Tool for maintaining wellbore penetration

A tool connected into a non-rotating wellbore drill string, for use in a subterranean wellbore extending from the surface to an end beneath the surface, which maintains an amount of penetration of a drilling bit operatively connected to a mud motor connected into the non-rotating drill string when an axial compressive load applied through the tool to the drilling bit by the non-rotating drill string is decreased, the wellbore containing wellbore fluids which exert a hydrostatic pressure on the tool, the tool comprising:

• (a) a tubular outer member having an inner surface and an outer surface;
• (b) a tubular inner member, having an inner surface and an outer surface, telescopically received in the outer member in a spaced relationship therewith such that the outer member and the inner member form a chamber therebetween having an upper portion and a lower portion, the outer member and the inner member being movable longitudinally relative to each other for an amount of relative longitudinal movement to permit telescoping of the outer member and the inner member between a fully contracted closed position and a fully extended open position;
• (c) means for transferring the axial compressive load from the outer member to the inner member including a first surface located on the outer member which contacts a second surface located on the inner member when the outer member and the inner member are in the fully closed position, wherein a first distance, designated as D.sub.1, is defined by the distance between the first surface and the second surface when the outer member and the inner member are in the fully open position;
• (d) means for connecting the outer member and the inner member into the drill string above the mud motor so that during normal drilling operations, the axial compressive load contracts the inner member and the outer member and is substantially transmitted to the end of the wellbore;
• (e) compressible, resilient extending means contained within the chamber for telescoping the outer member and the inner member towards the fully open position, which extending means become compressed during contraction of the outer member and the inner member during normal drilling operations so that when the axial compressive load applied through the outer member and the inner member is subsequently decreased during an interruption in normal drilling operations due to sticking or hangup of the drill string in the wellbore, the outer member and the inner member are urged to extend towards the fully open position in order to maintain an amount of penetration of the drilling bit, the extending means having a spring rate, designated as S, and a maximum compressibility defining a second distance, designated as D.sub.2, such that:
• D.sub.1 .times.S<W.sub.B, and D.sub.2 .gtoreq.D.sub.1
• wherein W.sub.B is an intended maximum weight on bit during normal drilling operations;
• (f) means for inhibiting rotational movement of the inner member and the outer member relative to each other;
• and
• (g) means for neutralizing the hydrostatic pressure of the wellbore fluids exerted on the extending means.