Steering Performance Analysis on BHA with Controllable Eccentric Telescopic Blocks in Coiled Tubing Drilling

In order to meet the requirement of steering control during drilling horizontal slim hole by coiled-tubing, a new method was proposed to control the wellbore trajectory by using controllable eccentric telescopic blocks. The bottom hole assembly (BHA) was equipped with independently controllable eccentric telescopic blocks that can be controlled for extension and retraction to generate displacement of BHA against the borehole axis, so as to change the mechanical properties of BHA. Based on the proposed BHA, a mechanical model and control equation were established to analyze steering performance of the coiled-tubing BHA with eccentric telescopic blocks by means of beam-column theory, and the solution for the specific cases was given by using the control equation. The calculation results showed that the bit drift force, azimuth force and its rotation angle within the inclination plane and azimuth plane are linearly related to the eccentric displacement of eccentric stabilizers at different WOBs. With the increase of the inner diameter of drill collars, the drift force and the azimuth force of bits decreased in cubic polynomial form, but the angle of bit rotation changed very slightly. It is shown that the mechanical properties of BHA can be changed by controlling the extension or retraction of the eccentric blocks by which the eccentric displacement of BHA produced against the borehole axis, so as to control the well trajectory.