A kick is defined as an unexpected and unwanted influx of reservoir fluid, oil, water, or gas, into the wellbore due to an underbalanced condition in which pressure inside the wellbore or bottom-hole pressure (BHP) is less than formation pressure.
Gas kicks are riskier than fluid kicks because of their high mobility in the wellbore.
Formation water kicks may be troublesome, but they rarely constitute a significant threat to the safety of the crew, the environment, the rig, or the wellbore. A saltwater kick may trigger an emergency response since an influx will be identified through an increase in fluid volume returning to the pits. A saltwater kick can be circulated out of the hole using fairly benign methods and the well pressure increased through a drilling fluid density increase to prevent further saltwater kicks. This increases BHP inside the wellbore to balance reservoir pressure.
Gas kicks are much more troublesome. The gas not only invades the wellbore but also begins to migrate upward due to the density difference between drilling fluid in the well and the gas “bubble.” In other words, it can migrate upward even if there is no further influx from the formation. As it moves upward, it retains the same pressure it had when it entered the wellbore.
Good kick control requires that some fluid at the surface be bled off to allow the gas to expand as the hydrostatic pressure on the bubble decreases (i.e., there is not as much drilling fluid above it as it moves up the hole). Without this pressure reduction and expansion, the bubble simply brings reservoir pressure along with it as it moves up the hole.
Many kicks occur as a result of swabbing. This effect occurs during the upward movement of pipe and the BHA as it is being pulled from the hole. Pulling pipe out of the hole requires that mud in the wellbore flow from around the BHA to fill the void below it created by the absence of the BHA. If the pipe is pulled too fast for this to occur, a slight suction is created on the formation each time a joint or stand of pipe is pulled from the well. In effect, the pipe and BHA behave like a plunger unstopping a drain. The pipe movement literally sucks reservoir fluids into the well.
Thanks to ScienceDirect