Wellbore characteristics of air drilling with hammer in thick conglomerate and its influence on hole deviation

Abstract

Air drilling with hammer is generally recognized as an effective drilling method, but the hole obtained in thick conglomerate formation is very irregular, and the control of hole deviation is very difficult. Although many studies have been carried out, the mechanism of this phenomenon has not been found out yet. In this paper, based on the numerical simulation of rock breaking process of air hammer, the influence of the heterogeneity of conglomerate on the wellbore regularity and hole deviation is analyzed. The results show that the direction of the bit will be changed due to the heterogeneous distribution of gravel particles, which make the wellbore with a severely irregular shape. On this basis, the finite element models of BHA in regular and irregular wellbores are established respectively, and the influence of wellbore irregularity on the stress of BHA is analyzed. It can be found the irregular wellbore is easy to form additional fulcrum, shorten the effective swing distance of pendulum bottom hole assembly. The additional fulcrum greatly changes the mechanical characteristics of the BHA, reducing the angle dropping force on bit, or even produce the angle building force.