Smaller Rigs Take on Hard Rock

Every time a hole is put in the ground, it is an adventure into the unknown. Even with geotechnical site investigations, there is no guarantee for what geological formation you may encounter.

So, what do you do when you are the owner of a 100,000- to 200,000-lb class drill rig and you hit rock? Drilling in rock is a challenge for even the most experienced driller sitting at the helm of a larger drill. So how will your 100,000- to 200,000-lb class rig take on the solid rock formation you have just encountered?

Fortunately, with advances in the field of horizontal directional drilling (HDD), there are now tools designed specifically for 100,000- to 200,000-lb size drill rigs that can take on hard rock. By ensuring you have outfitted your rig with the proper tooling, you can be successful in drilling hard rock.

Pilot Hole
The first phase of any bore is the pilot hole. A typical pilot hole drilled by a 100,000- to 200,000-lb class rig falls in the 5- to 7-in. range. Rig operators have several different technologies to choose from to drill the pilot hole:

• Mud Motor with Tri-Cone Bit
• Air Operated Hammer
• Dual Pipe Mechanical Systems  (Rod in Rod)

All three of these methods work equally well for drill rigs in this class. What is paramount during this pilot phase is accurate recordings of the rock strength while drilling to your exit location. This will provide critical information for your reaming operation in terms of joint (drill rod length) times, unconsolidated or consolidated formation areas or plastic type formations.

Reaming Process
With the completion of the pilot bore, the reaming process can begin. This is where outfitting your rig with the proper tools comes into play. The pilot bit is replaced with a hole opener to enlarge the borehole. There are several factors to consider when selecting the type of hole opener to use for the reaming process.

First, consideration should be given to the geotechnical site investigation, if one was performed, as well as any data received during the pilot bore. This evaluation should provide you with insight into how hard the rock is that you have encountered. You should also consider if the rock is broken or consolidated. There are only certain hole openers that are productive in broken rock due to the extra force that is applied on the cutters when rock fragments get caught between the cutters.

Finally, you need to establish a plan for the reaming process. The final hole size is determined by the pipe you will be installing. The typical rule of thumb is one and a half times the size of the pipe being installed. For example, a 12-in. pipe would require a final hole size of 18 in. Reaming passes are usually done in stages because hole openers have a limited amount of rock that can be cut per pass. The size of the pass (or cut) is determined by the size of the cones on your hole openers. Larger cones equals larger passes (or cuts).

Generally, hole openers smaller than 20 in. are typically built with 9 7/8-in. or smaller segments whereas a 20-in. hole opener can be built with 12 ½-in. segments. The Cone to Hole Opener Revolution Ratio is significantly reduced with larger diameter cones. A reduced revolution ratio increases the life of the bearings and seal and allows the hole opener to remain in the hole for longer periods of time leading to a more cost-effective project. The result is longer downhole time without the need to pullback and change out hole openers.  

Hole Opener Selection
There are three main categories of hole openers that you can select from:

• Bolt on or Threaded Segments
• Split Bit
• Solid Body Engineered Hole Opener

Bolt On/Threaded Segments
Bolt on or threaded segment hole openers are categorized by the way the cutter segments are attached to the body of the hole opener. These types of hole openers are appealing to some contractors for their ease in replacing cutter assemblies and lower costs. However, most bolted or threaded hole openers have only three cutter heads, which inherently makes them run rough due to the triangular shape. For less aggressive situations, they certainly have their place.

Split Bit Hole Opener
The name “split bit” hole opener refers to hole openers that have been welded together using portions of tri-cone bits. Split bit type hole openers have been in the drilling arena since oilmen started prospecting for oil. Either new or rerun (used) cutter segments derived from tri-cone bits are welded to a steel plate with a shaft running through the center.

Split bit hole openers also appeal to many contractors due to their lower upfront cost. However, just like the bolted on or threaded segments, the split bit hole opener segments tend to flex and crack due to the extensive amount of welding needed to join all the segments.

Solid Body Hole Opener
Hole openers designed with a solid body have engineered cutter segments welded into machined pockets which precisely align the cutters for maximum cutting efficiency.

This design incorporates a precision–machined solid body that is welded to a hardened shaft. Engineered arm and cone assemblies are lowered into place and welded for superior strength. This process eliminates all the welds that are needed to a make a fluid chamber and segment holder that the split bit design requires. The solid body design is also very rigid so cutter flex is virtually eliminated maximizing the energy directed to fracturing the rock.

One of the benefits of this design is the ability to replace the cutter segments, which allows for multiple cutting structures to be inserted into one body transforming an otherwise disposable piece of equipment into an asset. Solid body hole openers have a higher upfront cost due to their Engineered and rebuild-able design, but their overall cost per foot can make them the better choice in the long run.

Drilling Considerations
Controlling costs, maximizing efficiency and minimizing the risk of failed bores are important considerations for any contractor. Downhole tools have a significant impact on overall costs. The right hole opener selection reduces the stress that can be placed on your drill rig and supporting equipment. Wear on equipment is a cost that no contractor should overlook. Thus, it is imperative that contractors invest in durable downhole tools.

The least expensive tool may not always be the most economical decision for a given job, especially when drilling in hard rock. One high-quality hole opener could replace two, three or four less expensive hole openers. The higher quality hole openers provide extended runtimes before reaching their point of failure. Another advantage of using a premium hole opener is the integrity a solid body design offers. Cutter segments are welded directly into this rigid body design reducing the possibility of lost cutters being left in the hole. This eliminates any wasted time fishing for tools left in the hole.

There will always be a place for the less expensive hole openers in the HDD arena. These hole openers work well in the softer rock formations. However, they do not hold up to the high rotational torque and pull forces that are needed to fracture harder rock, where a solid body design excels.

Until recently, contractors with 100,000- to 200,000-lb class rigs did not have the option to select a solid body design hole opener. Due to rig limitations, their only choice was the less expensive hole openers. With advances in the HDD field, there are now solid body hole opener designs that are made specifically for the 100,000- to 200,000-lb class drilling rig.

This new hole opener option provides these contractors with the opportunity to confidently bid for jobs they used to shy away from due to hard rock. It also offers a solution for the smaller hard rock road bores that are being encountered on a more frequent basis in tight setup sites. Smaller rigs that require a smaller footprint can now drill through the harder rock with the stronger, solid body design that has only been used by larger rigs up to this point.

Owners of 100,000- to 200,000-lb class rigs can now play in the hard rock arena. They can bid on the jobs they once overlooked. They too can see the job through to the end.

Jeff Beste is manager of HDD services of Century Products.