Expandable reamers move from marginal to integrated components

Hole openers are not new to the industry. In the early days of rotary drilling, bitslost gauge as they traveled downhole. To allow for the setting of casing, this tapered effect had to be dealt with. In its origin, a reamer, or a hole opener as they were referred to then, was run to ensure adequate hole size throughout the well bore. As early as the 1930s, stationary reamers were used to open up pilot holes drilled for sidetracks and whipstocks.

Standard hole openers exist in fixed sizes and feature carbide metal teeth insert cutters with sizes ranging from 6-in. up to 26-in., typically with three or four cutters. As technology advanced, the addition of motors, MWD tools, and rotary steerable systems to the drill-string now called for an integrated system. The need for expandable reamers came into play. Today, reamers are designed to pass through a restriction and open up beneath it to expand the hole for the next series of casingto be run, providing sufficient room between the casing and the formation for the cementing to take place. By the mid-1990s, expandable reamers were becoming a viable type of tool. “This is really when the under-reamer came into its own,” said Les Shale, business development manager – Hole Enlargement, Baker Hughes.

The Troll field is one of Statoil’s key gas assets on the Norwegian Continental Shelf, containing approximately 40% of total gas reserves in the area. The Troll C platform produces from oil-bearing layers in the Troll West reservoirs. First oil was produced in September 1995. (Photo by Øyvind Hagen, courtesy of Statoil)

Reliability issues
“In the early days, under-reaming was a necessity,” Shale said. This portion of the work often ended in problems due to the lack of reliability inherent to most under reaming tools. “The early under-reamers were definitely one thing – unreliable,” he added. Secondly, typical reamers were lacking in cutting structure. “What would happen is when the tool would go into a soft formation, it would work fine, but as soon as it got into a formation that had any hardness to it, such as 10 to 15 thousand psi UCS-type formations and above, the hydraulics would allow the cutter blades to go in and out according to the formation hardness.” The end result would be an hourglass effect, which often effected casing running operations. Early under-reamers were renowned for unreliability, premature activation, and well bores that were not always conducive to smooth casing running operations.

The activation process also has been improved for the variety of expandable reamers available on the market today. Because these tools are designed to pass through a cased hole prior to “doing their job,” early activation can cause problems if the tool expands inside the casing string. In offshore projects involving high-pressure formations, damaged casing can wreak havoc on the economics of a well in terms of nonproductive time (NPT) as well as damage to the tool itself.

Typical activation for most under-reamer tools involves hydraulics, mechanical, or both. Many tools used shear pins in their early versions. Activation could be tricky, depending on the drilling conditions at the shoe. Constant weight on bit and vibration – apart from torsional and stick/slip – could create a great deal of bit bounce, which often led to early shearing of the shear pin activating the tool within the casing. This meant an extra trip for the driller along with NPT for the operator.

Baker Hughes' Hughes Christensen 14 3/4-in. GaugePro XPR expandable reamer has been used in several landmark applications. (Images courtesy of Baker Hughes)

Reliability today
One example of the positive developments in under-reamer technology can be found in Baker Hughes’ Hughes Christensen Gauge-Pro XPR expandable reamer. Part of the GaugePro XPR success comes from extensive use in several landmark applications. “We have learned to see what the problems were and now we can overcome those problems by introducing new tools – better mousetraps, basically,” Shale said.

Improvements in shear pins have been made by multiplying the number of shear pins. In some cases, reamer tools can have a pin that shears internally yet requires additional weight on the tool to push a sleeve out of the way before the tool can fully activate, using either hydraulics or mechanical means to activate the cutters for hole enlargement.

In the case of the GaugePro XPR, the system is ball drop-activated. “All the tools are ball drop-activated at the moment. This is the current generation of tools,” Shale said. The GaugePro XPR has a direct operation so that it maintains a dead-head pressure acting on a ball, but rather than using pins, the system relies on set screws. “These set screws are an engineered set that uses materials with a known tensile strength, and that tensile strength will shear at a certain psi that is acting on that tool,” Shale said.

Typically, the screws are made in batches from the same material, which is certified to a certain tensile strength. By testing each batch at a certain pressure, engineers can determine tensile strength within +/- 5%. With these engineered screws, the GaugePro XPR reamer shear screws work through a hydraulic method rather than mechanically. “This hydraulic method can be controlled much more accurately by allowing the pumps to stroke slowly so the pressure downhole builds up,” Shale said. At activation, the pressure drops. This signals that the internals of the tool have been activated and the nozzles are open, allowing fluid to emit from the ID ofthe tool into the annulus of the well bore, which provides proper cooling and cleaning functionality of the reamer.

