Bullheading Well Control Method in Drilling Operations – All Things You Need to Understand about It

What is Bullheading?

Bullheading well control is a well control technique used in specific scenarios during drilling operations to pump an influx back into the formation. This method involves displacing the casing with a sufficient quantity of kill fluid to push well fluids back into the reservoir. Successful bullheading requires unobstructed annulus flow and the ability to inject into the formation without surpassing pressure limitations such as the Maximum Allowable Annular Surface Pressure (MAASP). Formation breakdown may be acceptable in certain cases if it is preferable to other potential outcomes.  Bullheading may result in fracturing the exposed formation if injection pressures exceed the fracture gradient.

Situations for Bullheading

Bullheading may be considered in the following circumstances:

• Gas Volume at Surface: When conventional displacement methods would result in an excessive gas volume at surface conditions, possibly exceeding the capacity of the mud-gas separator.
• High H2S Content: When the influx contains unacceptable levels of H2S.
• Large Influxes: When a significant influx is encountered.
• No Pipe in Hole: When an influx occurs with no pipe in the hole.
• High Surface Pressures: When conventional methods could lead to excessive surface pressures, potentially exceeding the Maximum Allowable Surface Pressure (MASP) and risking casing failure near the wellhead.
• Pipe Off Bottom: When a kick is taken with the pipe off bottom and stripping back is not feasible.
• Pressure Reduction: To reduce surface pressures before further well control operations.

Note: The decision to bullhead must be made promptly after shut-in. Delays can allow gas migration upwards, reducing the likelihood of successful re-injection into the formation. Pumping may lead to formation fractures at weak points such as the shoe.

Key Factors in Bullheading

Bullheading should be considered only when standard well control techniques are unsuitable. Accurate information on formation injectivity or fracture characteristics is often unavailable. The feasibility of bullheading is determined by several factors:

• Type of Influx and Formation Permeability: Gas migration may require downward fluid velocity to exceed migration rates for effective displacement. Viscosifiers in the kill fluid may help limit migration.
• Openhole Characteristics: Low reservoir permeability may necessitate exceeding fracture pressure.
• Influx Position: The location of the influx in the hole.
• Well Control Equipment and Casing Pressure Ratings: Equipment and casing pressure limits must be known and not exceeded, accounting for wear and deterioration.
• Consequences of Formation Fracture: Evaluate the impact of fracturing open hole sections.
• Drill Pipe and Casing Pressure Limits: Ensure limits are not exceeded. Applying pressure to the outside of the innermost casing may help stay within burst limits.
• Filter Cake Quality: The integrity of the filter cake at the permeable formation.
• Drilling Fluid Displacement: Consider the effects of displacing large volumes of drilling fluids into potentially productive formations.

Bullheading Procedure

Bullheading procedures should be tailored to the specific conditions at the rig site. For instance, bullheading an H2S-containing influx may be necessary even if it causes a downhole fracture. Conversely, fracturing may be unacceptable with shallow casing, where broaching risks outweigh surface high-pressure risks.

Steps for Bullheading:

1. Calculate Fracture Pressures: With the well shut-in, calculate expected surface pressures that would cause formation fracture during bullheading.
2. Prepare Pressure Chart: Create a chart using strokes versus pumping pressure, especially if using heavier mud to reduce surface pressure.
3. Eliminate Surface Gas: If gas is present at the surface, use Lubricate and Bleed procedures before starting bullheading.
4. Pump Kill Fluid: Gradually bring the pumps up to speed to overcome well pressure and pump the kill fluid down the annulus. Monitor pump pressure throughout.
   • Caution: Avoid exceeding maximum surface pressures unless formation breakdown is tolerable.
5. Pump Rate: Pump fast enough to surpass gas migration rates.
6. Monitor Pressure: As fluids are forced back into the formation, the added hydrostatic pressure should decrease the pumping pressure. Record all pressure values.
7. Stop Pump and Monitor: Stop the pump (unless over-displacement is approved), shut-in the well, and monitor the situation.
   • Note: If pressure is observed, gas may have migrated up-hole faster than the fluid was pumped down, or the fluid density may be insufficient to kill the well.

In summary, bullheading is a high-risk but potentially effective well control technique for managing influxes during drilling by pumping kick fluids back into the formation. Its feasibility depends heavily on specific well conditions and risks like inducing fractures. Bullheading requires comprehensive planning, contingencies, and care in execution by experienced personnel. When justified for the situation, it can enhance safety and reduce costs, but should only be attempted after thorough analysis deems it the best available option given the elevated risks compared to conventional methods.

References

Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.

Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.

Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.