Developing reserves in the northern portion of Russia's Perm region frequently requires the operator to drill extremely hard carbonates which contain high concentrations of problematic chert inclusions. Drilling the chert with PDC bits has been challenging and in many cases impossible due to the cutters' inability to withstand high impact forces which resulted in severe cutter damage. Both rotary and motor BHAs have been utilized in an attempt to mitigate shock and vibration.
Rollercone TCI bits are a viable option but slow ROP and short runs combined with high total bit revolutions caused numerous trips for tool change out. It was evident that PDC's shearing action was the most efficient rock failure mechanism and had the greatest potential to drill extended meters at higher ROP compared to a roller cone's crushing action. The operator required a new type PDC bit that could efficiently drill long intervals of carbonate/chert formation at high ROP and a 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 hybrid CDE bit was tested and compared against two direct offsets in Russia's North Perm Unvinskoe field. Unvinskoe field was selected for the case study because the formations and application challenges are similar to several other northern Perm fields. The first well was drilled entirely with roller cones which required 10 runs delivering average meterage of 149/bit at an average ROP of 6.05 m/hr. The second well required three roller cones and three 913-standard type PDC bits delivering average meterage of 213.4/bit at an average ROP of 9.65 m/hr. The third well was drilled with one roller cone, one standard PDC with averages of 336m at 8.2 m/hr and two hybrid CDE bits with averages of 433m at 15.6 m/hr. Performance improvement for the CDE bit runs versus offsets runs in wells were: 1) Well-1 190% more meters, 157% faster ROP; 2) Well-2 103% more meters, 62% faster ROP; 3) Well-3 29% more meters, 90% faster ROP.