Research On Bullheading And Controlled Release Of Overflow Fluid In The Wellbore

Safety and risk issues in oil and gas production processes should be a top priority for blowout and well control.With the continuous extension and expansion of worldwide oil and gas development,especially the gas reservoirs with high hydrogen sulfide,abnormal high temperature and pressure,high permeability and high production,the difficulty of well control has been greatly increased.The requirements for well control are also gradually improved globally.Under some special field conditions such as blowout accident,it is considered that the fluid in the wellbore invasioned from formation can be pushed back to the formation completely.However,the research on pushing the wellbore fluid back to the formation is limited at present.In view of the limitations of some unknown formation factors and high risk wellbore conditions,the failure of operation or calculation may cause further accidents such as blowout out of control.Therefore,the study of well control methods for different wellbore conditions and different geological conditions is of great significance to reduce the accidents.In order to solve the well control problems of high-risk wells,firstly,this study takes the multiphase flow as the research foundation,and established the model of gas migration in non-Newtonian fluids to improve the accuracy of calculation.Secondly,with considering the characteristics of mud migration in formation and different kind of fluid distributions in the wellbore,the bullheading method for killing well is studied.The killing effect of the bullheading method under different geological features was evaluated,correspondingly.Thirdly,for the high-risks drilling conditions,a new method of pressing a part of the fluid back to the formation and another part to discharge the wellbore was studied to prevent further accidents.Main conclusions of this study are as follows:(1)The model of gas slip in non-Newtonian fluid in vertical pipe with considering the yield stress effect was established.The results for the field data(the data was from the well with the problem of sustained casing pressure)validation indicates that the new model of gas slip in non-Newtonian fluids can match the field data in perfectly.Furthermore,the cement permeability predicted by the new model in the well with sustained casing pressure problem is higher than previous model(model of gas migration in Newtonian fluids).In addition,the new model considered the effect of yield stress indicates that the higher the yield stress,the lower the magnitude of wellhead pressure rise.This model can be applied to the other calculation of gas-liquid two-phase migration.(2)The coupling model of wellbore and formation of bullheading method is established.According to the distribution of gas-liquid two phase fluid in the wellbore,the models of bullheading with less gas,ideal situation for large amount of gas,semiideal situation for large amount of gas,large amount of gas with considering the gas-liquid replacement were established,respectively.Also,combining the law of gas-liquid migration in high permeability,medium permeability and low permeability,the variation of wellbore parameters during the killing process of different reservoir permeability,thickness,porosity,gas saturation,and open-hole mud cake and mud displacement was obtained.The results show that at the situation of low permeability formation and with large amount of mud in the lower part of the wellbore,it can easily lead to further accidents.Therefore,it is not recommended to use the bullheading method to kill the well at this time.(3)A new method of pressing a part of the fluid back to the formation and another part to discharge the wellbore was studied.The bullheading method based on mud positive and reverse circulation is established by using the gas migration law in mud circulating process.The variation law of wellbore parameters in the killing process was analyzed,and the ability of further gas invation to the wellbore was evaluated.The results show that the new method can kill the wells faster than the reverse circulation method.In addition,in the process of positive circulation and reverse circulation,the killing mud can effectively reduce the permeability of the reservoir and block the reservoir.In this paper,the bullheading method is carried out,and the killing model under high-risk well conditions is established.It is suitable for guiding the on-site work and providing theoretical support for reducing the occurrence of drilling accidents,thus enabling the safe and efficient development of oil and gas.