Abstract: In China, to achieve deflection control while fast drilling with conventional drilling assembly, easing the bit in light pressure and high WOB pre-bending passive anti-deviation technique are often combined to realize hole deflection control in complex formations. However, low weight on bit (WOB) may lead to low rate of penetration (ROP), and high WOB pre-bending passive anti-deviation technique may make the drill string become instable and thereby cause underground complexities. So, for purpose of balancing the relationship among deflection control while fast drilling, ROP and drill string instability, it is necessary to select an optimal WOB. In this study, based on the vertical and horizontal bending beam and column bending theory, a WOB optimization model for deflection controlling and straight drilling with conventional drilling assembly in vertical well was built, which considers the influences of formation deflecting force on well deflection and the minimum comprehensive bit force. Relevant computer program was prepared. Moreover, field test was made in Well BUCS-44H in Missan Oilfield, Iraq. The results show that different range of WOB is applicable to formations with different deflections. In high and steep structures, drill tools with high angle drop capacity are used, and under the condition of pre-bent drill string, high WOB can be adopted to address the contradiction between ROP and deviation controlling and straight drilling. In addition, an optimal WOB is selected by fully considering the deflecting features in formations and those of drill tool itself. In this way, the instability of drill string can be avoided, and deflection control while fast drilling can also be achieved. This study has a certain guiding significance on deflection control on site.

Key words: deflection control while fast drilling, critical WOB, conventional drilling assembly, deflection control, vertical and horizontal bending beam method