Analysis of wellbore instability in vertical,directional,and horizontal wells using field data

摘要

An old offshore field produced using vertical and directional wells is being redeveloped by drilling horizontal wells.The experience gained while drilling vertical and directional wells is not useful for drilling horizontal wells,as the failure rate is 1 in 3 holes.Quantification of drilling problems in sixty wells show that majority are tight holes.Stuck pipes and hole pack offs are also significant in number.The major loss of productivity is due to stuck pipes.A preliminary study of shale in sections where problems occur,show no chemical reactivity.Petrographic analysis confirmed the fissile and brittle nature of shale with presence of open,partially healed microfractures and partings.Rock mechanical simulation predicted the safe mud weight window for horizontal wells as 76-90 PCF,depending on azimuth.However,all the horizontal wells analyzed in this study were drilled using the same mud weight window.Therefore,field based parameters like initial mud weight used for drilling,mud weight increment and problems per well were used to analyze wellbore instability,identify different instability mechanisms and design safe mud weight window for drilling horizontal wells.These parameters were used first on the drilling data of vertical wells to develop the procedure for the analysis of wellbore instability and identify the mechanisms of instability.The developed procedure was then applied to the drilling data of directional wells to show the dependence of mud weight on the inclination and azimuth of the well.Finally,the procedure was applied to horizontal wells data along with the concept of critical washouts to infer the safe mud weight window as 77-80 PCF in East-West and 82-85 PCF in North-South directions.The safe mud weight window is validated on another set of drilling data showing 90% success rate.The analysis confirms the existence of anisotropy in horizontal stresses and is extremely useful in cases where there is significant variation in mechanical properties of different layers of reservoir rock.