Approaching integration
As the industry has moved further offshore in recent years, the challenges have increased for under-reaming capacity. Where deep water once was considered to be 1,000 ft (305 m), common water depths for many drillers can reach 7,000 ft (2,134 m). In the southern Gulf of Mexico (GoM), Pemex has plans for several ultra-deepwater wells that could exceed 8,000 ft (2,438 m) between 2015 and 2020. The GaugePro XPR was successfully deployed in a world-record 31,400-ft (9,573-m) deepwater well in the GoM. As with most offshore operations today, the forces impacting on equipment is quite substantial.

One benchmark example of current reamer technology’s improved role in the integrated drilling process took place in Norway’s Troll field. Conventional drilling methods can be problematic in extended-reach horizontal wells.Known as one of the world’s most difficult drilling environments, Troll field presents many challenges. Due to the changes in formation structure, increased vibration is a common problem for drilling these wells. As the formation produces vibrations, the associated harmonics often can cause damage to the drillstring, its components, and surface equipment. The operator was limited to how much drilling could be done. “It was possible to use two or three bits and several trips to get to a certain position within the well bore because of the damage and the vibration to both the equipment and to the actual cutting structures of the drill bits and the reamers,” Shales said.

The GaugePro XPR is not a standalone tool. “We designed the reamer to work as an integrated system first of all by synchronizing the reamer with a Baker Hughes bit so that both tools would have similar designed cutting structures that could support one another,” Shales said. The bit is the most aggressive part of the drillstring; this was greatly improved through the integration of the GaugePro XPR. At Troll field, as the bit and bottomhole assembly (BHA) were in compression, drilling through a hard stringer or a hard section of the formation, the reamer cutters could be anywhere from 80 to 130 ft (24 to 40 m) above the bit because of the integrated BHA and application. Once the bit drilled the harder stringer or the section 80 to 130 ft later, the reamer would try to match this performance as it drilled through that same formation, enlarging that section.

While the bit had drilled through the harder section into a more homogeneous formation that was softer than the hard stringer, the bit would drill away much faster until the reamer came down to the point where the bit had drilled through the hard formation. As the reamer started to drill through the hard formation, the ROP would be slowed down dramatically. Meanwhile, the bit was still in the softer formation and would drill itself off the formation. So the bit would go from compression into tension, which means the lower BHA below the reamer was in tension. As the lower BHA is in tension, everything is hanging on the reamer, so all of the BHA – that 80 to130 ft below the reamer – did not have compression on it, allowing lateral and torsional vibration, causing damage to the BHA, bit, and reamer cutters.

Baker Hughes’ Hughes Christensen 12¼-in. Genesis Premium HCD506ZX PDC bit is synchronized with the reamer cutters.

By designing the bit and the reamer to be synchronized, Baker Hughes matched the bit and reamer cutter technology so as the bit drilled off in the softer formation, the bit did not just drill away, allowing the BHA to remain in compression. Contrary to consensus within the industry, simply matching cutter size in the bit to cutter size in the reamer does not provide a “matched” system and mitigate vibration. “We have known this for a while because the multiblades on the bits are much more aggressive than the reamer,” Shale said. Theactual loading per cutter on the bit was removing much more material than the reamer, so the bit was more aggressive and could drill off faster than the reamer. The results in Trollfield speak for themselves. The operator found savings through performance improvements of reduced stick/slip and whirl and improved wear on the reamer and other equipment.

Looking ahead
The under-reaming world is dramatically changing. As clients become more demanding, reliability will continue to be a driving force for this technology. A variety of applications are generating tools that are rapidly becoming more sophisticated. Most tools are ball-drop activated and, in the beginning, consisted of a single ball. Tools now are designed for multiple balls. “In other words, you can drop more than one ball, so you can open and close the tool. Closing the tool allows you to perform different operations,” Shale said.

Where reamers once were considered a stationary tool with a limited, but necessary application, they are rapidly being integrated into an ever advancing toolstring.

Editor’s Note: A portion of the technology discussed in this article was sourced from SPE paper #138708, “First Worldwide Horizontal Run and Eastern Hemisphere Application of an Expandable Reamer and Stabilizer BHA on Troll Field, Norway,” by Hugh D. Evans, SPE Baker Hughes, and Lydia E.B. Ulvedal, SPE, Statoil.