Oil Well kick

Kick is the inflow of formation fluid into the wellbore. Generally drilling is done under overbalance condition which means borehole pressure is higher than formation pressure. This overbalance keeps the formation fluid from entering into the wellbore. But sometimes due to different reasons wellbore pressure may decrease below the formation pressure (under-balance condition), which allows the formation fluid to enter into the hole. This influx is called kick.

Kick can happen during any operation drilling,tripping and circulation. If the kick is handled successfully kick is said to be killed and if the kick flows to the surface uncontrolled its called a blowout.

oil and gas kick

Wellbore pressure (under static condition) = Conversion factor*TVD*MW

From the formula it is clear that wellbore pressure depends on TVD ( vertical depth of mud column) and mud weight and if either of the two decrease wellbore pressure will also decrease.

Reason for decrease in wellbore pressure

  • Decrease in Mud column height:- height of mud column can decrease due to various reasons.
    • Lost circulation
    • While tripping out, volume of mud pumped into the hole should be equal to the volume of pipes pulled out. If we fail to do so mud column height will decrease
  • Mud weight can also decrease due to different reason
    • Dilution at surface
    • Gas cut mud. If the formation being drilled contains high % of gas than the mud returning to the surface will also have high % of gas and is called gas cut mud
  • While drilling, we may encounter high pressured zone which were not expected. This high pressure may exceed the pressure exerted by the drilling fluid and cause kick
  • ZonePressure gradient(psi/ft)
    Subnormal pressure<0.40
    Normal pressure0.43 to 0.50
  • While tripping out, if the speed of pulling out is high it causes pressure below the drill string to fall (swabbing) and can induce a kick

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