BI-MILL FOR MILLING AN OPENING THROUGH A WELLBORE CASING AND IN A PREPLANNED LATERAL DRILLING PATH IN DEPARTURE FROM THE WELLBORE AXIS

A bi-mill mills an opening through a wellbore casing in a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or rock formation and in the direction of the drilling path. The bi-mill provides increase debris removal and cutting surfaces and includes an assembly having a lead mill and a follow mill and attaches to the wellbore whipstock. The lead mill further includes fluid passageways leading from the lead mill and exiting the nose of the lead mill. Debris removal channels channel milled debris away from the lead mill and in the direction of the follow mill form in a helical configuration for guiding debris milled by lead mill inserts along the debris removal channels. The lead mill and follow mill continue milling through the wellbore casing and into the underground rock formation a distance sufficient to initiate the preplanned drilling path.