For the past five years, drilling engineers at Verkhnechonskneftegaz (VCNG), together with service company advisors, have made steady improvement in drilling performance by studying drilling records and making systematic improvements to the bottomhole assembly (BHA). Initially, tungsten carbide insert (TCI) drill bits were run on positive displacement motors (PDMs). The procedure evolved to the use of rotary steerable systems (RSSs) and PDC bits.

Challenged to improve drilling performance in its Eastern Siberian oil field, VCNG took a systematic approach. Instead of focusing on a single factor, such as the bit, the mud, the BHA, or the actual drilling techniques, each facet of the drilling operation was studied, from both its individual contribution to its collective performance aspects.

Cutting-edge technology

The specific challenge to overcome was the efficient construction of an 8½-in. directional interval penetrating alternating carbonate and evaporite sequences. Drillers found it challenging to maintain good directional control while achieving a high rate of penetration (ROP).

For help in drilling hard rocks, engineers specified a drilling system with the SHARC high abrasion resistance PDC drill bit and the PowerDrive vortex-powered RSS. The RSS gets its main drilling power from the rig’s top drive but is speed-boosted by an inline PDM located in the BHA.

To justify the economics of the RSS, the drilling team knew they would need to shave considerable time off the drilling curve. They hoped to do this through a combination of improved ROP and improved drill bit longevity. The goal was to drill the entire 8½-in. directional interval in a single bit run.

A new bit design

With the rest of the drilling system optimized, the drill bit design was analyzed using the IDEAS drill bit design platform. The analysis led the team to choose the proven SHARC PDC drill bit. The drill bit was shown to be very stable when powered by the high-speed RSS that had been chosen to drill the target formations. Initial field testing with the MDSi716 bit achieved the desired shoe-to-shoe runs using standard cutters.

Laboratory tests indicated that further ROP improvement could be attained by fitting the drill bit with the new ONYX II premium PDC cutters. This proved to be the ideal combination for the tough interbedded carbonate and evaporite sequences.

The principle benefit of the premium cutter is its improved wear resistance. Not only could it achieve the one-trip goal throughout the target section, but it could do so at unprecedented speed and efficiency. Hole quality and directional control were excellent. Average ROP improvement of 19% was achieved compared with offset wells. Two separate field records were attained: the longest interval was drilled at 2,645 m (8,676 ft), and the fastest shoe-to-shoe interval was drilled at 33.2 m/hr (109 ft/hr).

Throughout the campaign, the VCNG set the performance bar high with demanding operating parameters:

Bit rotational speed ranging from 165 rpm to 210 rpm;

Downhole weight-on-bit ranging from 2 to 18 tons;

Drilling fluid flow rates ranging from 396 to 528 gpm;

Mud weight 10.8 ppg (1.3 sg);

Hole deviation in ranging from 10 degrees to 30

degrees; and

Hole deviation out ranging from 80

degrees to 90 degrees.

Although the earlier generation ONYX PDC cutters have good wear-resistant qualities, a comparison with the new ONYX II cutters on separate shoe-to-shoe runs revealed dramatic improvement.

The ONYX II cutters (left) consistently outlast standard ONYX cutters in comparative tests on offset wells. (Images courtesy of Schlumberger)

Evolutionary improvements experienced

The systematic approach allowed the operator to develop a compelling set of statistics from which the incremental improvement of each change could be gauged. Over the interval in which the design changes were implemented and tested, steady improvement in ROP can be seen from the initial run that averaged 20 m/hr (66 ft/hr) to the final run that averaged 33.2 m/hr (109 ft/hr).

Likewise, a comparison of ROPs attained in intervals exceeding 2,000 m (6,560 ft) illustrates the clear advantage of the new ONYX II cutters.

The PDC bits used in these wells are specifically designed to minimize vibration. Vibration robs energy from the bit’s ability to cut rock and can cause damage to downhole drilling instrumentation. The bits also are designed to minimize cutter wear and damage. This maximizes cutter life and extends shoe-to-shoe performance while retaining sharp edges for higher ROP.

In this offset comparison of 8½-in. bit runs of greater than 2,000 m (6,560 ft), the ONYX II bits outperformed other bits.

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