The successful introduction of Rotary Steerable Systems (RSS) in the late nineties has spawned the exponential growth of the Variable Gauge Reamer (VGR). More particularly and where high-cost Rotary Steerables are run and stuck pipe is a concern, they are used to reduce problems when tripping or when running casing.

In the North Sea, VGR tools were originally used to enhance ECD, improve cementing or casing tolerances and help set sand screens or gravel packs. In other locations – such as West Africa – they were used to counter swelling shales or salts. Known as concentric hole openers in the United States, the VGR was initially designed to help reduce clearance between casing strings (Gulf of Mexico) and, when originally introduced by Trimax Inc., they were pitched against the early bi-centre bits and their alleged limitations, such as unpredictable directional control, fluctuating torque, over gauge hole size, etc.

VGR (cutter blocks out)

VGR (cutter blocks in)

The first generation VGR tools were weight set and relied on a simple shear pin arrangement to retain the cutters in the retracted position as the tool drilled through the casing shoe. Once in open hole weight would be applied, the shear pin would hopefully snap, and the cutters would expand and cut a larger hole size. Of course, shear pins are notorious for not always performing as required and several 9,000-foot North Sea reamer runs behind rotary steerables have been made with the cutters retracted when every indication said they were out. These early weight-set tools had no surface signal to inform the driller the cutters had expanded – that is, they relied solely on increased torque telling them that they really were enlarging the hole.

In truth, the VGR market did not really begin to take off until the introduction of RSS and the need to ensure the enlarged hole was concentric throughout the length of the run and today their utilisation above AutoTrak, PowerDrive etc. has become standard practice.

At the same time other weight-set VGR tools began to enter the market (Andergauge) but these tools followed the original route taken by Trimax – weight-set with shear pins to prevent the tools actuating inside the casing. It was not until the introduction of the hydraulically actuated VGR (Halliburton and Andergauge) and later the ability to do away with the shear pin altogether (Smith) that the industry realised the full potential of variable gauge reaming tools. Additionally, these second generation tools were beginning to offer fully bi-directional cutting blocks – that is, their cutting action was the same whether drilling or back reaming out of the hole.

The entry of two major bit companies into the VGR market (Halliburton Security and Smith Industries) meant that many years of pdc cutter expertise was finally applied to the design of VGR cutter blocks. This was seen as an important advance particularly when one considers the stresses placed on expandable tools (basically a tube with holes cut out of it) and the dire consequences if a cutter block is lost downhole or if the cutter blocks fail to retract prior to pulling into the shoe.

One bit company’s engineering manager recently commented that the bit/VGR cutter relationship was like entering a race with a thoroughbred (the bit) tied to an old nag (the VGR). It’s not until you can get both bit and VGR working in harmony that you see the best results from the variable gauge reamer.

Axial and lateral vibration forces are one of the biggest concerns seen to date with VGR tools and it’s not uncommon for some tool bodies to show cracks after only 200 hours down hole. Cutters blocks have also been known to be left down hole – as a recent CHO run in Brazil demonstrated – and the advantage of having a tried, tested but above all reliable downhole actuation mechanism can not be underestimated.

The other requirement is the need to reliably retract the cutter blocks when pulling back into the casing shoe. There is little use drilling 9000 feet with expanded cutter blocks if it is then not possible to get out of the hole because those same cutter blocks will not retract into the VGR’s body. This has happened with both weight-set and hydraulically actuated tools (North Sea) and is without doubt the greatest fear for today’s drilling engineer.

New VGR tools are currently being designed and engineered which will address many of the problems highlighted above and in particular the need to reduce the stress on cutter blocks whilst ensuring those same blocks retract first time every time when the tool is pulled back into the shoe. The shear pin will become a thing of the past and the use of already tried and proven hydraulic actuation mechanisms will allow new VGR tools to offer variable gauge cutting positions – the ability to change the cutting gauge downhole, for example. (Today’s VGR cutters are either fully retracted or full extended and, because the original weight-set tools were designed before the introduction of rotary steerables – now the biggest market for today’s Reamers – meaning the standard cutting change may be larger than necessary for certain applications.)

The third generation of VGR tools will be similar to the new generation of variable gauge stabilizers and will offer a larger overall cutting range but in addition will give the driller the choice of two or more cutting positions. This ability to choose cutter range downhole not only reduces the volume of unnecessary cuttings coming up over the shakers but also the amount of cement needed after running the casing. In addition, these new tools will provide guaranteed surface indication of cutter block position, will be oil filled (thereby reducing internal wear and tear) and will ensure cutters retraction on demand. The new VGR tools will also be multi-functional – that is, the driller can expand/retract the cutter blocks as many times as required and this ability also adds a new dimension to VGR runs of the future. Gone will be the need to drop a ball and hope the tool has opened; the new tools will be as simple and easy to use as the new generation of variable gauge stabilizers such as the hydrastab.

There is currently a race on to develop better and more reliable VGR tools and, with the global RSS market continuing to expand, it seems certain there will be a major shake up within this part of the industry.

The service company majors have already seen the advantage of having a reliable and field proven mechanical mechanism that can multi-task inside a number of downhole tools. It’s something the car industry has been doing for a decade or more, such as when the same chassis platform is used to construct a varying number of different car types. Volkswagen, Seat and Lada, for example, all use the same tried and tested platform for a number of their best selling cars.

With drilling tools becoming ever more sophisticated (electronic and electro-mechanical) and therefore more expensive if lost in hole the requirement for increased mechanical reliability has never been greater. Multi-tasking mechanisms – especially in the Variable Gauge Reamer and Variable Gauge Stabilizer business – make much sense and it should not be too long before we see a multi-positional variable gauge stabilizer – that is, 12 1/4- to 15-inch that is purpose-designed to run with today’s ever more sophisticated bi-centre bits.