A kick occurs when the hydrostatic pressure of the mud column in the well is less than the formation pore pressure, providing that the well is capable of producing. The article summarise the effects which may indicate that a kick has taken place:
Primary Indicators
- Increase in mud return flow
Note: An increase in mud returns indicates that a kick is being taken. In this case the well shall be closed in as quickly as possible in order to minimise the size of the influx and record shut in pressures. If shut-in pressures are recorded then secondary well control shall be implemented.
Secondary Indicators
- Sudden increase in penetration rate (drilling break).
- Mud contaminated by gas, salt water, or oil.
- Reduction in pump pressure.
- Reduction in drillpipe weight.
- Sudden loss of circulation, or mud returns
- Unexplainable pit level changes
- Increased background gas
- Torque and drag, Fill on connections
- Heaving shales
These effects are cautionary signals which should be investigated with a suitable flowcheck.
Detection of Over-Pressured Formations
The trends exhibited by various parameters, e.g. penetration rate and flowline temperature, can provide significant warning of the presence of over pressured formations, if sufficient care in their interpretation is taken.
On exploration wells, such information is particularly valuable, and the Drilling Supervisor (or Mud Logging Contractor) should keep up to date plots of penetration rate and flowline temperature against depth.
Drilling Breaks
A drilling break is an important indication in deeper wells that an over pressured porous formation might have been penetrated, as such a break will occur when the value of the formation pressure approaches the hydrostatic pressure of the mud column. This reduction in overbalance makes the bit drill faster. To check that the formation pressure does not exceed the hydrostatic pressure of the mud causing the well to start flowing, a flow check shall be held after drilling not more than 2 ft into the drilling break.
Flowline Temperature
The presence of over pressured shales could be shown by an increased temperature gradient, which will be reflected in the flowline mud temperatures.