The Study Of Borehole Collapse Time For Longmaxi Shale In Sichuan

In view of the collapsibility of shale wellbore in Longmaxi formation in Sichuan Province,the paper investigates the dual-porosity and dual-permeability elastic model and dual-porosity and dual-permeability chemical elastic model of shale formation in abroad,and the physical and chemical properties of shale formation in Longmaxi formation.Based on the research results of the collapse pressure and collapse time,the TTD-WSS software was used to study the poroelastic properties(Biot coefficient,mud viscosity,matrix permeability of the formation,undrained Poisson's ratio)that affect the collapse time,the formation and mud chemistry properties(mud activity,formation fluid activity,membrane efficiency,formation temperature),and fractured formation properties(fracture compressibility,fracture permeability,fracture volume fraction,overall compressibility of fractured formation,inter-porosity flow coefficient)for quantitative analysis.Based on the field drilling situation of YS108H19-1 horizontal well in Sichuan,it was concluded that when using water-based mud,the collapse time of the shale wellbore of Longmaxi formation is between 10-20 days,and the drilling period is generally about 15 days.Collapse may occur during the period;when oil-based mud are used,the collapse time of the shale wellbore of Longmaxi formation will be more than 30 days,and it may reach 60 days.It will not collapse during drilling period.Therefore,it is recommended to use oil-based mud.