The cutters in the center of a standard polycrystalline diamond compact (PDC) drillbit are highly inefficient due to the nature of the cutting action of a PDC bit. The cutters in the center of the bit are rotating around a very tight radius, and the cutting edges on these central PDC cutters are simply not able to engage the rock formation in an efficient manner. This results in the cutting structure in the center of the drillbit consuming a lot of energy while destroying very little rock.

With the MicroCORE cutting system, these center cutters are removed, and the energy being transmitted to the bit in the form of weight on bit and RPM is more efficiently used to destroy rock. The end result is higher drilling rates.

The MicroCORE cutting system, part of Tercel’s premium PDC drillbit product line, is a technologically evolved cutting structure from the standard PDC drillbit. The technology that is a part of this system has developed significantly over the last two years as Tercel’s engineering staff has studied performance and dulling characteristics from hundreds of runs from around the globe. The result is a product that consistently drills faster than offset PDC drillbit runs while also continuously producing large intact cuttings that can be used for advanced geological insight.

Figure 1 shows a cutaway of the MicroCORE cutting system. The traditional cutting structure is interrupted in the center of the bit, which allows an uncut core to build up in the center of the bit (1 and 2). This small core is efficiently broken off as the bit progresses forward (3) and then is hydraulically evacuated out the junk slot (4).

FIGURE 1. The cutaway of the Tercel MicroCORE cutting system illustrates the progression of a core (red) being cut while drilling ahead and then removed out the junk slot. (Source: Tercel Oilfield Products)

Bakken play

In North Dakota’s Williston Basin, the Bakken play has been a very active area for oil and gas exploration. Tercel has been working with a variety of operators in the area to improve their drilling efficiency and enhance their overall operations. As part of this work, an E&P company ran a standard Tercel PDC bit and two different designs of a Tercel PDC bit with the MicroCORE cutting system in the same area in the 8¾-in. vertical interval.

The 8¾-in. vertical interval typically starts at a measured depth (MD) of 701 m (2,300 ft) and generally runs to the kickoff point at around 3,049 m (10,000 ft) MD, which varies depending on the well location in the basin. This interval is characterized by fairly soft but highly interbedded formations in the shallower sections of the interval. As the interval progresses, there are many hard stringers of sandstones and limestones interbedded along with competent shale sequences.

A comparison was made between three Tercel bits—a standard PDC bit, a PDC bit with the MicroCORE cutting system and a PDC bit with a MicroCORE cutting system that was optimized for the application based on drilling performance and dulling characteristics. When comparing the standard PDC bit to the first MicroCORE PDC run, a noticeable improvement in ROP was observed throughout the run.

Then, as the design was further optimized, that bit run produced a noticeable improvement in the ROP even vs. the first iteration of the MicroCORE design.

Eagle Ford play

Improved drilling performance was achieved in the Eagle Ford play in South Texas. In this example, the operator was drilling the 8¾-in. production interval. This involves drilling the curve and lateral of these horizontal wells on a steerable motor. In this application, the operator is drilling the curve at around 12 degrees per 30 m (100 ft) and then drilling a 2,134-m (7,000-ft) lateral without making a trip for a bottomhole assembly or bit change.

For a particular operator drilling in Webb County, Texas, the Tercel MicroCORE bit runs were compared to PDC bit runs from other manufacturers. Most of the drillbits in this comparison were drilling to the same depth and generally pulled after reaching total depth for the well. However, there is a marked difference in the ROP for the MicroCORE cutting system bits relative to the standard PDC bits. Specifically, the average ROP of the non-Tercel bits is around 22 m/hr (72 ft/hr), while the Tercel bits averaged about 28 m/hr (92 ft/hr). This results in a time savings of about 24 hours to drill this interval. With a typical spread cost in the U.S. land drilling market, this saved about $75,000 in drilling costs per well.

Tunu Field

In Figure 2 there is a performance comparison of all Tercel bits in the same application. This interval is in the 8½-in. section of the Tunu gas and condensate field in Indonesia. This is a directional interval consisting of interbedded sandstones, shales, siltstones, coal and traces of limestone.

FIGURE 2. Three Tercel bits used in the same interval in the Tunu Field, Indonesia, are compared. The third run (right) shows a step-change improvement (50% boost in ROP) using the MicroCORE design. The improved dull condition of the bit exhibited no broken or chipped cutters. (Source: Tercel Oilfield Products)

The general unconfined compressive strength of the rock in the interval is relatively low, but it ranges from 8,000 psi to 15,000 psi throughout the interval. The nature of the rock and directional demands for the interval often resulted in impact damage on the PDC cutters on the bit.

In Figure 2, the comparison being made is between three Tercel bits. Runs one and two are standard cutting systems, while run three employs the MicroCORE cutting system. From the first to the second run, some improvements were made in the design of the bit that increased performance.

However, there was a step-change improvement (50% boost in ROP) in performance when the MicroCORE design was used in the third run. The improved drilling efficiency showed in the improved ROP, and the improved stability of the bit is illustrated by the improved dull condition of the bit exhibiting no broken or chipped cutters.

As the MicroCORE cutting system is introduced to new applications, the Tercel engineering staff continues to develop design practices that improve ROP and help drill longer intervals. These updated design characteristics then become established as best practices and are incorporated into all new products manufactured.

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