Bullheading

If normal well killing techniques are not possible or will result in critical well control conditions bullheading may be considered whereby mud/influx in displaced/squeezed back into the weakest open hole formation. Bullheading is a critical option of well killing and authorisation to proceed with this method of well killing must generaly be obtained form the Drilling Superintendent.

It shall only be used under the following circumstances:

• 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:

• 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.
• MAASP due to formation fracture pressure and equipment rating.
• Formation permeability.
• Position of influx in open hole
• The consequences of fracturing a section of the openhole.

Note: Bullheading is done only if the intermediate or production casing is set.

Preparation for bullheading

The following preparation work shall be undertaken  for bullheading method :

• 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.