Optimizing directional drilling performance in U.S. shale plays and around the world continues to focus on increasing the hydraulic efficiency of the drilling system consisting of the drill bit and the directional drilling tool (RSS/Motor). Bit/tool balling is one of the prevalent causes of reduced system hydraulic efficiency, which leads to bit ROP degradation.
This paper details knowledge gained from laboratory tests of drilling under pressure in a drilling simulator. The tests are designed to replicate the balling that occurs in the field and determine which variables cause the system to ball-up.
Tests took place at various depths of cut (DOC) to determine the effect cutting size has on system balling. An 8 ¾-in?? five-blade PDC bit with polished and non-polished cutters and bottom-hole assembly (BHA) setup was used in the tests. Drilling parameters were chosen to vary cutting size with depth of cut. Catoosa shale was chosen for the tests because it presents the best opportunity to replicate field observations.
Tests show cutting size as the most critical factor affecting the system's hydraulic efficiency. Larger cuttings cause more balling because of tight clearances between the bit/tool and the borehole. Tests also show that polished cutters keep the system from balling or packing off at twice the DOC as standard cutters. The cuttings produced by polished cutters are half the size as standard ones even at more than twice the DOC enabling higher ROPs before the bit or tool is packed off.