Above: A delegation visits a U.S. energy production company to see firsthand that pneumatic drilling is safe, faster than rotary and is often the more economical choice in many deep-hole applications. As long as hard rock formations that require weighted drilling fluid to control the formation are accessed, tricones will have their place in the drilling industry. In all other conditions, today’s drillers have alternatives. These include new polycrystalline diamond compact (PDC) bit designs for both air and mud drilling in soft rock and sediment and new hammer designs that make directional and horizontal deep-hole drilling faster and more cost effective.
For decades, drillers have known that the pneumatic down-the-hole (DTH) hammer drilling technique is generally two to five times faster than the rotary mud technique in rock. The harder the rock, the more efficiently hammer drilling is when compared to tricone rotary mud drilling.
However, today’s range of hammer designs is not limited to hard rock. Rates of penetration (ROPs) as high as 500 feet per hour have been accomplished in soft formations. There are some ultra-soft materials, most likely organic in composition, in which there would be no advantage to using a hammer. Hammer drilling ROP is limited only by cutting management. In other words, you can drill only as fast as you can effectively clear chips from the bottom of the hole with compressed air exiting the face of the bit.
Pneumatic Deep-Hole Directional Drilling
Directional drilling with DTH hammers has recently improved how unconventional oil and gas formations are drilled. This not only includes drilling vertical top holes ahead of conventional rigs but also forming the curve to horizontally drill the production laterals. A patented directional hammer drilling system was developed around existing DTH products. The hammer in this new system required little modification. Once the blow-down sequence was fine-tuned to control the downhole directional motor rotation when off bottom, the main adjustments were to the drill string. Jet subs manage the annular pressures and control the downhole motor while on bottom, drilling ahead. Air enters the annulus, providing the extra lift necessary to return the cuttings up a column that eventually becomes thousands of feet long.
While weighted mud cannot be used with a hammer, other additives can. Mixtures of water and polymer make the water denser, helping it lift less dense rock chips from the bottom. A hydrocyclone can be used to help manage drilling fluids, expelling the fluid before it reaches the working chambers of the hammer, which would otherwise inhibit its performance if allowed to flow through it. Making the hammer curve the hole is accomplished using a directional motor run with air. The directional hammer research and development project was conducted in the Marcellus Shale and included a drilling company and an oil and gas operator. The drilling company and a drill bit manufacturer shortened the motor to accommodate the hammer’s length, providing a sharper radius. Word of this 8-inch-class hammer’s success in the Marcellus Shale quickly spread. Today, hammer sizes range from 6 to 12 inches in diameter and include designs that are highly effective in deep-hole applications. Special DTH bits have also been designed to endure the trip to total depth even for holes that are a few thousand feet long. Not only are the well bores drilled faster, trip times that can exceed 12 hours or more are minimized. Roughly speaking, wells are drilled in half the time using hammers instead of tricones.
PDC Drill Bits
PDC bit lines provide another aggressive alternative to tricones. Rotary PDC technique provides a wide overlap in application with pneumatic hammers but proves most valuable in ultra-soft material—usually organic material that is too soft for hammers. Even at 500 feet per hour, the driller could not keep the DTH bit on the bottom. In this type formation, PDC bits become the better value.
However, they are only a better value if they are quality bits. Bit technology has improved through the years. One improvement is an alloy steel computer numerical control (CNC) machined body for maximum blade and body strength that provides a solid base for several other key design features. Tungsten carbide hard-facing in critical wear areas provides erosion and abrasion resistance and gauge protection. A parabolic profile optimizes cutter quantity and bit stability. The bit’s cutters must be perfectly balanced. Unbalanced cutters cause bit whirl, causing the bit to work inefficiently, which shortens its useful life, wastes energy and results in an imprecise hole. PDC or Hammer? How does a driller know in advance which drilling method will be most effective? In many instances, hammers will be the better choice unless the formation is extremely soft, with occasional exceptions. Kevin Mallin, a drilling consultant with Geolorn LTD of Callender, Scotland, helps plan and execute deep-hole drilling strategies at global locations for on- and offshore operations. He discussed one well for which the plan changed unexpectedly from hammers to PDC bits. In this example, the job was drilling holes to provide grout-filling access to underground mine workings. The grouting would stabilize ground prior to construction that would take place over the site. Normally, the holes would be drilled with either top hammers or DTH tools through the sandstone layers into the voids. However, once they started drilling, operators discovered an “altered” basalt flow that trended into a dyke, a T-formation in the ground profile. The basalt was brittle with ductile properties. Large pieces would break away, causing the DTH hammers to stall. Mallin decided to change to PDC bits using an air/foam flush and a stiff bottom hole assembly. The PDC bits in this case cut cleaner and much more quickly, since they were also drilling through roof timbers of the workings. Mallin believed that the basalt responded to shearing more readily than crushing, despite the rock’s compressive strength in excess of 250 MPa. Mallin said he gets great results using PDC bits in softer sedimentary and clastic rocks all the time. He recommends DTH for rock with hardness above 100 MPa. He believes the driller needs to have a greater understanding of what occurs down the hole to use PDC bits effectively. In most stable formations, except when the material is exceedingly soft, hammers are the ideal choice over tricones. If a hammer cannot be used, PDC and rotary mud are a good Plan B.