The Kick: Causes and Initiation

Lesson Summary

A kick is the entry of formation fluids into the well when bottomhole pressure drops below formation pressure. We explore the most common causes during drilling and tripping.

This lesson is from the Well Control track and lasts 14:30 minutes. Watch the video first, then review the key points and test your understanding with the questions below.

Key Points

1. 1Failure to fill the well while pulling pipe
2. 2Swabbing during rapid pulling out of hole
3. 3Insufficient mud weight
4. 4Lost circulation and drop in mud column
5. 5Penetrating an unexpectedly overpressured formation

Written Lesson

The two conditions required for a kick to occur

Formation fluid enters the well only if two conditions are met: bottom hole pressure (Bottom Hole Pressure) becomes less than formation pressure, and the formation is permeable enough for the fluid to move. Primary well control (Primary Well Control) means that the hydrostatic pressure of the mud remains higher than the formation pressure at all times. Therefore, every cause of a Kick is actually a way of lowering bottom hole pressure, or encountering an unexpectedly higher formation pressure than planned.

BHP (static) = 0.052 × MW × TVD

BHP (circulating) = 0.052 × MW × TVD + APL

First cause: Failure to fill the well while pulling pipe

When we pull pipe from the well, the volume of steel comes out with it, causing the mud level to drop unless we compensate. Every foot the mud level drops reduces hydrostatic pressure at the bottom. This is why we pull pipe using the trip tank (trip tank) and compare the volume actually taken by the well to the calculated volume on the trip sheet (trip sheet).

Worked example: we pull 10 stands of 5 in pipe dry (a stand is about 93 ft, so 930 ft). Steel displacement is about 0.0071 bbl/ft, so the volume needed to fill the hole is 930 × 0.0071 ≈ 6.6 bbl. If we do not fill: the capacity of 9-5/8 in casing is 0.0732 bbl/ft and the remaining pipe occupies 0.0071 bbl/ft, so the level drops 6.6 ÷ (0.0732 − 0.0071) = 6.6 ÷ 0.0661 ≈ 100 ft. With 10 ppg mud the pressure loss is 0.052 × 10 × 100 = 52 psi. That alone can be enough to start a kick if the overbalance is small.

Second cause: Swabbing

If we pull pipe too quickly, the BHA acts like a piston in a cylinder: it draws mud beneath it upward and momentarily reduces bottomhole pressure, allowing formation fluids to enter the wellbore. The risk increases with a (Balled Bit), tight clearances between tools and the wellbore wall, and high mud viscosity and (Gel Strength). Its primary sign: the well takes less mud than calculated on the trip sheet. Conversely, Surging when running pipe in quickly increases pressure and may break down a weak formation causing losses, which in turn can lead to a Kick.

Third cause: Insufficient mud weight

Mud weight may be too low for several reasons: encountering an overpressured layer unexpected by the well program, diluting mud with water at surface, (barite sag) in deviated sections, or gas and water contamination. (Gas-cut mud) appears very light at surface because gas expands near the wellhead, but the bottomhole pressure drop is usually small. However, we always treat it as a warning: we monitor the pit and check the well.

Fourth cause: Lost circulation

If a weak or fractured formation swallows mud, the mud level drops in the well, reducing hydrostatic pressure on all exposed formations. Here, another higher-pressure layer may flow. Therefore, upon total losses, we fill the well from the top with the lightest available fluid, such as water, and record the volume, because a partially filled well is better than one where the mud level is unknown.

Fifth cause: Pump shutdown and loss of frictional pressure

During circulation, mud frictional pressure in the annulus, known as APL, is added to the hydrostatic pressure, with the total expressed as ECD. When pumps stop to make a connection, this portion disappears.

Worked example: a well at 10,000 ft TVD with 10 ppg mud: hydrostatic pressure 0.052 × 10 × 10,000 = 5,200 psi, annulus friction 200 psi, so bottomhole pressure while circulating is 5,400 psi. If formation pressure is 5,300 psi, the well is stable while the pumps run, but when they stop, bottomhole pressure becomes 5,200 psi, which is 100 psi below 5,300 psi, so the well flows on every connection. This is called connection gas or a connection kick, and the fix is to raise the mud weight, not to rely on ECD.

Other causes not to be overlooked

• Shallow gas before setting the BOP, where response time is extremely short.
• Cement setting phase: the slurry loses its ability to transmit full hydrostatic pressure as it transitions to a solid.
• Drilling close to a producing or charged neighboring well.
• In offshore drilling, disconnecting the riser removes the mud column between the rig and the seabed.

The figures and examples in this lesson are for educational purposes. On the job, always follow your company procedures and accredited training center instructions.

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