When faced with a formation containing chert, a roller cone insert (RCI) bit is the first type of drillbit technology that comes to the drilling engineer’s mind. The ability of the RCI bit to crush chert is proven and usually has good results. Another option is to try a diamond-impregnated bit, which may require the use of a turbine, adding to the cost of the operation. Polycrystalline diamond compact (PDC) bits may be used and will show a high initial ROP, but the ROP will soon be reduced by the damaging effect chert has on PDC cutters.

Lately, several hybrid technologies have emerged that promise better performance than conventional drillbits. National Oilwell Varco (NOV) developed the FuseTek hybrid drillbit product line to combine the strengths of PDC and diamond-impregnated drillbit technologies to drill in harsh applications with higher performance than either technology by itself.

The most recent success was in the central part of Bakersfield, Calif., for a major operator, where a series of FuseTek bits were run. The technology has shown excellent potential since the first run using an 8½-in. FT716 bit equipped with Helios impact cutters was completed with a motor and RSS. The bit drilled 250 m (820 ft), from 3,710 m to 3,960 m (12,170 ft to 12,990 ft), in 40.5 hours at a rate of 6 m/hr (20.2 ft/hr) through difficult shale and chert formations.

The mud log in this run showed that the bit drilled three major chert packages, one of which was more than 122 m (400 ft) thick. The bit drilled 91.4 m (300 ft) into the most difficult chert interval and was still drilling at 5 m/hr (16 ft/hr) when it was pulled to correct an RSS failure. The rig dull grade was 7-7-WT-A-X-I-CT-DTF, which, when taking into account the footage drilled, is a very good dull condition.

After this run, the bit shoulder retained a significant amount of cutter volume and impregnated-diamond material. The nose of the bit suffered the majority of the wear, with most of the cutters worn down to the blade top and a small ring-out around the nose and cone area. The dual-cutting structure of the bit promoted the extended drilling interval and protected the cone of the bit from further damage.

Well comparison

This well was compared to the most recent competitor bit run, which only drilled 114 m (373 ft) in 32.5 hours at a rate of 3.2 m/hr (10.6 ft/hr). The performance of the hybrid bit was notable because standard PDC bits stopped drilling completely in this formation when a 1 to 2 wear flat developed. Looking back on a standard performance of 30.5 m (100 ft), this was considered a good run.

The 8½-in. FT716 bit drilled through chert stringers at 1.3 m/hr to 1.5 m/hr (4 ft/hr to 5 ft/hr) as normal but then picked up to 4.6 m/hr to 9.2 m/hr (15 ft/hr to 30 ft/hr) in the softer zones. The highest ROP recorded was 27.4 m/hr (90 ft/hr), which occurred at the start of the run.

Since this initial success, these hybrid bits have completed nine runs averaging 137 m (451 ft). This is a 27% improvement over the average competitor bit run and a 30% improvement in average ROP, increasing from 3.8 m/hr to 5.1 m/hr (12.6 ft/hr to 16.6 ft/hr).

The success of NOV’s hybrid bit technology in comparison to the standard PDC bit performance clearly shows the technology’s advantage in providing solutions for this problematic area.

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