DRILLSTRING INSTABILITY PHENOMENA STUDIED BY SUPERIOR ANALYSIS TECHNIQUES, RESONANCE MODELLING

Drillstring instability phenomena leads to damage all parts of the drillstring, wellbore instability and reducing the rate of penetration (ROP). The bottom hole assembly (BHA) configuration is a main factor in optimizing the drilling operations. Therefore, should be designed to minimize the vibration levels in the lateral, axial and torsional directions. This can be done, by avoiding the rotation of the drillstring at the natural frequency which called the resonance. In this thesis the vibration of drillstring was studied under the impact of weight on bit and rotation drillstring. Thus, the lateral vibration has been chosen as the most important and centered element, because it is increased dramatically with the variation of the rotary drilling speed. The design of current bottom hole assembly (BHAs) components need utilizing of sophisticated analytical methods that can solve the complex and time-consuming equations. The Finite Element Analysis (FEA) is the most common way that used to evaluate the behavior of the drillstring vibration by means of mesh discretization of a continuous body into small components. Two softwares were employed for a superior analysis techniques ANSYS and LANDMARK.ANSYS software has been used to investigate the lateral vibration of drillstring in a vertical well, and to determine the critical speeds of the drillstring that should be avoided. consequently, the resonance can be prevented and be away from severe downhole vibration which lead to drillstring damage. The Simulation was first carried out by benchmark model before proceeding to deal with the actual case studies by implementing the parametric study (drill string length, weight on bit, range of frequencies). The analysis by ANSYS was applied in two stages. First stage of modal analysis was performed to determine the natural frequencies of the drillstrings for three sections of well ZB-202 (17 ½‶, 12 ¼‶ and 8 ½‶) .The second stage of harmonic analysis was executed to obtain the frequency response at a varying bottom hole assemblies (BHAs) for well ZB-202.The critical rotary speeds that should be avoided were obtained from the aforementioned analysis can be listed as following, for the drillstring section 17 ½‶ resonance occurs at frequency 3.9 Hz and rotary speed of 234 revolution per minute (RPM), for the drillstring section 12 ¼‶ the resonance occurs at frequency of 5.08 Hz and rotary speed 304.8 RPM and for the drillstring section 8 ½‶ the resonance occurs at frequency of 2.58 Hz while rotary speed was 171 RPM .The study focused on 12 ¼‶ hole section of well ZB-202 with the entire details because it is the longest and most problematic section such as wellbore instability and lost mud circulation The second software was LANDMARK used to determine the torque, effective tension and weight on bit (WOB) which directly influence the lateral vibration.