Study On Operation Limit Of Downhole Tubular String In Horizontal Well Engineering For The Fuling Shale Gas Field

The engineering mode of large cluster horizontal well,which requires multiple horizontal wells to be drilled on one platform and expanding the length of horizontal section in the reservoir as long as possible,is employed in the development of the Fuling shale gas field,and it has achieved remarkable benefits.In order to further promote the development of the Fuling shale gas field,the length of horizontal section of horizontal well is planned to be increased from 3000 m to 5000 m.However,with the further expanding of the horizontal section,the problems of string buckling,high friction and torque,frequent back pressure and difficult borehole cleaning are prominent,which seriously restrict the horizontal drilling efficiency and horizontal drilling limit.In view of the above problems,this paper carried out the study on the prediction of forces and operation limit of downhole tubular string.Firstly,an amended torque & drag model was built considering the effect of curvature discontinuity of well trajectory and stiffness discontinuity of tubular string.Secondly,a real-time inversion method of friction factors was presented by introducing the weight coefficient related to the drilling process.Thirdly,the prediction model of operation limit of tubular string and the real-time evaluation method of operation risk were established accounting for the various constraints on the surface and underground,which realized the evaluation of operation risk and the identification of the main constraint of short-term,medium-term and longterm.Finally,a platform for forces analysis and prediction of operation limit of tubular string was developed with the calculation accuracy of more than 90%,which was applied to the drilling process of horizontal shale gas well in Fuling.The results show that the friction factor of open hole is significantly higher than that of cased hole and fluctuates greatly during drilling,which is the main constraint of operation limit.In order to achieve5000 m horizontal section,the friction factor of open hole shall not be higher than 0.31,and advanced technologies such as rotary steering system and rotary floating liner running should be adopted.This study can provide quantitative calculation guidance and basis for optimization design and safe and efficient drilling of shale gas horizontal wells with long horizontal section.