The most common secondary indicators that an influx has occurred are:
• Drilling break
• Gascut mud
• Changes in pump pressure
a. Drilling Break
A drilling break is an abrupt increase in the rate of penetration and should be treated with caution. The drilling break may indicate that a higher pressure formation has been entered and therefore the chip hold down effect has been reduced and/or that a higher porosity formation (e.g. due to under-compaction and therefore indicative of high pressures) has been entered. However an increase in drilling rate may also be simply due to a change from one formation type to another. Experience has shown that drilling breaks are often associated with over pressured zones. It is recommended that a flow check is carried whenever a drilling break occurs.
b. Gas Cut Mud
When gas enters the mud from the formations being drilled, the mud is said to be gascut. It is almost impossible to prevent any gas entering the mud colom but when it does occur it should be considered as an early warning sign of a possible influx. The mud should be continuously monitored and any significant rise above low background levels of gas should be reported. Gas cutting may occur due to:
• Drilling in a gas bearing formation with the correct mud weight
• Swabbing when making a connection or during trips
• Influx due to a negative pressure differential (formation pressure greater than borehole pressure).
The detection of gas in the mud does not necessarily mean the mudweight should be increased. The cause of the gas cutting should be investigated before action is taken.
c. Changes in Pump Pressure
If an influx enters the wellbore the (generally) lower viscosity and lower density formation fluids will require much lower pump pressures to circulate them up the annulus. This will cause a gradual drop in the pressure required to circulate the drilling fluid around the system. In addition, as the fluid in the annulus becomes lighter the mud in the drillpipe will tend to fall and the pump speed (strokes per min.) will increase. Notice, however, that these effects can be caused by other drilling problems (e.g. washout in drillstring, or twist-off).
Institute of Petroleum Engineering, Heriot-Watt University