DUAL BHA DRILLING SYSTEM

The invention relates to apparatus and methods useful for drilling lateral boreholes into a formation surrounding a main borehole. Proposed technique can be used for drilling of laterals which does not require such significant interruptions to deploy the small BHA and is based on the use of a drilling apparatus, comprising a drill collar forming part of a primary drilling assembly and having an outward opening groove in the side thereof, a secondary drilling assembly, comprising a tubular drill string connected at one end to the drill collar, a drilling motor mounted in the drill string, a drill bit mounted at the other end of the drill string and connected to the drilling motor, wherein the secondary drilling assembly is mounted in the drill collar so as to be movable between a first position in which the drill bit is seated in the groove, and a second position in which the bit projects laterally from the groove in the side of the drill collar.