Over-core recovery of a drill string bogged in a longhole

Underground in-seam directional drilling regularly encounters zones of unstable strata. In some cases, borehole collapse leads to the bogging of the drill string. With the high cost of in-hole equipment at risk and the likely loss if not recovered at the time, over-coring has become an established form of recovery. Due to in-hole friction of the over-core rods with the borehole and the bogged rods exacerbated by the deviations within a directionally drilled borehole, the depths from which over-core recovery has been successful has been limited. This paper explains the prior installations, precautions, considerations and procedures employed for a successful over-core recovery of a 717 m long drill string bogged in an in-seam borehole.