| Gas drilling has been widely utilized and become one of the important means to explore the oil-gas field effectively in terms of its advantages on shortening the drilling period, lowering the damage to reservoir and solving the problem of lost circulation. However, a key issue in gas drilling is the extremely serious failure of drillstring that will directly affect the availability of the drilling operation, associated with the enormous economic losses. This problem is mainly caused by the vibration of drillstring and the erosion with high-speed cuttings. In fact, owing to the weak effects of the gas density and frictional resistance during gas drilling, the drillstring vibrates intensely at the longitude in the wellbore with the complex lateral vibration, the torsion vibration and the whirling, leading to the complicated dynamic stress on the drilling pipe. In addition, due to the comprehensive vibration forms, the location of centroid of drillstring in wellbore changes at random, which makes the annulus random changing space. Considering that the gas velocity becomes greater and the flow is very close to the turbulence, the gas possesses the complex turbulent quantities with changing space, which is more complex than the turbulent problem with fixed boundary. As a result of the erosion on drillstring with the cuttings carried by the high speed gas, the intensity of drillstring is reduced and the fatigue failure is intensified. In this thesis, the flow characteristics and the turbulent behaviors for the annular gas were studied, and the erosion of cuttings on the drilling pipe and the cuttings removal pipe with different shape were analyzed. Then the forming and evaluation of the vortex around the drilling pipe with the vortex generator were explored, and the investigation of its effect on the flow in horizontal hole section was taken.Firstly, based on the control equations of the computational fluid dynamics, the simplified motion equations of the annular gas flow were derived for gas drilling. The turbulent model involved the wall functions was proposed which is suitable to the annular gas flow during gas drilling, and the numerical method was addressed to solve the flow equations, with introducing the procedure of the numerical simulation.Secondly, the flow behaviors in the vertical and horizontal hole section, such as the variation of the gas velocity, were investigated through consideration of the drilling pipe in the center, with eccentric non-rotation or rotation, so as to explore the turbulent characteristics of annular gas, for example, the turbulent energy, wall friction factor and the wall pressure coefficient etc. Combined with the motion equations of the cuttings and the erosion model of the drilling pipe, the trajectory of the cutting and the erosion rate of the cutting to drilling pipe were both simulated, with the consequence of achieving its erosion laws. Meanwhile, the effects of the eccentricity, the penetration rate, the grain grade and the cuttings shapes on the erosion rate peak were discussed. Moreover, the flow behaviors, the cutting motion trajectory were analysed qualitatively, and the erosion in the cutting removal elbow with the bend of 90°and the tees were studied.Finally, for the sake of the cuttings bed probably appeared in the lateral segments, we first proposed the modification of the drilling pipe with the vortex generator, to control the flow field around the drilling pipe. The numerical simulation was made to analyze the formation and evaluation of the vortex around the vortex generator for the improved drilling pipe with the central and the eccentric rotation case, and the effect of the vortex generators on the behaviors of the annular gas flow was studied. The numerical results show that the vortex generator can not only increase the tangential velocity of the annular gas but also move the cuttings ,accumulated at the low side of the well hole, into the vortex near the center of the annulus, then the cuttings bed could be cleaned effectively.
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