The ultimate goal for a drillbit is more footage at a higher ROP. Generally, these two performance benchmarks are inversely correlated. The higher ROP of a drillbit typically reduces its life and, conversely, the ROP is reduced to increase the drillbit life. By truly understanding the mechanics of how a cutter fails rock, the Stega efficient layout technique has managed to deliver both increased footage and ROP.

The drillbit industry has long understood the concept of bit depth of cut (penetration per revolution) and its relation to drilling efficiency. The depth of cut can be used to optimize performance and mitigate drilling dysfunction. From a drilling efficiency standpoint, low depth of cut leads to inefficient drilling and dysfunction such as bit chatter, vibration and whirl. A high depth of cut leads to efficient drilling but can induce stick/slip if not controlled.

New layout

The Stega efficient layout technique has taken the depth of cut from the bit level to the cutter level. Instead of using a global bit depth of cut, a Stega layout breaks down the individual cutter depth of cut and then optimizes for each cutting element.

When analyzing a drillbit, it is critical to understand the forces on each cutter. A key finding of individual cutter testing is that forces are related to a combination of both engagement area and perimeter length of engagement. Managing the combination of cutter engagement area to cutter perimeter of engagement has unlocked the potential for backup cutters on a drillbit.

The updated cutter force models have led to a backup cutter layout in which the backup cutters no longer trail their parent cutter on the same blade. Backup cutters placed in the traditional sense on the same blade as the parent cutter will experience a very low cutter level depth of cut with a high perimeter of engagement, which is due to the bottomhole pattern generation and yields very inefficient use of the backup cutters. When the backup cutters engage, the weight on bit required increases and the cutter is more susceptible to wear and damage. By moving the backup cutter to a different blade, a completely different engagement shape can be created due to other cutters on the bit carving up the bottomhole.

The real key to the efficient layout technique is exposure. If the backup cutters are not underexposed from the primary cutters, it is likely that drilling efficiency will suffer from the start. Likewise, if they are underexposed too far from the primary cutting structure, they will not do any work until it is too late and again drilling efficiency will suffer. Through dull evaluation and a detailed understanding of the application, Halliburton’s Design at the Customer Interface (DatCI) process is used in combination with cutter force and wear models to place the backup elements at the correct location to provide maximum life and ROP.

Case studies

Drillbits designed utilizing the Stega efficient layout technique have been proven around the globe in some of the harshest drilling environments. In an interbedded transitional application in the North Sea, Stega was utilized to minimize impact damage to the shoulder of the drillbit by better-distributing cutter loads while transitioning from a 5-kpsi shale to a 40-kpsi limestone at an inclination of 50 degrees.

The drillbit was able to complete the section with one bit and one bottomhole assembly (BHA) versus two to three drillbits for the offset wells leading to a 37% increase in footage over the best offset. It also increased the ROP by more than 14% over the best offset and came out with a 1-1-CT dull condition rating while the average dull through the transition section was pulled with a 4-4-BT dull condition.

Additionally, Stega outperformed its offsets in an abrasive sandstone in Argentina. In the application, bits were pulled after only 381 m (1,250 ft) of drilling due to extreme abrasive wear. The large wear flats were leading to very low ROPs of only 1.8 m/hr to 3 m/hr (6 ft/hr to 10 ft/hr). The bit was redesigned keeping the same primary cutting structure but adding backup cutters using the new Stega layout technique. The goal of this design was not to absorb impact load from transitional drilling but rather slow down the progression of wear as the wear flats were the reason for the bits being pulled. The underexposure of the backup element was customized through the DatCI process to keep the cutting structure sharp longer. This resulted in a bit that drilled 97% more footage at an equivalent ROP of the offsets by adding backup cutters.

In yet another application, a Stega backup cutter bit was used in the Permian Basin with the goal of reducing a curve lateral for an Upper Wolfcamp target from two BHA runs to a single BHA run. This application utilized a conventional motor to drill the curve and then push as far as possible into the lateral before tripping for the ROP and finishing the lateral with a new bit. When the 8½-in. GTD64DC bit with backup cutters placed in the Stega layout configuration was used, the results were outstanding. Not only did the bit meet the goal of a single BHA run but made it in record ROP compared to the offsets. The GTD64DC completed the 2,585-m (8,483-ft) section in one run while the best offset managed only 1,839 m (6,033 ft), a 41% increase in footage over the best offset. It was anticipated that if the bit completed the section in one run that the ROP would be sacrificed, but that was not the case. The backup bit improved ROP by 19% over the best offset and nearly 25% better than the average offset.