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
Saudi Aramco is developing gas fields in various areas utilizing vertical and horizontal wells with rotary as well as motor BHAs. This application requires Saudi Aramco to drill laterals through an extremely tight sandstone formation with inclination varying anywhere from 72° to 90°. The highly abrasive formation is causing inconsistent PDC bit performance while drilling the 5 7/8-in. horizontal section resulting in exceeding the authorization for expenditure (AFE) of the wells. Saudi Aramco, in collaboration with the bit company, analyzed the field data, which showed extensive cutter damage, with abrasive wear being the most common dull characteristic. The cutter wear was causing short runs and frequent trips to change out the bit. A step change in bit design was required to extend bit life and increase rate of penetration (ROP) in areas where poor drilling performance is expected. The fixed PDC design creates an inherent limitation because only a small portion of the cutter contacts the formation, and drilling efficiency declines as the cutter wears. The resulting wear flat generates a high degree of frictional heat, which can break the diamond-to-diamond bonds in the cutter diamond table and lead to accelerated cutter degradation. An R&D initiative was launched to investigate different methods to enable a PDC shearing element to fully rotate while drilling to increase overall cutting efficiency and bit life. Several different retention methods were investigated, and a specialized fixed housing that is brazed into the bit blade was developed. The rolling cutter assembly has essentially the same outside diameter (OD) as a standard PDC cutter, which helps maintain design flexibility and cutter placement options. The newly designed 5 7/8-in. PDC was manufactured with strategically positioned rolling cutters and was run in a gas well. The objective was to achieve an average ROP of 4.7 ft/hr and drill a minimum of 240 ft of lateral hole section to achieve proposed 5% reduction in cost-per-foot. The bit achieved a record ROP of 14.85 ft/hr while drilling 527 ft of the abrasive sandstone formation. After achieving positive results from tests in lateral sections, the new bit design was also tested in a two vertical sections through the same tight sandstone formation on rotary assemblies. In these applications, the new bit design performed better than those in offset wells, with one of the two drilling from shoe to shoe for the first time in that field. This paper will discuss the application and evolution of rolling cutter bits, along with the results achieved by bits fitted with these cutters in Saudi Arabia.
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