Tripping pipe into and out of a well constitutes a commonplace activity in completion and workover operations. However, alarming statistics reveal that a significant number of kicks occur during these trips. Consequently, gaining a profound understanding of the fundamental principles associated with tripping is imperative in completion and workover endeavors.
What is Surge?
The descent of tubing into the well (tripping in) generates pressure exerted at the well’s bottom. As the tubing is introduced into the well, the well’s fluid must ascend to exit the volume being encroached upon by the tubing. This simultaneous downward movement of the tubing and upward movement of the fluid, often referred to as the piston effect, leads to a rise in pressure at any given point in the well. This pressure escalation is commonly termed surge pressure.
What is Swab?
Conversely, the ascent of tubing from the well (tripping out) impacts the pressure applied at the well’s bottom. When withdrawing pipe from the well, fluid must descend to fill the void left by the tubing. The collective result of the tubing’s upward movement and the fluid’s downward movement is a reduction in bottom hole pressure. This pressure decrease is known as swab pressure.
Key Parameters Affecting Surge and Swab
Both surge and swab pressures are influenced by several key parameters:
- Velocity of the pipe, or tripping speed
- Wellbore geometry (annular clearance between tools and casing, tubing open-ended or closed off)
- Fluid viscosity
- Fluid density
- Fluid gel strength
It is evident that higher tripping speeds lead to increased surge and swab pressure effects. Similarly, greater fluid density, viscosity, and gel strength amplify the tendency for surge and swab effects. Additionally, downhole tools like packers and scrapers, characterized by minimal annular clearance, further accentuate surge and swab pressure effects.
Accurate determination of surge and swab pressures can be achieved through the utilization of drilling hydraulic calculator programs, or by referencing hydraulic manuals.
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.