How Snubbing Units Work IN THE OIL AND GAS INDUSTRY
History of Snubbing
Units
Snubbing units have evolved into one of the most capable and
efficient well servicing tools in the oil & gas industry. In the 1920's, the need for a rig to work with
pressures at surface drove the invention of the snubbing unit. The first snubbing unit was primarily designed
to work in well control situations to "snub" drill pipe and or casing
into, or out of, a well bore when conventional well killing methods could not
be used. The first snubbing unit relied
on the draw works of the companion rig to supply its' power. A series of sheaves, cables and counter
weights were rigged up so that as the rig's traveling blocks hoisted up, the
snubbing unit would snub in the hole. Conversely,
when the traveling blocks on the rig were lowered, the snubbing unit would snub
out of the hole. As you can imagine, this required close communication with
several different contractors in order to perform the work safely and
efficiently.
Components of a Snubbing
Unit
One of the main components of a snubbing unit is the slip. Stationary and travelling slips are operated
in sequence to grip the pipe as it is snubbed into the well. Typically, a minimum of four slip bowls are
used in snubbing operations. Two slip
bowls are designated for "pipe light" operations. Pipe light is when
the well bore forces are greater than the tubular weight in the well bore. The other two slip bowls are designated for
"pipe heavy" operations. Pipe
heavy occurs when either enough pipe has been snubbed into the well bore and
fluid weight inside of the pipe is greater than the snub forces acting against
the pipe in the well bore.
While snubbing into the hole, there is a transition point the
tubular goes through from being pipe light, to pipe heavy. This transition is an equilibrium typically
referred to as the "balance point". The balance point occurs when there is enough
pipe weight in the wellbore to equal the snub forces generated against the
pipe. In certain instances, thousands of
feet of pipe can be moved with minimal effort since the pipe weight is at an
equal state with the snub forces. Snubbing
contractors calculate this snub force and add in a friction factor from the BOP
and wall contact on either a casing or tubing string. If done correctly, the snubbing contractor can
predict when this balance point will take place and can properly prepare for
it.
Modern snubbing units are powered by sophisticated hydraulic
systems. These hydraulic units typically supply all power required by the components
of a snubbing operation. With a better understanding of hydraulics and modern
advances, companies have been able to harness this hydraulic energy to develop
precision controlled snubbing units. These units move tubulars into and out of
a well bore by use of a "multi cylinder jack"; a snubbing jack comes
in many sizes depending on the task at hand. They are usually denoted in size
by the snubbing unit description (i.e. 460K, 340K, 200K, etc). The 460K
snubbing unit has the ability to lift 460,000 LBS and a snubbing capacity of
230,000 LBS. Most snubbing units can typically snub half of their lift rating.
Assume you had a well with 10,000 PSI at surface and wished to snub in a string
of 2 3/8" tubing. The snubbing
contractor can calculate the snub force, add in their respective friction
calculations and project the snub force to overcome will be approximately
51,000 LBS. This would put a 120K
snubbing unit to close to its maximum capacity of 60,000 LBS snub loading. The safest bet would be a 150K or 235K
snubbing unit.
Well control is taken very seriously by snubbing contractors. The BOP is the only barrier between the well
bore and personnel. Depending upon well
conditions, pressures and work performed, the BOP stack configuration varies
greatly; there can be a minimum of three BOP's and in some cases, up to ten. All of this is determined in the pre-job phase
of the operation.
Pipe handling is performed by the snubbing units "gin
pole" and "pipe winches". The gin pole is typically telescoped out in
excess of 40ft above the snubbing unit. With
the use of dual tubing winches, multiple joints of pipe can be handled
simultaneously, speeding up the operation.
The snubbing "basket" is the platform where the
snubbing personnel work. The basket
contains all of the necessary hydraulic controls to operate all the features of
the snubbing unit, as well as a large bank of BOP's and hydraulic valve
controls.
Snubbing Unit
Applications
Today's snubbing units can be employed to provide a wide range
of services. In essence, a snubbing unit is a hydraulic rig that can do
everything a rig can do, plus it can perform under pressure in an under
balanced live well state. This is especially critical to the operators in the
Haynesville Shale, which is known for HPHT wells. With the use of the snubbing
units' hydraulic rotary, the unit can be employed for fishing, milling,
drilling, side tracking or any task needed to remove bridge plugs, cement or
deepen wells.
Advantages
The industry has become more aware of damages caused by heavy
kill weight fluids and mud. This has helped make snubbing units more popular in
a completion and workover role, versus its' traditional use as a well control
response tool. With the advances in drilling technologies in the unconventional
shale market, the benefits of snubbing units have become very apparent. These
types of completions often have laterals extending out thousands of feet. With
costly stimulations used to help extract the gas more efficiently, operators
often times do not wish to turn around and load the well with heavy fluids to
complete the well dead.
Coiled tubing has its limitations in reach, due to wall to wall
mechanical friction in horizontal wells. Often times the coiled tubing units
cannot reach TD or supply the needed weight on bit to mill up composite plugs
typically used in completions.
Another clear advantage to using a snubbing unit is its' small
footprint, which is critical on the tight locations in the unconventional
shale's. Moreover, the small size and ease of mobilizing is especially useful
and cost effective with offshore wells.
In conclusion, with the snubbing unit's size, ability to handle
pressure, rotary capabilities, rigidity of jointed tubing and minimal wall
contact, snubbing units have become the chosen resource for these types of
completions.