A drilling liner having a core bit at its bottom end is carried along with a pilot bit on an inner bottom hole assembly driven by a downhole mud motor. In one embodiment, the motor is powered by mud carried by an inner string. Alternatively, the inner string may be omitted and the flow of mud through...http://www.google.es/patents/US6196336?utm_source=gb-gplus-sharePatente US6196336 - Method and apparatus for drilling boreholes in earth formations (drilling liner systems)
Method and apparatus for drilling boreholes in earth formations (drilling ...
