A kick is an unwanted flow of formation fluid (gas, oil or water) into the wellbore. It happens when the pressure of the fluid in the well is lower than the pressure of the fluid in the rock. If a kick is not detected and controlled in time, it can become a blowout, one of the most dangerous events in drilling. This article explains what causes a kick, how to recognize it, and the basic steps used to control it. It is an educational overview. On a rig, always follow your company’s well control procedures and the instructions of certified well control personnel.
What Is a Kick?
While drilling, the drilling mud keeps the formation fluids in the rock by pressing on them with its hydrostatic pressure. This is called primary well control. When the formation pressure becomes greater than the hydrostatic pressure of the mud, formation fluid flows into the well. This is a kick. It is the opposite of overbalance; see key pressure definitions.
Kick occurs when: Formation pressure (PF) > Hydrostatic pressure of the mud (PH)
Main Causes of a Kick
- Mud weight too low. The mud column pressure is less than the pore pressure. This can happen when an unexpected high-pressure zone is drilled. See formation pressure.
- Not keeping the hole full when tripping. When pipe is pulled, the mud level falls by the volume of the steel removed. If the hole is not filled, the hydrostatic pressure drops.
- Swabbing. Pulling the pipe too fast reduces the pressure below the bit and can draw formation fluid into the well.
- Lost circulation. If mud is lost to a weak formation, the mud level falls and the hydrostatic pressure decreases.
- Gas-cut mud. Gas that enters the mud lowers its density.
- Drilling into abnormally pressured formations without enough warning.
Warning Signs of a Kick
Early detection is the key to safe control. The signs are divided into primary (more reliable) and secondary indicators.
| Type | Sign | What it means |
|---|---|---|
| Primary | Increase in flow rate out of the well, with pump rate constant | Extra fluid is entering the annulus |
| Primary | Pit gain (increase in total mud volume) | More fluid in the system than was pumped in |
| Primary | Well flows when the pumps are off (flow check) | The well is flowing on its own |
| Primary | Hole not taking the correct volume of mud when tripping | Fluid is entering, or not filling correctly |
| Secondary | Drilling break (sudden increase in penetration rate) | A softer or higher-pressure zone may have been entered |
| Secondary | Drop in pump pressure and increase in pump strokes | Lighter fluid in the annulus |
| Secondary | Gas-cut or chloride-cut mud | Formation fluid mixing with the mud |
What to Do: The Basic Sequence
When a kick is suspected, the crew follows the shut-in procedure for the operation in progress. In outline:
- Stop drilling and pick up the bit off bottom (space out).
- Stop the pumps and check for flow.
- If the well is flowing, close the blowout preventer and the choke line.
- Notify the supervisor and record the shut-in pressures and the pit gain.
- Start the kill operation using a planned well control method.
The shut-in drill pipe pressure (SIDPP) and shut-in casing pressure (SICP) are read after the pressures have stabilized. The pit gain is the volume of influx that entered the well. The smaller the pit gain when the well is shut in, the easier the kill is.
Key Calculations
Formation pressure
The drill pipe is full of mud of known density, so the formation pressure at the bottom is:
PF = 0.052 × MW × TVD + SIDPP
Kill mud weight
KMW (ppg) = MW + SIDPP ÷ (0.052 × TVD)
Example: a well is 10,000 ft TVD and is drilled with 10 ppg mud. After shut-in, SIDPP is 400 psi.
- Formation pressure = 0.052 × 10 × 10,000 + 400 = 5,200 + 400 = 5,600 psi
- Kill mud weight = 10 + 400 ÷ (0.052 × 10,000) = 10 + 0.77 = 10.8 ppg (rounded up)
See the hydrostatic pressure formula for the basic relation behind these numbers.
Common Kill Methods
| Method | How it works | Main feature |
|---|---|---|
| Driller’s method | Circulate the influx out with the original mud first, then circulate kill mud | Two circulations; can start quickly |
| Wait and weight (engineer’s) method | Weight up to kill mud first, then circulate once | One circulation; needs time to mix kill mud |
During the kill, the choke is adjusted to keep the bottom hole pressure slightly above the formation pressure, so that no more fluid enters, and below the fracture pressure of the weakest formation. The fracture limit at the casing shoe is established with a leak-off test.
Prevention
- Plan the mud weight and casing program from reliable pore and fracture pressure estimates; see well planning.
- Monitor pit volumes and flow rates continuously.
- Fill the hole correctly while tripping and use a trip tank.
- Control tripping speed to avoid swabbing.
- Run regular well control drills and keep the BOP equipment tested and in good condition.
- Keep the mud properties within the program.
Frequently Asked Questions
What is the difference between a kick and a blowout?
A kick is an influx of formation fluid into the well that is detected and controlled. A blowout is an uncontrolled flow of formation fluid to the surface.
What is swabbing?
Swabbing is a reduction in the bottom hole pressure caused by pulling the pipe out of the hole too fast. It can draw formation fluid into the well.
Why is gas the most dangerous kick fluid?
Gas expands quickly as it rises and is lighter than mud, so it reduces the hydrostatic pressure further and can reach the surface fast.
What does SIDPP tell us?
The shut-in drill pipe pressure shows how much the formation pressure exceeds the hydrostatic pressure of the mud in the drill string. It is used to calculate the kill mud weight.
Key Takeaways
- A kick happens when formation pressure exceeds the hydrostatic pressure in the well.
- The most reliable warning signs are a flow increase, a pit gain, and flow when the pumps are off.
- Shut the well in quickly, read SIDPP and SICP, and kill with the planned method.
- Kill mud weight = original mud weight + SIDPP ÷ (0.052 × TVD).
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