| Deepwater casing drilling,which uses casing instead of conventional drill pipe to apply torque and weight on drill bit during deepwater drilling process,is an innovative drilling technology that can be used to drill and case the well simultaneously.Deepwater casing drilling has a great advantage over conventional drilling method.On one hand,it can be used to reduce the non-product time(NPT)related to trip the drill pipe and run the casing,thus improve the drilling efficiency;on the other hand,the“smearing effect” of casing drilling can help to improve the stability of the wellbore.However,to take full advantage of casing drilling technology in deepwater well construction,a lot of problems need to be settled.Since the drilling assembly is directly exposed to marine environmental load due to the lack of protection from the drilling riser,the dynamic response of the whole assembly should be analyzed to guarantee its safety & reliability.Besides,regarding the “smearing effect” of casing drilling,its strengthening effect on the wellbore should be comprehensively quantified.Based on the study stated above,the safe mud weight window of casing drilling should be derived to provide a theoretical guidance for the safe operation of deepwater drilling.A mathematical model to predict the axial dynamic behavior of the landing string assembly during the landing operation of conductor is established.The effect of guide base plate and bottom hole assembly is incorporated by introducing the Dirac function and step function.The effect of fluid frictional force acting on the outer wall of the pipe element and the fluid drag force on the accessory structures are considered.By iterative calculation,the axial dynamic behavior of the landing string assembly in frequency domain is obtained.According to the collapse resistance of landing string,the maximum allowable heave amplitude of drillship under different heave frequency is evaluated.The study shows that,the hydrodynamic drag force and surface frictional force can suppress the longitudinal vibration of the landing string assembly only in near resonant conditions;when the drillship’s heave frequency is lower than the first longitudinal resonant frequency of the assembly,the maximum allowable heave amplitude of drillship is large,and it decrease sharply with the increase of the heave frequency;neglecting the bottom hole assembly will result in large error in predicting the axial dynamic behavior of the landing string assembly.By considering the geometric nonlinear effect,the Finite Element models to investigate the mechanical behavior of pipe assembly during the jetting operation of deepwater structural casing are established.For different boundary condition in different operational stage,the deformation and stress state of the pipe assembly under the of marine environmental load is analyzed.Corresponding sensitivity analysis is carried out to investigate the key environmental factors that affect the stress state of the pipe assembly.The study shows that,during the jetting landing operation,the stress on landing string is much larger compared with that on conductor;the mean value of tensile stress and effective stress decrease from top to bottom along the landing string,while the extreme value has a little change;the maximum value of both the mean and extreme values of bending stress appear at the bottom of landing string,and the extreme value is significantly larger than the mean value.Establishing a 3D axial-transverse coupled model to investigate the nonlinear dynamic behavior of drill pipe assembly during the drilling operation of deepwater surface hole section.The deformation and stress state of the pipe assembly under the combined influence of weight on bit perturbation,wave & current force and force exerted by the internal flowing fluid are analyzed.The study shows that: during the casing drilling operation in deepwater surface section,the effective stress,tensile stress and bending stress on the drillstring that exposed to marine environmental load fluctuate substantially;the effective stress and tensile stress on the landing string are larger and the bending stress on the casing is larger;the maximum values of the effective stress and tensile stress appeared at the bottom of the landing string and the maximum values of the bending stress appeared at the bottom of the casing.For casing drilling technique,the strengthening effect of “smearing effect” on the wellbore are comprehensively quantified and a model for predicting its corresponding safe mud weight window is established.The effect of temperature,external mud cake efficiency and internal mud cake efficiency on the safe mud weight window is analyzed.The study shows that: The temperature increases of the wellbore wall caused by the friction between the drillstring and the wellbore has a certain strengthening effect,but the effect is not obvious;the dense mud cake formed on the wellbore wall can make the collapse pressure of the wellbore lower and the fracture pressure higher,and also make the re-initiation pressure of the microfractures on the wellbore wall lower.The reduced contamination of the rock pore space around the wellbore due to the "smearing effect" also helps to enhance the strengthening effect of the casing drilling plugging effect. |
