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 is connectable to or includes a drill bit; a releasable well string interconnection means for selectively interconnecting the lower and upper well string parts above the lower end of the lower well string part; and an auxiliary tool arranged such that it can be passed along the passageway and comprising 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, and wherein the auxiliary tool is arranged such that it 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 working position in which at least part of the auxiliary tool is not radia