The Research Of Optimization And Safety Evaluation On Drilling Tools In Ultra Deep Wells

Deep and ultra deep well drilling has gradually become the main means of oil and gas fields to increase reserves and production at home and abroad.This paper analyzes the cause of drilling tool failure in ultra deep well drilling,and provides theoretical guidance for drilling safety in ultra deep well drilling.Drilling tool failure is the most common drilling accident.Every drill stem fracture accident will cause huge economic losses.The drilling tools in ultra deep well are subjected to heavy load,complex downhole environment and difficult handling of accidents,which become the key factors restricting the development of ultra deep wells.The Shunbei oilfield is located in the territory of Shaya County,Xinjiang.The depth of the reservoir is about 7500m.There are currently over a dozen wells deployed in the block that are all ultra deep wells.Several ultra deep wells that were developed in the previous period have caused a lot of time and economic losses due to frequent drilling tool damage.This article starts with the failure mechanism of drilling tools and mainly introduces three types of failure modes:drill string excessive deformation,drill string surface damage and drill string fracture.The main well structure and drilling tool combinations used in ultra deep wells in the Shunbei Oilfield were introduced,and failures of the drilling tools in the block were counted.There were 51 drill pipe failures and 30 drill hole failures.Drilling tools fell 3 times from fish accidents,two of which occurred in the upper drill pipe,and 1 occurred in the connection between the centralizer and the drill collar.The main forms of drill rod failure are:damage of male and female thread,damage of thread step surfaces,damage to the shoulders of drill pipe joints and puncture,and the failure site is mainly the upper drill pipe.The main form of drill collar failure are:damage at the threaded joints,bending of the body,and wear of the body.The failure location is mainly near the drill collar and drill pipe links.By analyzing the failure,it is concluded that the main reason for the failure of drilling tools in ultra deep wells is the fatigue breakdown of drilling tools.Under the effect of high temperature and corrosive environment,fatigue failure will be more serious.In order to study the causes of fatigue failure of drilling tools in ultra deep wells,this paper first analyzes the factors that affect the fatigue strength of ultra deep well drilling tools.The reverse movement of the drill string causes the drill to produce lateral vibration,which is a major factor in generating the cyclic load.The natural frequency of the transverse vibration of the drill has a certain influence on the bending stress of the drill,and will be affected by the pressure difference and temperature of the drill string.The static load of drilling tools will have a greater impact on fatigue load,and the fatigue limit of drill string will be greatly reduced when the average stress is higher.Drilling tools are more rapidly fatigued under corrosive conditions,especially in formations containing hydrogen sulfide,which are prone to sulphide stress corrosion cracking and accelerates the fatigue failure of drilling tools.The high temperature environment of ultra deep wells will lead to a faster corrosion rate of the drilling tools and at the same time reduce the ultimate strength of the drilling tools.Finally,the size,structure,surface quality and material composition of the drilling tool were analyzed,and the influence on the fatigue strength of the drilling tool was obtained.The strength checking is the most important part of the drilling design.This paper first checked the static load strength of the second drilling tool set in well Shunbei 1-2h,and the strength of the drilling tool complied with the design standards.Then the fatigue strengths of the upper and lower drill collars were separately checked.The fatigue safety factor of the upper drill pipe was 1.29,and the fatigue safety factor of the lower drill collar was 1.09.The results were all less than the allowable safety values,indicating the possibility of the fatigue failure.In order to explain the influence of various factors on the fatigue strength,the control variables are analyzed for the factors such as depth,rotation speed,stress concentration factor,fatigue limit and natural frequency respectively.It is concluded that the lower fatigue strength of the upper drill rod is mainly due to the high axial stress of the drill string,while the lower drill collar fatigue strength is mainly due to the greater bending stress of the drill string.In order to solve the service life of the drill string under a certain stress state,the article first adopted the name of the legislation to predict the life of the upper drill pipe of the well 1-2h in Shunbei.According to the S-N curve and the stress condition of the drill string,the fatigue life of the drill rod is estimated to be about 4.145×10~7cycles.Then,the fatigue life of the drill string is solved by the fatigue crack propagation method.Using the Forman model,the fatigue life of the drilling tool was calculated to be 8.98×10~6cycles.Finally,the finite element software ANSYS workbench and Design Life module in NCODE software were used to predict the life of the drill rod.The fatigue life of the drill was 6.33×10~5cycles.The control variables of the fatigue crack propagation method and the finite element method are respectively calculated,and the fatigue life of the drill rod under different stress amplitudes and average stress is obtained,and combined with ultra deep well,the factors are analyzed.At the end of this paper,the optimization of size structure,optimization of material properties,anti-corrosion methods and drilling parameters were carried out respectively.The optimization of drilling tool combinations was carried out for ultra deep wells in the Shunbei block,which provided a reference for the design of ultra deep wells.