Drilling the 12¼-in tangent section (55-60°) through thick carbonate lithology in Qatar's offshore North Field is exceptionally challenging. In spite of bottom hole assembly (BHA) and parameter adjustments, the operator regularly experiences severe impact induced PDC cutter damage resulting in reduced bit life. Lithology (Hith to Khail) is predominantly interbedded carbonate/anhydrite, dolomite/argillaceous limestone with unconfined compressive strength (UCS) ranging between 9000-30,000psi. The operator wanted to improve rate of penetration (ROP) by stabilizing the bit body and reducing micro-balling tendencies. The objective would be to complete the 6000ft section in one bit run at a minimum ROP of 2000ft/24hr. An FEA-based modeling system, rock strength identification program and computational fluid dynamics (CFD) analysis was used to investigate several 12¼-in PDC bit designs. The results of the simulations led to a new MSi716 design with back-up cutters strategically positioned on all blades to provide maximum durability in critical wear areas. After reviewing the results of the analysis, the operator requested additional bit design changes including a 6-in tapered gauge and managed depth-of-cut in the cone/nose areas to reduce reactive torque. A parameter sensitivity study was conducted to determine the optimal operating window (WOB/RPM) and to ensure the new PDC design would be compatible with a unique high-torque RSS which allows for higher weight on bit than standard rotary steerable systems to maximize rate of penetration potential.The new design was run on the high-torque RSS/motor system and drilled 6,001ft of 12¼-in section at a high ROP of 139.4ft/hr setting a new North Field record. The average daily footage was 2572ft besting the old mark by 38%. Additionally, the run set a single 24hr footage record of 3100ft/day, 29% better than benchmark. Based on this outstanding bit performance the operator made these design changes a standard requirement for PDC bits in this application. The new design also helped standardize mechanical specific energy (MSE) values for future ROP gains.An important reference in the fast drill optimization process and MSE index is monitoring changes in formation unconfined compressive strength (UCS). In North Field operations, a baseline MSE index of 100,000psi or lower was established for the 12¼-in hole section. An increase in MSE above the baseline value that exceeds the change in rock strength is a strong indication of drill bit dysfunction. [2][3][4][5][6][7] Challenge (12¼-in Hole Directional Section) The 12¼-in directional hole section normally begins in the Hith formation at 5000ft and transitions to the Khail formation with TD at 11,500ft (Figure 3). Lithology predominantly consists of soft to medium-hard carbonates including anhydrite, dolomite and limestone. The lithologies are highly interbedded and have an unconfined compressive strength range between
