Inserting or removing tubing from oil and gas wells is a common part of well control procedures and is referred to as "tripping". If the well has pressure at the surface, the pressure will push against the tubing and try to eject the tubing from the well. The magnitude of this upward force is equal to the wellÍs surface pressure times the cross sectional area of the tubing being tripped. The fluids in the well will provide a buoyant force that also tries to eject the tubing from the well. The magnitude of this force is equivalent to the weight of the fluid displaced and is obtained by multiplying the fluid density times the volume of the tubing in the fluid. There is also a downward force on the tubing equal to the weight of tubing in the well. This force tends to push the tubing down into the well.

For a well under pressure, it is common to initially need to supply an external force to the tubing to overcome the well pressure and buoyant forces. In this case the tubing string is referred to as being "light". However, once a sufficient amount of tubing has been inserted, the weight of the tubing in the well exceeds the upward forces, and an external force must be applied to prevent the tubing from falling into the well. A tubing string in this condition is referred to as being "heavy". Inserting or removing tubing from a well in the heavy condition is referred to as "stripping". Inserting or removing tubing from a well in the light condition is referred to as "snubbing". Snubbing is therefore the operation of inserting or removing tubing from a well when an external force must be applied to prevent the tubing from being ejected from the well.

CALCULATION OF BALANCE POINT / SNUB FORCE:

Every well that a snubbing unit works on is subject to wellbore pressure. This pressure is constantly pushing up on the plug that is in the bottom of the tubing. To compensate for this upward thrust on the tubing, the balance point of the tubing must be determined. This upward force is known as the snubbing force.

An understanding of the "Calculation of Balance Point" must be a part of the general knowledge for every person on a snubbing unit. This calculation lets a person know when the pipe will be in a transitional state.

To determine exactly at what point transition will occur is very difficult when a few variables are present, like bag drag, fluid in the hole, flowing the casing, etc.

Due to the constantly changing factors the calculations give the snubbing operator an approximation as to when transition will occur within a couple joints plus or minus.

This is the most precise way to calculate:

• Determine the OD of the tubing and square it.
• FORCE CALCULATIONS: We try to keep our calculations in imperial form kPa s become too large a number in snub force. 60.3MM TUBING 2.375 2.375 tubing diameter X 2.375 tubing diameter = 5.64 tubing area.
• Multiply the answer from step 1 by the constant 0.7854. This equals the area of the bottom of the tubing. 5.64 tubing area X 0.7854 constant = 4.43 sq.inch / snub force factor
• Multiply the answer from step 2 by the well pressure. This equals the pounds of thrust upon the tubing. 4.43 snub force factor X 500 psi well bore pressure = 2214.8 LBS snub force/thrust
• Take the weight of tubing per foot and divide it into the lbs of thrust up determined in step 3. This gives you the length of pipe in feet to reach the balance point [properties of tubing in HIGH ARCTIC tally books]. 2214.8 lbs snub force / thrust divided by 4.70 lbs per foot = 471.23 ft
• Divide the answer from step 4 by the average length of a joint of pipe which is 31 ft. 471.23 ft / 31 feet = 15.2 jts