Underreamer and method of use

1. An underreamer to be used within a wellbore drilling assembly, the underreamer comprising:

a tubular body providing an axial flowbore and a plurality of longitudinal pockets;

a pair of removable guide inserts installed longitudinally within each of said longitudinal pockets, each guide insert having at least one linear projection;

a collapsible blade installed within each of the longitudinal pockets between the pair of removable guide inserts and having linear grooves in opposing sides thereof corresponding to each linear projection on the pair of guide inserts whereby each linear groove engageably contacts a corresponding linear projection; and

the collapsible blade translates in a substantially linear path between an extended position and a retracted position in response to a change in pressure within the axial flowbore.

2. The underreamer of claim 1 having a mandrel longitudinally disposed within the tubular body and having a plurality of load fingers engageably contacting the collapsible blades to manipulate the collapsible blades between the retracted and the extended positions by longitudinal translation of the load fingers in response to the change in flowbore pressure on the mandrel.

3. The underreamer of claim 2 further comprising a biasing spring opposeably contacting the mandrel to maintain the collapsible blade in the retracted position when there is no pressure within the flowbore.

4. The underreamer of claim 1 wherein the linear path of translation is characterized by an acute angle departing from the central axis of the underreamer upstream from said longitudinal pockets.

5. The underreamer of claim 1 wherein the linear path of translation is characterized by an acute angle departing from the central axis of the underreamer downstream from said longitudinal pockets.

6. The underreamer of claim 1 wherein each pair of removable guide inserts are hardened.

7. The underreamer of claim 1 wherein each collapsible blade is a stabilizer pad.

8. The underreamer of claim 1 wherein each collapsible blade has a trailing edge including a cutting surface thereon.

9. A method of enlarging a borehole comprising:

installing at a distal end of a drillstring a collapsible underreamer having a tubular body, an axial flowbore with a mandrel installed therein, and a plurality of longitudinal channels with a pair of removable guide inserts, a collapsible blade, and a guide insert lock installed longitudinally therein;

pressurizing the axial flowbore of the underreamer to engage opposing sides of the collapsible blade with the pair of removable guide inserts and substantially linearly translate each collapsible blade to an extended position; and

rotating the drillstring with each collapsible blade in the extended position to enlarge the borehole.

10. The method of claim 9 comprising changing a pressure within the axial flowbore to retract each collapsible blade, and retrieving the collapsible underreamer through an under gauge string of casing.

11. The method of claim 9 further including replacing the collapsible blade in the field by:

disconnecting the collapsible underreamer from the drillstring;

removing the mandrel;

removing each collapsible blade;

inserting replacement collapsible blades

reinstalling the mandrel; and

reinstalling the collapsible underreamer onto the drillstring.

12. The method of claim 9 further including shortening a radial extension of the collapsible blade by

disconnecting the collapsible underreamer from the drillstring;

removing the mandrel;

removing guide inserts;

removing the guide insert lock;

reinstalling a longer guide insert lock longer than the removed guide insert lock;

reinstalling the guide inserts;

reinstalling the mandrel; and

reinstalling the collapsible underreamer body onto the drillstring.