| The Bottom Hole Assembly (BHA) is an assembly of drill-bit, drill collars, and stabilizers at the bottom portion of a drillstring. It is well established that the direction of drilling is largely determined by the deformation of BHA in the borehole. In this dissertation, a three-dimensional static equilibrium analysis of BHA was carried out to determine the force vector acting on the bit while drilling. Experimental observation suggested that the action of drilling was to reduce the force acting on bit. Based on this observation, a bit-rock interaction model was developed for calculating the three-dimensional wellpath. A computer program for on-site analysis of BHAs and wellpath prediction was developed in this study. The calculated wellpath was compared with the well survey data, which were kindly made available to us by industries, and an excellent agreement was observed.;The whirling motion of drill shaft was also studied. Whirling motion may be regarded as lateral vibration of the drill shaft; the motion which is known to cause fatigue failure of BHA, change of the borehole shape, and introduce uncertainty in drilling direction, should be avoided while drilling. In this dissertation, the effect of drilling fluid and internal damping of drill collar material on the whirling motion of drill shaft were studied by modeling the drill shaft as a simple spring-mass system. The results from simulation study have demonstrated that the fluid motion aids backward whirling of the shaft, while the internal damping in the drill collar aids forward whirling. Furthermore, the amplitude of whirling decreases with increasing density and viscosity of the drilling fluid. These conclusions were found to be consistent with field experience.;The BHA analysis and wellpath prediction presented in this study can be used in conjunction with any rock formation which has a structural integrity. Despite uncertainties in rock properties, the computer program developed in this study is expected to give a reasonable estimate of BHA forces and wellpath. |
