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Well Control Theory : Causes of Kicks

Primary well control is the use of drilling fluid density to provide sufficient pressure to prevent the influx of formation fluid into the wellbore.
An entry of oil, gas or water into the wellbore. When the bottom hole pressure becomes less than formation pressure and the permeability is great enough, formation fluid will enter the wellbore causing a "kick ".
There are 5 major causes for the loss of primary well control (kick).

Failure To Fill The Hole Properly While Tripping

As the drillstring is pulled from the hole, the mud level drops due to the volume of pipe being removed. As the mud level drops the hydrostatic pressure may be reduced enough to lose primary well control allowing formation fluids to enter the wellbore.


The hydrostatic pressure in the wellbore will always be reduced to some extent when the drillstring or full gauge tools are being pulled from the hole. The reduction in hydrostatic pressure should not be such that primary control is lost. Swabbing is caused by one or more of the following:
  • High pulling speeds.
  • Mud properties with high viscosity and high gels.
  • Tight annulus BHA-hole clearance, or restricted annulus clearance
Lost Circulation

When lost circulation occurs, the drilling fluid level will drop and a reduction in hydrostatic pressure in the wellbore may cause the loss of primary well control. Loss of circulation may result from one or more of the following:
  • Cavernous or vugular formations.
  • Naturally fractured, pressure depleted or sub-normally pressured zones.
  • Fractures induced by excessive pipe running speeds.
  • A restricted annulus due to balling of BHA or sloughing shales
  • Excessively high annular friction losses.
  • Excessive pressures caused by breaking circulation when mud gel strength is high. Mechanical failure (casing, riser, etc.)
The hydrostatic pressure in the wellbore will always be reduced
Insufficient Mud Weight

When the hydrostatic pressure due to drilling fluid density is less than formation pressure of a permeable zone, formation fluids will enter the wellbore. This may occur due to the following:
  • Drilling into an abnormal pressure zone.
  • Dilution of the drilling fluid.
  • Reduction in drilling fluid density due to influx of formation fluids, in particular gas.
  • Settling of weighted material.
  • Failure to displace riser to kill mud after circulating out a kick.
  • Pumping long column of low weight spacer while cementing.
  • After cementing while WOC. Cement losses hydrostatic pressure as is starts to set.
Loss Of Riser Drilling Fluid Column

On floating unit operations, the loss of the drilling fluid column in the riser may result in a reduction of hydrostatic pressure in the wellbore and may cause the loss of primary well control. This loss of riser hydrostatic column could be due to:
  • Accidental disconnect.
  • Riser damage.
  • Displacement of riser with seawater.
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Well Control Theory : Causes of Kicks Reviewed by Djoernalist on 12:58 PM Rating: 5

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