There are several situations when a kick is induced by swabbing effect. Today, we are going to discuss swabbing and well control situation. Swabbing is a condition when the string is pulled out of the well and it creates temporary bottom hole pressure reduction. If the hydrostatic pressure reduction is large enough to create underbalance condition, the well will eventually flow.
When you swab the fluid in, the swabbed fluid may not necessarily cause pit gain or the well flowing because the volume swabbed in is not significant. However, if you have several swabbed-in fluid, the well will finally flow.
Figure 1 – Take Swabbed Kick
It is quite tricky to recognize the swabbing volume and the most trustable method to detect is by tracking hole fill volume. For example, if the volume displacement for 10 stands pulled is 8 bbl but the hole fill volume is just only 6 bbl, 2 bbls of kick may possibly be swabbed in while tripping out. Once the swabbing is detected, you need to trip back to the bottom and circulate bottom up even though the well is not flowing. If you don’t go back to the bottom, it will be very difficult to control the well off bottom once the swabbed gas moves up to shallower depth of the well.
In some situations, you may need to consider performing a short trip operation to determine the effect of bottom hole reduction by swabbing and loss of equivalent circulating density. The short trip becomes very critical when you drill into an unknown pressure zone. For this case, the result from the short trip will tell you whether you need to raise mud weight or not.
Factors that increase a chance of swabbing in are as listed below;
Balled up Bit and BHA – The balled up bit/BHA acts like an excellent piston and this will cause greater swabbing effect. If the well is at near balance condition, the well will have more chance to be underbalance due to swabbing.
Formation pressure vs hydrostatic pressure – If the hydrostatic pressure is equal to or slightly above formation pressure, the well can be swabbed in so easy. In order to mitigate this issue, you need to have overbalance margin (trip margin) more than pressure reduction by swabbing.
Mud Properties – poor mud properties as high rheology, high viscosity, high gel strength, etc have high tendency to induce swab kick while pulling out. It is very critical to monitor the drilling fluid properties and personnel should have action plans to keep the mud in a good shape.
Pulling Speed – Faster tripping speed, higher chance to swab influx. It is very critical to monitor the well while pulling out and the pulling speed must not induce the well control situation.
Larger OD of Drilling Tools – Larger tools as fishing tool, coring tool, drill collar, mud motor, etc enhance swabbing tendency. Carefully tripping with larger tool is a key success to prevent the problem.
Swelling/Heaving Formations – Swelling and heaving formation will reduce wellbore diameter resulting a small clearance between an open hole and a BHA or a bit. While pulling out with a small clearance, it has higher chance to swab in the well.
How To Minimize Swabbing
There are several items which can minimize swabbing as listed below;
- Keep the mud in good condition
- Pull out of hole with reasonable speed
- Add lubricant additives and maintain good drilling hydraulic to prevent bit/BHA balled up
- Add chemical to prevent clay swelling in water based mud or use oil based mud drilling into clay formation
- Pump out of hole instead of pulling out
Reference books: Well Control Books