Drilling advances

	Vol. 227 No. 3
LES SKINNER, PE CONTRIBUTING EDITOR

Snub drilling. Snub drilling, or “jack drilling” is not a new concept. Snubbing units have been used in some form of drilling from the time that hydraulic jacks replaced the old cable and pulley systems used for years on drilling rigs. Now, their use as compact drilling rigs has come to the attention of those hungry to poke a hole in the ground, but lacking a derrick, substructure and drawworks to do so.

“Snubbing” is generally considered to be the act of running pipe into or pulling it out of a well that has pressure at the surface. The snubbing unit is suited for this work since it can push pipe in the hole against well pressure that is trying to push it back out of the hole. At some point when the buoyed weight of the string in the hole equals the expulsion force plus friction, the string is called “pipe heavy.” It can then be lowered in the hole under its own weight. Still, since there is pressure on the well, running the remainder of the hole in the well is considered snubbing. Block and tackle gear can’t do this job since they rely on gravity to pull the pipe into the ground.

Snubbing units have been used extensively in service operations for many years. They can contain the pressure, within limits, so there may not be a compelling reason to kill the well. Live well work has become essential in many wells for a simple reason. Once some of these old dogs are killed, production can never be restored due to excessive fluid loading and near-wellbore damage.

Operators are using snubbing units routinely for sidetrack/ re-drilling work to recondition old wells. With oil at $14 per barrel, these were hardly economical in the past. Now at $60+ per barrel, a lot of older wells are becoming attractive candidates for reworking, but not if it means losing the hole.

One type of grass roots-to-granite drilling that is clearly suited for a snubbing unit is underbalanced drilling (UBD). By definition, there must be pressure at the surface in UBD. If there were not, it would either be balanced or overbalanced. So, with even moderate pressure at the surface, snubbing must take place before the drill string can be run into or pulled from the hole. Some UBD projects employ a “rig assist” snubbing unit as part of the drilling package just for this purpose.

OK; if there’s going to be a snubbing unit there anyway, why not use it to drill? After all, a snubbing unit can move the pipe up, down and around like a rig. It simply uses hydraulic cylinders to hoist and lower the pipe and a hydraulic motor to turn it.

Snub drilling has gotten a bad rap over the years due to excessive daily cost and slow tripping speeds. They are not generally equipped with all the ancillary equipment vital to the drilling process such as pumps, pits, kelly hoses, electric generating equipment and a derrick in which to rack pipe in thribbles (three segments per stand). All of this equipment must be rented as separate items, manifolded together somehow with the snubbing jack and made to operate as a single integrated unit. Generally, more rented equipment implies greater cost.

Tripping speed seems to be the greatest problem, however. Traditionally, snubbing units can only pick up, run, pull and lay down single Range 2 pipe joints. This correlates to a lot of manual handling of singles, some degree of risk both to the pipe and to the people doing it and, of course, time. Generally, more time also implies greater cost.

Various snubbing companies have met the challenges in different ways. One company attached a special basket, complete with finger-boards, to their jack and installed a telescoping boom that can handle two joints of pipe at once. This provides racking capability, which reduces trip time. The pipe can literally be run and pulled as fast as the jack can cycle, using this system. Other companies have erected towers or some type of scaffolding to allow vertical pipe racking. Generally, these require more room on the location.

Larger snubbing units are equipped with a hydraulic rotary table mounted on top of the jack. This is often used to drill out fill or to mill on a stubborn piece of junk for a limited time. They are usually not well suited to long-term rotary motion – they simply aren’t made for it. Their life’s blood comes from the same source as the rest of the snubbing jack, the hydraulic power pack. Hydraulic fluid heats as it transfers horsepower from one unit to the other. For long-term drilling, an oil cooler or a second rotary motion source will probably be required with its own hydraulic power pack, such as a power swivel – the ubiquitous “rotary table in the sky.”

Cost for the equipment spread is also becoming more tolerable for snub drilling as drilling rig rates continue to soar. Rigs that would bring only enough money to buy the owner a new set of tires for the pickup a few short years ago are now going for daily spread costs that would make a robber baron gasp. Equipping a snubbing unit for drilling may be economically practical, especially if it’s the only thing available.

Tripping speed? This is still a problem, since a jack simply cannot move as fast as a block and tackle assembly. In UBD or potential live well drilling, the pipe can only be tripped so fast anyway. In these operations, tripping speed is limited to that allowed by the rotating control head, whatever style it may be. The newer snubbing units with pipe racking capability can actually trip pipe as fast as a drilling rig in these types of operations. I’ve seen it.

What about setting surface casing? Unlike a free-standing drilling rig, a snubbing unit normally rigs-up on a wellhead with the casing supporting the weight of the entire stack. Solution: most snubbing companies now have their own substructures, sturdy steel frames that support the unit until the first casing string is run and cemented. Then, the snubbing unit is rigged up on the newly installed wellhead. Snubbing units still require anchors and guylines, however. 

These advances have pointed many operators toward snub drilling. Most of them formerly thought of a snubbing unit as a last resort – the worst piece of equipment that could show up on a location. They are beginning to reconsider.

Once operators realize that creative use of available equipment is needed in times of extremely high drilling rig use, a snubbing unit may be the rig of choice, if that hole in the ground is going to get drilled at all.