The primary indicators of a kick are as follows:
1. Flow rate increase
2. Pit volume increase
3. Flowing well with pumps shut off
4. 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:
1. 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.
2. 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.
Institute of Petroleum Engineering, Heriot-Watt University