A “Kick” or “Wellbore Influx” is undesirable flow of formation fluid into the wellbore and it happens when formation pressure is more than hydrostatic pressure in wellbore.
Several causes of Kick (Wellbore Influx) are listed below:
1. Lack of knowledge and experience of personnel (Human error)– Lacking of well-trained personnel can cause well control incident because they don’t have any ideas what can cause well control problem. For example, personnel may accidentally pump lighter fluid into wellbore and if the fluid is light enough, reservoir pressure can overcome hydrostatic pressure.
2. Light density fluid in wellbore – It results in decreasing hydrostatic pressure. There are several reasons that can cause this issue such as
• Light pills, sweep, spacer in hole
• Accidental dilution of drilling fluid
• Gas cut mud
3. Abnormal pressure – If abnormally high pressure zones are over current mud weight in the well, eventually kick will occur.
4. Unable to keep the hole full all the time while drilling and tripping. If hole is not full with drilling fluid, overall hydrostatic pressure will decrease.
5. Severe lost circulation – Due to lost circulation in formation, if the well could not be kept fully filled all the time, hydrostatic pressure will be decreased.
Lost circulation usually caused when the hydrostatic pressure of drilling fluid exceeds formation pressure. There are several factors that can cause lost circulation such as
• Mud properties – mud weight is too heavy and too viscous.
• High Equivalent Circulating Density
• High surge pressure due to tripping in hole so fast
• Drilling into weak formation strength zone
6. Swabbing causes reducing wellbore hydrostatic pressure.
Swabbing is the condition that happens when anything in a hole such as drill string, logging tool, completion sting, etc is pulled and it brings out decreasing hydrostatic pressure. Anyway, swabbing can be recognized while pulling out of hole by closely monitoring hole fill in trip sheet.
Reference book: Well Control Books