Abstract: Automatic control of borehole trajectory is one of the most frontier research areas in drilling engi-neering. This paper presents the results of a study on
the realization of automatic trajectory control with controllable eccentric to improve the precision of con-trol and reduce drilling cost. The controllable eccen-tric is a kind of downhole guided drilling tool consist-ing of a central axle, measuring and controlling sys-tem units, eccentric blades and eccentric shell,which can be controlled within the wellbore and can be used with rotary drilling. Automatic trajectory control is realized during drilling by way of adjusting the force of eccentric blades against wellwall and the
length of the blades at all time. When controllable eccentric is used in drilling practice, trajectory con-trol can be performed without tripping frequently.
This feature of the method has great significance and prospects in accomplishing the goal of highly-dense cluster drilling, all kinds of directional drilling, hori-zontal drilling and emergency drilling.

Key words: Borehple trajectory, Control system, Method, Controllable eccentric