Primary Indicators of a Kick
The primary indicators of a kick are as follows:
• Flow rate increase
• Pit volume increase
• Flowing well with pumps shut off
• Improper hole fillup during trips
a. Flow rate increase :
While the mud pumps are circulating at a constant rate, the rate of flow out of the well, Qout should be equal to the rate of flow into the well, Qin. If Qout increases (without changing the pump speed) this is a sign that formation fluids are flowing into the wellbore and pushing the contents of the annulus to the surface. the flowrate into and out of the well is therefore monitored continuously using a differential flowmeter. The meter measures the difference in the rate at which fluid is being pumped into the well and the rate at which it returns from the annulus along the flowline.
b. Pit volume increase :
If the rate of flow of fluid into and out of the well is constant then the volume of fluid in the mud pits should remain approximately (allowing for hole deepening etc.) constant. A rise in the level of mud in the active mudpits is therefore a sign that some other fluid has entered the system (e. g. an influx of formation fluids). the level of the mud in the mudpits is therefore monitored continuously. the increase in volume in the mud pits is equal to the volume of the influx and should be noted for use in later calculations.
c. Flowing well with pumps shut off :
When the rig pumps are not operating there should be no returns from the well. If the pumps are shut down and the well continues to flow, then the fluid is being pushed out of the annulus by some other force. It is assumed in this case that the formation pressure is higher than the hydrostatic pressure due to the colom of mud and therefore that an influx of fluid is taking place. There are 2 other possible explanations for this event:
• The mud in the borehole will expand as it heats up. This expansion will result in a small amount of flow when the pumps are shut off.
• If a small amount of heavy mud has accidentally been pumped into the drillstring and the mud in the annulus is being displaced by a u-tubing effect
d. Improper Hole Fill-up During trips
As mentioned earlier, the wellbore should to be filled up with mud when pipe is pulled from the well. If the wellbore overflows when the volume of fluid, calculated on the basis of the volume of drillpipe removed from the well, is pumped into the well then fluids from the formation may have entered the well.
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 overpressured 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).