If normal well killing techniques with conventional circulation are not possible or will result in critical well control conditions, bullheading may be considered. Mud/influx are displaced/squeezed back downhole into the weakest exposed open hole formation.
Bullheading may be considered when the following well control situations occur:
- H2S influx cannot be handled safely by rig personnel and equipment.
- A combined kick and losses situation is experienced (downhole annulus bullhead rates must exceed the gas migration rate to ensure the situation does not deteriorate further).
- Kick calculations show that MAASP will be largely exceeded and probably results in a detrimental well control situation when the well is killed conventionally. In this case, only the influx needs to be squeezed back.
- Large influx which may result in excessive volume of gas or excessive pressure at surface if circulated out.
- Influx is likely to contain high concentration of H2S.
- Pipe is off bottom and cannot be stripped in.
- No pipe in the hole.
Factors which may affect the feasibility or success of bullheading:
Bullheading is not a routine well control method. In many cases it will be doubtful whether the well can be killed by squeezing back the influx into the formation.
A permanent loss situation may be created by pumping fluid immediately below the shoe. Each case must be judged on its own merits, considering such variables as:
- Formation permeability -The formation must have good permeability/porosity to allow squeezing. The actual kick can be used as an indicator (inflow performance prior to closing in the well and the speed of pressure build-up after the well is closed-in).
- Type of influx -A gas is easier to squeeze back than a liquid. Also, the higher the viscosity of a liquid, the more difficult it is to squeeze.
- Contamination of influx with mud -If the influx is contaminated with mud (which will be the case with most kicks), squeezing will be much more difficult, because of the plastering qualities of the mud and the possible presence of cuttings.
- Position of influx -If the influx has migrated, or has been circulated up over a certain distance, mud below the influx will have to be squeezed ahead of the influx, assuming that the weakest formation is where the influx came from.
- Discrete or strung out influx. An influx sustained while drilling may be contained in a large mud volume which may be difficult to squeeze away.
- Strength of the formation - Squeezing should be performed with the aim not to fracture any formation. In principle, the surface squeeze pressure should not exceed the pre-calculated MAASP. If heavier kill mud is pumped down the annulus, MAASP should be adjusted accordingly.
- Casing burst strength at surface and wellhead/BOP pressure rating - The casing burst strength should always be taken into account when bullheading is considered. An appropriate safety factor should be selected which depends on the condition of the casing and possible casing wear.
- MAASP due to formation fracture pressure and equipment rating.
- The consequences of fracturing a section of the openhole.
Note: Bullheading is done only if the intermediate or production casing is set.
Bullheading has the following disadvantages:
- small chance of successfully squeezing the influx back into the formation;
- fluid will go to the weakest formation which may not be the formation where the influx came from;
- there is a potential risk of fracturing formation anywhere along the open hole section which can lead to an (internal) blowout situation. In the case of shallow casing setting depths this could lead to cratering;
- high pressures may have to be applied to surface/subsurface equipment.
Even if squeezing fluid back into the formation is possible to some extent, it may not be possible to kill the well completely. Different well control techniques may then have to be employed.
It is obvious that, the earlier bullheading is being implemented, the better the chances of success. Bullheading is not a routine operation and competent authorities need to be consulted prior to implementing this well control method.
Bullheading is an acceptable method in killing completed gas wells (i.e. actual producing wells or production tested cased exploration wells).
Operational considerations for bullheading include the following:
- When high pressures need to be applied, the cementing unit should be used for better control and adequate pressure rating.
- Large mud volumes and LCM pills should be available in case major losses are experienced during bullheading.
- A kill line connection above the bottom pipe rams of the BOP stack should be used to be able to isolate the annulus in case of a kill line failure.
- A check valve should be installed in the kill line.
- Assess the migration rate.
- Calculate MAASP for the current mud weight, taking equipment rating into account.
- Ensure enough mud volume is available to displace the estimated top of the influx to TD, with 50% excess.
- If time permits, raise the mud weight by 0.3 - 0.5 ppg.
- Line up one mud pump on the annulus.
Bullheading Procedure
- Establish injection pressure by pumping down the annulus at a slow rate. Keep pump rate constant and plot the injection pressure versus the volume.
- Injection pressure shall not exceed MAASP. If injection pressure continues to increase, stop pumping and observe. A decrease in pressure indicates that bullheading is successful. (The pumping rate selected shall be greater than the migration rate).
- If pressure decreases when shut down, continue bullheading at maximum rate.
- Continue pumping to overdisplace the top of the influx to TD by 50%. If gas is strung out in the mud, the effective influx height will be considerably greater than for a discrete influx.
- Shut down and observe the well. Drill pipe and annulus pressures should be approximately the same.
- Raise mud weight (if necessary) and circulate using Wait and Weight Method until annulus is clear of influx.