A kick occurs when the formation pressure is greater than the hydrostatic pressure of the drilling fluids column in the well. The kick is a positive indicator that formation fluids are entering to the wellbore and well control procedures have to be followed to get back the well under control.
Kick Detection while drilling:
Two changes in the mud circulating system can be caused by the flow of the formation fluids into the wellbore.
- Mud level increase in the active mud system
- The return flow rate from the well exceeds the flow rate pumped into the well
The mud circulating system is closed system, and any increase in the level can be interpreted by taking a formation fluids influx which can be confirmed by a visual observation or using measuring instruments.
The visual observation is performed by placing a pit level marker in the mud tank, and asking a member of the drilling crew usually the derrick man to monitor the level. This method can not be reliable because it seems to be difficult to keep a constant watch on the level marker especially during trips when most kicks happen.
Using the measuring instrument is more accurate and reliable method. It is based on placing many level pit measuring instruments and mounting some recorders on the driller's console and controlled by the mud logging monitoring system. This method can allow a constant watch on pit levels during tripping and drilling.
Good communication between the crew members is an important point to avoid severe and risky well control situations. Any change volume in the mud tanks should be communicated between the different members of the crew. The drillers should make sure that any mud transfer in the surface system has to be notified before the transfer and after stopping adding the volume in order to re-set the level monitoring system.
Detecting a kick while tripping
During tripping, the flow will cause an improper filling of the well. If any anomalies are noted on the trip sheet, a flow check has to be performed:
- If the well is flowing then the well has to be shut in
- If the well is not flowing the drill string should be run back to the bottom in order to circulate a bottom up volume.
Trip tanks are the safest and most reliable method of monitoring mud volumes during tripping. Filling the well continuously is highly recommended when tripping out of the hole. The trip tank is used to ensure the correct mud displacement by the drill string, and any simple anomaly in filling up the well can be easily detected by the measuring system due to the limited volume which can be contained by the trip tank. The tripping has to be ceased when discharging the trip tank to avoid any confusion if the well taking a kick during this period of time.
Situations that can mask a kick
• Mud weight adjustments while drilling.
• Mud transfers while drilling.
• Partial lost circulation.
• Solids control equipment and degassing mud.
• Spills and leaks in surface equipment.
• Drain back.
• Pump start up and shut down.
Kick Behavior in Drilling Fluids
If the drilling fluid is an oil based system and under the well bore conditions, a small gas kick can become a serious well control situation. The gas can become dissolved and miscible. The reason behind this behavior is that the gas remains in solution until it reaches its bubble point.
The problems in OBM is that if the formation fluids enter the wellbore, it is impossible to detect where top of the gas bubble is. In extreme cases where the gas bubble can be 6000 ft to 7000 ft away from surface, the drillers cannot realize any changes. Under this condition it may be recommended to consider a drilling break as a positive sign. If the flow return is detected when performing the flow check, the well has to be shut in, and the gas can be then circulated through the choke in safe manner.
In the figure presented below, a three barrel of gas have entered the wellbore at 10000ft, but nothing can be detected on the surface, till the gas is circulated up to 2600 ft, then gas expands rapidly and there is a serious situation of blowing the mud out of the hole and making the well underbalance. This situation can be less severe in water based muds because the gas will expand much earlier.
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| Figure 01: Gas Behavior in Drilling fluids |
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