and its collapse strength reduction is the biggest along the horizontal section,so it is easy to be sheared.The low strength bedding planes leads to the anisotropy of shale rock strength.It is found that the axial compressive strength is the lowest when the core angle of the shale is 30 degrees.The statistical damage constitutive model based on Weibull distribution can well simulate the deformation of real core under external force.And the damage factor of underground compressed formation increases linearly with strain;non-uniform in-situ stress,shale formation damage,wellbore temperature change and pump pressure have great influence on the effective stress of casing.When fracturing fluid pumped into the formation,huge thermal stress caused by the temperature change are gradually accumulated at the heel due to the barrier of ultra-low permeability shale.In the process of multi-stage fracturing,stress concentration appears in the heel segment of horizontal casing,and the closer to the heel the more serious stress concentration is.Therefore the heel segment of horizontal casing is more easily deformed.Increasing the casing strength of the horizontal section near the heel can effectively reduce the probability of casing failure during fracturing.The results of this paper provide a theoretical basis for casing failure near the heel segment and guidance for maintaining wellbore integrity during horizontal shale gas wells fracturing.