Unique Backup Cutter Layout Method Leads to Step Change in Bit Durability and Performance when Transition Drilling Through Hard Limestone Formations at High Inclinations

Abstract

In the mature fields in Norway's North Sea, extended-reach drilling (ERD) well designs were implemented to reach distant reservoir target zones. Resulting from this change, the 12.25-in. overburden section, which drills through mostly shale with various hard and thin dolomitic limestone sections, had to be modified from a 5,000- to 7,000-ft vertical section to a 10,000- to 12,000-ft section at a tangent inclination anywhere from 35 to 70°. Drill-bit designs that used to drill the vertical overburden section consistently in one run currently require up to three bits to complete the modified tangent sections caused by excessive damage to the diamond cutting structure. An extensive study into the offset ERD wells and corresponding bit dull conditions helped identify that the cutting structure damage originated from high-point load impacts to the shoulder of the bit while transitioning through the thin, hard dolomitic limestone formations; the damaged cutters were being tangentially overloaded. Through further bit modifications, it was determined that this type of damage could not be addressed using typical bit design modifications (e.g., increased cutter density, increased blade count, etc.); therefore, a unique design solution was developed and implemented. This solution involved combining analytical simulations and a dull condition analysis to develop an improved method for using the backup cutting structure to support the high tangential loading on the primary cutting structure during hard, high-inclination transitions. In most dual-row cutting structure designs, the backup cutters are on the same radial position as their parent primary cutters and are underexposed, engaging the formation only at a specific depth of cut or after the primary cutters are worn down a certain amount. When positioned as such, the backup cutters do nothing to support the tangential loading on the primary cutters. By moving the backup cutters actively on profile with the primary cutters and offsetting them to their own radial positions, the backups could now actively support the high tangential loads occurring on the primary cutters when engaging the hard dolomitic limestone. This simple but unique modification enabled the bit to handle the transitions and drill the entire next 11,000-ft+ ERD section in one run while also providing the fastest rate of penetration (ROP) to date, with an increase ranging from 30 to 40% compared to previous runs. The success has been repeated a dozen more times since, each time providing a new record ROP for an ERD section. The improved dull condition could also allow for more efficient designs in the future for further improvements to drilling performance. The successful reduction of tangential impact damage could have implications for various bit designs globally that incur a similar dull condition.