Abstract: A well string assembly includes an upper tubular well string part having upper and lower ends between which there is formed a passageway, a lower well string part having upper and lower ends, which lower end is spaced apart from the lower end of the upper well string part and may include a drill bit. An auxiliary tool is arranged such that it can be passed along the passageway. It comprises an operating means for manipulating the well string interconnection means so as to disconnect the lower well string part from the upper well string part. The auxiliary tool can be passed, after disconnecting the upper and lower well string parts, through an opening at the lower end of the upper well string part, so as to reach a position in which at least part of the auxiliary tool is not radially surrounded by a part of the well string.