Well Control: Causes of Kicks

One of the functions of using drilling fluids is controlling the formation fluids to do not enter into the wellbore. The hydrostatic pressure generated by the column of mud is considered as the primary control or barrier. The mud weight has to be at an adequate level to overbalance pore pressure. If this overbalance is lost, then the formation fluids can enter to the wellbore. 

A kick is defined as an intrusion of unwanted fluids into the wellbore. This can happen when the formation pressure exceeds the effective hydrostatic pressure of drilling fluids 

An influx is defined as the intrusion of formation fluids into the wellbore which does not immediately cause formation pressure to exceed hydrostatic pressure.

Causes of Kicks and Influxes

Failure to keep the hole full while tipping

A reduction in the bottom hole pressure can occur when the fluid level falls when pulling the drilling string out of the hole. If this reduction exceeds the trip margin or the safety overbalance factor, a kick can happen. The trip tank is used while tripping to ensure that the hole is taking the correct mud volume after filling the well. If the well fails to take the theoretical mud volume while filling, then a flow check has to be performed. It is very important to the drilling crew to closely monitor the displacement and the fill up volume when tripping. 

Swabbing and Surging

Swabbing is defined as the reduction of bottom hole pressure below the formation pressure due to the effects of pulling the drilling string which allows formation fluids to enter into the wellbore.

The reduction of this pressure is caused by the friction between the drilling fluids and the drill string being pulled. The swabbing can also be caused by getting balled up tools which are full gauge like the bits, stabilizers or reamers. When pulling the string out of the hole, the balled up tools can create a piston effect which can seriously affect the bottom hole pressure.

Surging is when the bottom hole pressure is increased due to the effects of running the drill string too fast in the hole. This can generate mud hole losses if the bottom pressure exceeds the formation fracture pressure. 

Fig 01- Sketch of Swabbing and Surging

There are many factors which can affect the swabbing or surging: 

- Pulling speed
- Mud properties
- Hole geometry

If there is any doubt about swabbing fluid formation and the well is not flowing, a non-return valve has to be installed and bit has to be returned to the bottom. Once on bottom, a bottom up circulation has to be performed and return mud should be checked for contamination.

If the well is flowing during the flow check, then the following actions should be taken:

- Install a non-return valve. 
- Shut the well in
- Prepare for stripping
- Strip the string to the bottom
- Circulate the well, and check mud for contamination 

Fig 02- (a) Safety valve  (b) Grey valve

Loss of circulation

Loss of circulation can be also one the reason behind taking a kick. When loss circulation happens, the mud column height is shortened leading to a reduction of the bottom hole pressure. If this reduction make the pore pressure greater than the hydrostatic pressure, then the formation pressure will enter to the wellbore.

Insufficient mud weight

The kick can be detected when drilling through a permeable zone which has a higher pressure than the pressure exerted by the mud column. If the overpressure zone has low permeability, then traces of the formation fluids should be detected in the returns. If the permeability is at a high level, then the risk is greater and the well should be shut-in just after getting the flow during the flow check.

Shallow Gas Sands

The top hole formation can involve layers of shallow sands which can contains gas and water. Kicks from shallow sands can be very hazardous. There are many reasons which can lead to such situations like: poor cement jobs, casing leaks, improper abandonment and underground blowouts.

Special Situations

Drill Stem Test (DST) 

A drill stem test is performed by setting a packer above the formation to be tested, and allowing the formation fluids to flow. Down hole chokes are used to limit the surface pressures and flow rates to the capabilities of the surface equipment. During the test, the string is filled with the formation fluids, at the end of the test, these fluids must be circulated out of the hole to get back the overbalance by proper well control techniques. Failure to follow the correct procedures to kill the well could lead to a blowout.

Drilling Into an Adjacent Well

Drilling in offshore, where a large number of wells are drilled from the same platform, is a potential problem. if the penetrated well is a production well, the formation fluids will enter the wellbore of the drilling well causing a kick. 

Excessive drilling rate through gas sand/limestone

When drilling through gas sands or limestone, the gas cut can affect the mud weight. The severity of the situation depends on the penetration rate, porosity and permeability. Degassing using the surface degasser is very important in order to prevent the contaminated drilling fluids to be pumped again in the hole which can lead to greater bottom hole hydrostatic pressure reduction resulting in formation fluids to start flowing. 

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