Research On The Stability Of Horizontal Well Lining In Shale Formation

Wellbore instability is a serious problem in shale formation and also one of main restrictions for oil and gas development.The reason of collapsing in shale formation is its strong hydration ability and high anisotropy.Due to its high anisotropy,shale strength is affected by weak plane,increasing difficulty for well track design.Strong hydration makes its physical property and mechanical property sensitive to drilling fluid,which makes drilling and logging process more difficult.Therefore,take shale formation in one oilfield as object,wellbore stability of shale formation was analyzed based on experiment,numerical calculation and logging data.The substance of this paper is as followed:(1)Based on experiments,shale structure,mineral composition,CEC value and specific surface were tested.Those experiments show clay mineral is abundant in shale and so many micro-cracks were observed.Besides,value of specific surface and CEC were high.(2)After the analysis of shale structure and physical-chemical property,shale physical property and mechanical property before and after soaking drilling fluid were analyzed based on experiments,which can get the clear influence of drilling fluid on shale.This influence is clear in 4 days,then becomes stable.(3)Based on experiment results,logging data was rectified to remove interference from hydration.After logging revision,mechanical parameters were obtained based on logging data.This revision indicates that hydration has little effect on shale density,but has strong impact on acoustic time and resistivity.After revision,in term of experiments,mechanical parameters were calculated.Meanwhile,Eaton method and combined spring model were applied to predict pore pressure and in situ stress.Eventually,the profile of pore pressure,in situ stress and mechanical parameters were established,which offer reference for collapsing pressure prediction and wellbore stability analysis.(4)Based on in situ stress,pore pressure and mechanical parameter,linear theory and coordinate transformation were applied to get the stress state around wellbore.According to stress state,Hoek-Brown criterion was conducted to predict collapsing pressure in shale formation.In addition,according to oilfield situation,we focused on the influence of drilling time,well track and weak plan on wellbore stability.The result indicates that collapsing pressure grows in area with abundant weak plane.With the increasing of drilling time,collapsing pressure increases dramatically in 4 days,then becomes stable.In certain depth,the well along with horizontal minimum principal stress is most stable.From above researches,the comprehensive analysis was conducted in the paper.The method of predicting pore pressure,in situ stress and collapsing pressure is practical in shale formation,which can be reference for well track and drilling fluid design in oilfield.