Swab and Surge

Swabbing is the process by which formation fluid is sucked into the borehole by the upward movement of the drill string. Swabbing is observed when the bit is covered with drill cuttings and their is very less annular clearance between drill string and annulus. In this situation if the drilling string is pulled out it acts like a big piston and creates a region of low pressure below the bit.This low pressure region will suck the formation fluid into the borehole.

During trip up swabbing effect is observed,formation gases are sucked into the borehole which when circulated to surface will increase the gas level and is termed as Swab gas or trip gas.If the swab effect is high we may even take a kick.

surge and swab pressure definitionsurge and swab pressure definition

Surging is the opposite of swabbing. It is the process by which borehole fluid is pushed into the formation.Surging creates a region of high pressure below the bit which forces the borehole fluid into the formation.If this surge pressure is high enough it may exceed formation fracture pressure creating fracture and cause loss of drilling mud so both high swab and surge pressure should be avoided.

Swab and surge pressure will depend on:-

  • Speed of Drilling movement
  • Viscosity of drilling mud
  • Annular clearance between drill string and borehole
  • Mud cake thickness
  • Hole cleaning

Practices to avoid high swab and surge pressure:-

  • Drill string Movement should be slow but not too slow otherwise we will be wasting to much time on running in and pull out.
  • Proper hole cleaning should be maintained
  • Mud Cake should be as thin as possible-Mud cake thickness can be controlled by controlling solid content in drilling mud