lower end of a drill string 145 after a cementing operation as illustrated the wellbore 10 is lined with a string of steel pipe called casing 15 the casing 15 provides support to the wellbore 10 and facilitates the isolation of certain areas of the wellbore 10 adjacent hydrocarbon bearing formations the casing 15 typically extends down the wellbore 10 from the surface of the well to a designated depth an annular area 20 is thus defined between the outside of the casing 15 and the wellbore 10 this annular area 20 is filled with cement 25 pumped through a cementing system 50 to permanently set the casing 15 in the wellbore 10 and to facilitate the isolation of production zones and fluids at different depths within the wellbore 10 the cementing system 50 generally includes a float shoe 55 disposed at the lower end thereof to prevent reverse flow or u tubing of the cement 25 from the annulus 20 into the casing 15 the cementing system 50 also includes a shoe track 60 which is a space defined between the float shoe 55 and a float collar 65 the principal function of the shoe track 60 is to ensure that the float shoe 55 is surrounded in high quality cement and that any contamination that may bypass cement plugs 70 80 is safely contained within the shoe track 60 after the casing 15 is permanently set in the wellbore 10 the expandable bit 100 is employed to drill through the cementing system 50 and to subsequently drill the wellbore 10 to another designated depth as illustrated.