Debris bridge monitoring and removal for uphole milling system

A bottom hole assembly (BHA) contains a motor and a section mill for milling in an uphole direction after blade extension with circulating fluid through the BHA. Below the section mill is sensing equipment to detect location of a bridge formed by the cuttings or swarf from the section mill. A secondary mill oriented for cutting in a downhole direction is located at the bottom of the BHA for use in removal of the bridge. The sensing equipment delivers in real time data as to the density of the bridge so that decisions to interrupt the section milling and to lower the secondary mill to the bridge can be made in real time. Cement is pumped and displaced by a wiper plug to plug and abandon the hole.