Well Control - Pipe Surge/swab

Pipe Surge/swab

The total pressure acting on the wellbore is affected by pipe movement upwards or downwards.Tripping pipe into and out of a well is one other common operation during completions and workovers. Unfortunately, statistics indicate that most kicks occur during trips. Therefore, understanding the basic concepts of tripping is a major concern in completion/workover operations. Downward movement of tubing(tripping in) creates a pressure that is exerted on the bottom of a well. As the tubing is being run into a well, the fluid in the well must move upward to exit the volume being entered by the tubing.The combination of the downward movement of the tubing and the upward movement of the fluid (or piston effect) results in an increase in pressure at any given point in the well.This increase in pressure is commonly called Surge pressure. Upward movement of the tubing(tripping out) also affects the pressure which is imposed at the bottom of the well. When pulling pipe from the well,fluid must move downward and the replace area which was occupied by the tubing. The net effect of the upward movement of the tubing and the downward movement of the fluid creates a decrease in bottomhole pressure. This decrease in pressure is referred to as Swab pressure. Both surge and swab pressures are affected by the following parameters:

• Velocity of the pipe,or tripping speed
• Fluid density
• Fluid viscosity
• Fluid gel strength
• Well bore geometry (annular clearance between tools and casing, tubing open ended or closed off)

The faster pipe is tripped, the higher the surge and swab pressure effects will be. Also, the greater the fluid density, viscosity and gel strength, the greater the surge and swab tendency. Finally, the downhole tools such as packers and scrapers,which have small annular clearance, also increase surge and swab pressure effects. Determination of actual surge and swab pressures can be accomplished with the use of WORKPRO and DRILPRO calculator programs or hydraulics manuals.