Bullheading is one of the well control methods which may be utilized in some occasions in order to control the well. Concept of bullheading is to pump kicks back into formations by using kill weight fluid. People usually use this method when normal circulation is impossible and volumetric method is not feasible to perform.
When May You Consider Using the Bullheading Well Control Method?
- When the kick size is very big so you may not be able to control the excessive volume coming to the surface.
- When you need to reduce surface pressure in order to start further well control operations.
- When there is a possibility to exceed surface pressure and volume gas on the surface if the conventional methods (drillers’ method, wait and weight and volumetric) are performed.
- When there is no pipe in the hole while taking influx.
- The influx contains high level of H2S which can cause safety of personnel on the rig.
- When there is no feasible way to strip back to the bottom in order to kill in the flux below.
For every drilling operation, decision to perform bullheading must be discussed because if the well is shut in and wait for a long time before making decision to bullhead the well, it might be very difficult to perform because the surface pressure at that time may increase so high due to gas migration. The chance of pushing the kick back into reservoir becomes smaller.
Note: Bullheading may or may not fracture formations.
There are some factors affecting the feasibility of bullheading as listed below;
Reservoir permeability – pumping fluid back into low permeability reservoir takes longer time than pumping into high permeability zone. It might require breaking the formation in order to successfully bullheading the well.
Surface pressure rating – rating of surface equipment as BOP, wellhead, casing, etc will limit the maximum allowable pumping pressure.
Type of influx – Gas influx will migrate and it will increase surface pressure, however, liquid influx (oil or water) will not cause increasing in surface pressure because it will not migrate.
Procedure of Bullheading (Example)
This procedure below will give you only overview of how to perform bullheading therefore you must need to add the site specific information before conducting the actual work.
- Determine surface pressure limitation of surface equipment.
- Calculate surface pressure which will fracture formation during bullheading operation.
- Prepare a bullheading pressure chart representing strokes pumped vs pumping pressure.
- Ensure correct line up.
- Bring the pump to speed at low rate to overcome surface pressure.
- Slowly increase pump rate to the planned pump rate.
- Closely monitor tubing, casing pressure to ensure that pressures will not exceed the equipment limitation at any stage of operation.
- Slow down pump rate when the kill fluid close to reservoir. You will see surface pressure decrease over time while pumping kill mud into the well because the kill weight mud will increase hydrostatic pressure.
- Observed pressure increase when the kill weight fluid is pushed into formation.
- Shut the pump down and shut in the well.
- Monitor pressure. Bleed trapped pressure if required.
We wish you would get more understanding about the bullheading well control. Additionally, we will demonstrate some calculations related to this topic. Please feel free to leave any comments.
Reference books: Well Control Books