SNUBBING

Snubbing is the process of running or pulling tubing, drillpipe, or other tubulars in the presence of sufficient surface pressure to cause the tubular to be forced out of the hole. That is, in snubbing the force due to formation pressure's acting to eject the tubular exceeds the buoyed weight of the tubular. As illustrated in Figure 6.1, the well force, F _w, is greater than the weight of the pipe. The well force, F _w, is a combination of the pressure force, buoyant force, and friction force.

Figure 6.1: Snubbing

Stripping is similar to snubbing in that the tubular is being run into or pulled out of the hole under pressure; however, in stripping operations the force resulting from the surface pressure is insufficient to overcome the weight of the string and force the tubular out of the hole (Figure 6.2).

Figure 6.2: Stripping

Snubbing or stripping operations through rams can be performed at any pressure. Snubbing or stripping operations through a good quality annular preventer are generally limited to pressures less than 2000 psi. Operations conducted through a stripper rubber or rotating head should be limited to pressures less than 250 psi. Although slower, ram-to-ram is the safest procedure for conducting operations under pressure.

Some of the more common snubbing applications are as follows:

• Tripping tubulars under pressure

• Pressure control/well-killing operations

• Fishing, milling, or drilling under pressure

• Completion operations under pressure

There are some significant advantages to snubbing operations. Snubbing may be the only option...