Conical Diamond Element Enables PDC Bit to Efficiently Drill Chert Interval at High ROP Replacing Turbine/Impregnated BHA
Authors: Pak Mikhail , Azar Michael , Bits Smith , Pahl Joerg 
Published: January 2016

Developing reserves in Kazakhstan's Chinarevskoe field frequently requires the operator to drill an 8.5-in. vertical hole section through extremely hard carbonates which contain high concentrations of problematic chert inclusions. The first significant chert layer is encountered at 3050-3110m, which has 10-50% chert content. Penetrating the chert interval with PDC bits has been challenging and in many cases impossible due to the cutters' inability to withstand high impact forces, which resulted in catastrophic cutter damage. The application of impregnated bits on high-speed turbines (1300-1400 RPM) led to significant improvement in run lengths and this BHA became the standard approach.1 One impregnated bit could drill approximately 550m at 3.4m/hr. The hole section typically requires three bits to reach TD.

However, the impregnated bit generated very small cuttings and high frictional heat altered chip appearance hampering lithology characterization and efforts to identify formation properties. Additionally, the centrifuge was unable to separate the extremely small cuttings from the drilling fluid, resulting in a gradual increase in mud weight over the interval creating the need for constant mud replacement.

Tests with PDC were once again attempted, but when the bit encountered a high percentage chert interval, the bits were damaged and pulled in poor dull condition (7-6-CR). After repeated attempts with PDC, the operator realized wellbore verticality was compromised, which required correction run with impreg/turbine BHA with not optimum parameters leading to extra drilling time. The drilling team conceded the chert layers could not be drilled with a conventional PDC/motor BHA. The operator required PDC bit that could efficiently drill long intervals of carbonate formation at high ROP with a durable cutting structure that would remain intact when encountering the chert intervals.

To develop a durable bit solution, engineers used an FEA-based modeling system to implement an innovative hybrid-type bit by strategically positioning multiple conical diamond elements (CDE) and conventional PDC cutters across the entire cutting structure from bit center to gauge. This new-style cutting structure leverages the CDE's aggressive shape and superior impact and wear resistance to create a high-efficiency plowing/shearing action that improves performance in difficult formations. The analysis indicated the new hybrid bit would improve performance and be capable of surviving shock and vibration while drilling the hard carbonate/chert formations.

The CDE bit was field tested on rotary steerable BHA and drilled 615m at an ROP of 9.1m/hr through the carbonate formation that contained 10-40% chert inclusions throughout the entire interval. Compared to offsets drilled with an impreg/turbine BHA, the CDE bit drilled 12% more meters at 166% higher ROP while providing good wellbore quality and came out with excellent dull condition (1-1-CT). The bit generated much larger cuttings enabling wellsite geologists to readily identify lithology type and properties. The larger drilled cuttings were easily separated by the centrifuge saving the operator $63,000USD in mud replacement costs. The high ROP of the CDE bit saved the operator six-days of rig time reducing cost/meter by 26% for a total savings of $192,000USD.