Study On Drilling Conductor Mechanical Behavior Of Shallow-water Exploration Well

China’s marine shallow-water exploration wells generally adopt jack-up drilling rigs with a water depth span of 20-120 meters.The length of the free section can be more than 100 meters from the wellhead to the mudline.Different from the jacket production platform,it is very difficult to control the stability of the conductor during drilling process.With the deepening of marine exploration and development,a large number of small and marginal oil and gas reservoir have been discovered with 880 million tons in Bohai Sea.In order to evaluate these oil and gas reservoirs more accurately,exploration well prolonged tests have become a very effective tool.The exploratory well prolonged test period is usually several months or even more than one year.The operation period will be affected by the extreme sea conditions,and may cross the ice period in the Bohai Sea,while in the South China Sea and the East China Sea,may encounter severe typhoons.The general conventional exploratory well design will not consider these extreme conditions.Once a conventional exploratory well requires extended testing operations,it will bring even more severe challenges to the stability of the wellhead and conductor.In order to reduce the operation risk,it is necessary to carry out theoretical research on the mechanical behavior of shallow-water drilling wells and to provide a reliable basis for the design and operation safety of shallow-water exploration wells.For the interaction characteristics of jack-up drilling rigs,wellheads,conductors,and submarine soil foundations,based on the theory of simply supported beams,the deflection and deformation equations and traps for wellbore tubulars above and below the mud line for exploration interceptors were established.Based on the boundary conditions of the actual work environment and the work load,the above model was solved using the finite difference method,and the influence rule of the structure,load and subsea soil parameters on the anchoring end of the conductor was obtained.The design of the conductor running depth provides a theoretical basis.Prolonged test operation requires high requirements on the strength and stability of the wellhead.It is required that the wellhead maintain sufficient residual strength after the drilling operation to cope with the subsequent test operations.Therefore,the damage to the wellhead should be reduced or avoided as much as possible during drilling operations.Taking the constraints of the ultimate bearing capacity of the formation,the strength of the conductor and the ring plate,the stability of the conductor,and the bottom formation fracture pressure as the constraints,a calculation model for the casing running speed and the siting speed was established.Considering the differences in strength of the two adhesive surfaces of the conductor & cement ring,cement ring & soil layer,based on the effect of the height of the cement ring and the size of the conductor pipe on the bearing capacity of the conductor,a conductor bearing capacity was established.The calculation model of depth into the mud provides a scientific basis for wellbore structure design.Based on the characteristics of shallow-water exploration wells prolonged test conditions,a mechanical analysis method for the wellhead stability of the prolonged test wells for offshore exploration wells was proposed.The mechanical behavior of the conductor was developed from the environmental load,the nature of the subsea soil,the wellbore structure,and the position of the casing suspension point.Sensitivity analysis revealed the effect of various factors on wellhead stability.For the condition that severe ice loads may be encountered during the test,the stress of the conductor in the glacial and non-glacial conditions was analyzed and compared.The force models under the glacial and non-glacial conditions were established respectively.Based on the conductor under ice load,the strength of the conductor was checked against the various factors such as the size of the conductor,the height of the cement return,the return period of the wind flow,and the ice return period.According to the distribution characteristics of the ice age in the Bohai Sea,the extended safe operation window was obtained.This paper establishes a set of design and operation method of shallow-water exploration wells.The research have achieved good results and can be a guide for exploration wells.