Study Of Wellbore Stability And Fracture Characteristics Of Bashijiqike Reservoir In Keshen 5 Gas Field,Tarim Basin

The deep Cretaceous fractured sandstone reservoir contains rich natural gas resources in Keshen 5 gas field of Tarim Basin.But due to the complex structure,deep buried reservoir,high pore pressure,natural factors of fracture development,leads to severe borehole instability at progressive exploration and development stage.The complex problem of high accident rate,long drilling cycle,seriously affected the process of exploration and development in this area.On the basis of understanding the basic geological characteristics of the work area,combined with the field wellbore instability characteristics,the shape characteristics of debris,the depth characteristics of borehole failure,and the correlation between well leakage and fracture development are analyzed.The stability mechanism of the reservoir wellbore was determined by the natural cracks,which were damaged in the drilling process.Under the conditions of high density drilling fluid,the formation lossed and invaded by drilling fluid,which caused the increase of fracture formation pore pressure.As the effective stress reduced,the borehole collapse pressure improved,resulting in borehole instability block.For the fractured sandstone borehole instability mechanism,the thesis research on the massive rock with weak wellbore stability influence of literature.The problem of well wall instability with drilling fluid intrusion not only has the strength anisotropy,but also has time dependence.The method aims to establish the fluid flow of pores and cracks in the fluid solid coupling model whereas the operability of the model is bad in practice.According to the new understanding of the wellbore instability mechanism,the paper proposed to determine the fracture formation loss pressure,avoid formation loss,which is effectively method to solve this type of fracture formation wellbore stability.The study analysisd the rock mechanics characteristics,reservoir fractured characteristics,optimized the rock failure criterion and stress calculation model.A method considering the crack strength anisotropy of collapse pressure and loss pressure calculation was established to provide a reasonable safe mud density window for drilling engineering.Based on the deep understanding of wellbore instability mechanism in Keshen 5 gas field,putting forward to reduce the density of drilling fluid,reducing formation loss,avoiding the natural fracture again damage,and considering the influence of strata collapse pressure and fracture pressure loss calculation method was established.The field practice shows that,after reducing the density of drilling fluid to a reasonable range,the wall instability of the gas field is obviously reduced,and the complex problems in the field can be effectively solved.