When tricone bits are balanced - both in force and in volume - stresses are reduced, leading to longer wear and increased performance.

Unlike polycrystalline diamond compact (PDC) bits, roller cone designs are inherently difficult to balance due to the geometric space limitation, rolling cones and the large number of design variables. To overcome this difficulty, optimization technology has been developed that enables the balanced condition of a roller cone bit to first be evaluated and then balanced both by volume and by force. Volume balancing equalizes rock removal among all the cones, while force balancing ensures all cones are subject to nearly the same loads of weight-on-cone, bending moment and force-on-bearing. Thus, not one single cone or bearing system is over-stressed.

Simply put, when all three cones/bearings are subject to the same forces, the cutting structure is said to be "force balanced." When all three cones remove the same amount of rock during drilling, the cutting structure is said to be "volume balanced." An "energy balanced" (EB) bit means that it is both "force balanced" and "volume balanced."

Advantages of energy-balanced roller cone bits

Balancing the amount-of-rock-drilled/ forces-per-cone improves the cutting structure-to-rock interface dynamic of a roller cone design. As demonstrated in lab and field tests, EB bits generate smoother bottomhole patterns which minimize impact damage and enhance scraping action. Smoother, low-vibration drilling also improves directional responsiveness and reduces the risk of arm fatigue failure and damage to sensitive string components.

The bits can incorporate patented oriented tooth and/or chisel-shaped inserts set perpendicular to the scraping direction to remove a greater volume of rock and increase penetration rate (ROP). Oriented teeth provide uniform stress by preventing high loads on the corners of inserts, which reduces chipping and broken teeth. Uniform stress also means little or no torque on cutters, reducing the risk of insert loss from rotation. Also, while conical or hemispherical inserts can't be oriented, the cutting structures and bearing systems can certainly still be energy- and force-balanced. Thus, not every bit will have oriented teeth, but they all will benefit from both energy and force balancing design techniques.

The process for energy balancing has resulted in roller cone bits with tougher cutting structures, which increases cutting efficiency, seal reliability rates and the service life of the entire bit. Dull conditions have improved and, as a result, more aggressive bit designs can be developed, making it possible to recommend one IADC code softer when specifying EB bits. For example, in an application for a 517 IADC standard bit, a 447 or 437 IADC bit may now be recommended.

Case histories

Norway. In one recent application, a 24-in. EB insert bit set a footage record in the Oseberg field. In these long wells, the 24-in. sections are extended and can reach lengths up to 6,560 ft (2,000 m), requiring very long bit life. EB Bits had to be developed that could drill these longer sections without compromising ROP.
The result demonstrated extremely good longevity, drilling 4,077 ft (1,243 m) in 66 hours to achieve the longest single run footage performance to date in the Oseberg area. The entire section was drilled in only two runs, compared to previous similar sections that required as many as eight runs. Based on this performance, the bit design eliminated a minimum of three bit trips or approximately 30 hours of drilling rig time, resulting in estimated savings of $195,000.

In one Ovhor Field well, the objective was to drill the 16-in. hole section from surface to section total depth (TD) at 6,160 ft (1,878 m) measured depth in one roller cone bit run, thus eliminating one complete round bit trip. In this case, the EB bit drilled the interval of 6,160 ft (1,878 m) in a single run, achieving an average ROP of 67ft/hr. The bit was pulled at total depth (TD) with a dull grading of 3-3-WT-A-E-I-BU-TD. In this single-run footage performance the bit saved more than 1 day of drilling time, a savings of over US $60,000 for the run.

Canada. In this application in Northeastern British Colombia, the formation consists of highly interbedded sandstone and shale sequences with frequent coal lenses, which require exceptional ROP and durability from the 83/4-in. bits being used. The application typically requires use of roller cone insert bits which can maintain high ROP in the softer shale and coal sections yet be tough enough to maintain their cutting structures through the harder, interbedded sections.

To optimize performance through these formations, where compressive strengths range from 2,000 to 35,000 psi, the original 517S IADC design was analyzed and modified to make tooth orientation and force balancing changes.

A total of 13 of the EB bits were run in comparison to 13 offset bits and 8 bits of the original non-balanced design. The EB design drilled the interval at an average ROP of 86.6ft/hr (26.4 m/hr), while offsets averaged 74.1ft/hr (22.6 m/hr), and the original design averaged 67.6ft/hr (20.6 m/hr). Overall, the bit drilled farther in fewer hours than previous bits used in this interval, achieving an ROP 17% higher than the offset bits and 28% higher than the original design.

Algeria. In another example, a 121/4-in. EB soft formation insert bit was run in Algeria and achieved an ROP 15% higher than the average for the field. The custom-designed 12 1/4-in. bit incorporated balanced cutting structure and optimized insert orientation to promote faster ROP in this application. Run with controlled parameters, as per the customer program, the bit drilled 3,576 ft (1,090 m) in 68.25 hours including connections, completing the section at 52.4 ft/hr (16 m/hr) to achieve one of the top three performances in the field and produce savings of $10,550. The dull grade was 2-2-WT-E-I.

Middle East. In a development well project in the Middle East, an EB design was introduced to drill the entire 1,500 ft (457 m) horizontal section. To evaluate performance against the best offsets in this application, two 61/8-in. bits were run in consecutive wells. The first EB bit drilled 1,522 ft (464 m) at an average ROP of 34.6 fr/hr (10.5 m/hr), while the second drilled 1,500 ft (457 m), averaging 29.1ft/hr (8.9 m/hr). These two 61/8-in. bits drilled on average 10% faster than the two best offsets and saved $9,649 for an equivalent interval drilled.

As these case histories confirm, the performance advantages of a balanced bit - faster drilling and longer bit life - are not limited to PDC designs. With advanced analytical capabilities, EB roller cone bits are delivering faster penetration rates over longer intervals, with less wear overall.

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