Importance

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Abstract

A method and liner assembly for drilling into unstable or depleted formations is provided that maintains control of the wellbore against caving such as where unconsolidated formations are penetrated and/or minimizes fluid losses such as to underpressured formations where differential pressures exist. The method and liner assembly herein includes the provision of a liner having a portion thereof that is drillable so that after setting of the liner, drilling can continue deeper into the unstable formations with minimal damage to the bit used to drill out the liner drillable portion. In one form, the liner has a shoe that includes cutter mounting blades, each having a set of cutters thereon. Relief slots are formed in the blades between cutters so that as the shoe and its blades are being drilled, the drill bit will cut through the slots, releasing the shoe cutters for transport up to the surface by the drilling fluid thereby minimizing damaging contact of the bit with the shoe cutters. Preferably, the shoe has a bi-center and anti-whirl design. In another form, the liner has preassembled therewith a whipstock and a pre-formed window of drillable material adjacent the whipstock, so that after drilling into the unstable formation with the liner assembly and setting it therein, subsequent drilling beyond the liner occurs by running a drill bit downhole and drilling until it engages the whipstock that guides it to the window for drilling therethrough.

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First Claim