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).

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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.

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During normal drilling operations the volume of fluid pumped into the borehole should be equal to the volume of mud returned and when the pumps are stopped the fluid should neither continue to flow from the well (this would indicate that a kick was taking place) nor should the level of the mud fall below the mud flowline. The latter can be observed by looking down the hole through the rotary table.

If the top of the mud drops down the hole then the height of the colom of mud above any particular formation is decreased and the borehole pressure at that point is decreased. It is therefore essential that the height of the colom of mud is continuously monitored and that if the colom of mud does not extend to surface then some action must be taken before continuing operations.

The mud colom height may be reduced by :
•    Tripping
•     Swabbing
•     Lost circulation

a. Tripping :
The top of the colom of mud will fall as the drillpipe is pulled from the borehole when tripping. This will result in a reduction in the height of the colom of mud above any point in the wellbore and will result in a reduction in bottom hole pressure. The hole must therefore be filled up when pulling out of the hole. The volume of pipe removed from the borehole must be replaced by an equivalent volume of drilling fluid.
b. Swabbing :
Swabbing is the process by which fluids are sucked into the borehole, from the formation, when the drillstring is being pulled out of hole. This happens when the bit has become covered in drilled material and the drillstring acts like a giant piston when moving upwards. This creates a region of low pressure below the bit and formation fluids are sucked into the borehole. (The opposite effect is known as Surging, when the pipe is run into the hole).

The amount of swabbing will increase with:
•     The adhesion of mud to the drillpipe
•     The speed at which the pipe is pulled
•     Use of muds with high gel strength and viscosity
•     Having small clearances between drillstring and wellbore
•     A thick mud cake
•     Inefficient cleaning of the bit to remove cuttings.

c. Lost Circulation :
Lost circulation occurs when a fractured, or very high permeability, formation is being drilled. Whole mud is lost to the formation and this reduces the height of the mud colom in the borehole. Lost circulation can also occur if too high a mud weight is used and the formation fracture gradient is exceeded. Whatever the cause of lost circulation it does reduce the height of the colom of mud in the wellbore and therefore the pressure at the bottom of the borehole. When the borehole pressure has been reduced by losses an influx, from an exposed, higher pressure, formation can occur. Losses of fluid to the formation can be minimised by :
•    Using the lowest practicable mud weight.
•    Reducing the pressure drops in the circulating system therefore reducing the ECD of the mud
•     Avoid pressure surges when running pipe in the hole.
•     Avoid small annular clearances between drillstring and the hole.

It is most difficult to detect when losses occur during tripping pipe into or out of the hole since the drillpipe is being pulled or run into the hole and therefore the level of the top of the mud colom will move up and down. A Possum Belly Tank (or trip tank) with a small diameter to height ratio is therefore used to measure the amount of mud that is used to fill, or is returned from, the hole when the pipe is pulled from, or run into, the hole respectively. As the pipe is pulled from the hole, mud from the trip tank is allowed to fill the hole as needed. Likewise when tripping in, the displaced mud can be measured in the trip tank ( Figure 6). The advantage of using a tank with a small diameter to height ratio is that it allows accurate measurements of relatively small volumes of mud.

trip tank

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The mudweight is generally designed such that the borehole pressure opposite permeable (and in particular hydrocarbon bearing sands) is around 200-300 psi greater than the formation pore pressure. This pressure differential is known as the overbalance. If the mud weight is reduced the overbalance becomes less and the risk of taking a kick becomes greater. It is therefore essential that the mudweight is continuously monitored to ensure that the mud that is being pumped into the well is the correct density. If the mudweight does fall for some reason then it must be increased to the programmed value before it is pumped downhole.

The mudweight will fall during normal operations because of the following:
1. Solids removal
2. Excessive dilution of the mud (due to watering-back)
3. Gas cutting of the mud.

a. Solids removal :
The drilled cuttings must be removed from the mud when the mud returns to surface. If the solids removal equipment is not designed properly a large amount of the weighting solids (Barite) may also be removed. The solids removal equipment must be designed such that it removes only the drilled cuttings. If Barite is removed by the solids removal equipment then it must be replaced before the mud is circulated downhole again.
b. Dilution :
When the mud is being treated to improve some property (e.g. viscosity) the first stage is to dilute the mud with water (water-back )in order to lower the percentage of solids. Water may also be added when drilling deep wells, where evaporation may be significant. During these operations mud weight must be monitored and adjusted carefully.
c. Gas cutting :
If gas seeps from the formation into the circulating mud (known as gas-cutting) it will reduce the density of the drilling fluid. When this is occurs, the mudweight measured at surface can be quite alarming. It should be appreciated however that the gas will expand as it rises up the annulus and that the reduction in borehole pressure and therefore the reduction in overbalance is not as great as indicted by the mudweight measured at surface. Although the mud weight may be drastically reduced at surface, the effect on the bottom hole pressure is not so great. This is due to the fact that most of the gas expansion occurs near the surface and the product of the mudweight measured at surface and the depth of the borehole will not give the true pressure at the bottom of the hole. For example, if a mud with a density of 0.530 psi/ft. were to be contaminated with gas, such that the density of the mud at surface is 50% of the original mud weight (i.e. measured as 0.265 psi/ft.) then the borehole pressure at 10,000ft would normally be calculated to be only 2650 psi. However, it can be seen from Figure 5 that the decrease in bottom hole pressure at 10,000 ft. is only 40-45 psi.

bottom hole pressure

It should be noted however that the presence of gas in the annulus still poses a problem, which will get worse if the gas is not removed. The amount of gas in the mud should be monitored continuously by the mudloggers, and any significant increase reported immediately.

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Secondary control is required when primary control has failed (e.g. an unexpectedly high pressure formation has been entered) and formation fluids are flowing into the wellbore. The aim of secondary control is to stop the flow of fluids into the wellbore and eventually allow the influx to be circulated to surface and safely discharged, while preventing further influx downhole. The first step in this process is to close the annulus space off at surface, with the BOP valves, to prevent further influx of formation fluids (Figure 2). The next step is to circulate heavy mud down the drillstring and up the annulus, to displace the influx and replace the original mud (which allowed the influx in the first place). The second step will require flow the annulus but this is done in a controlled way so that no further influx occurs at the bottom of the borehole. The heavier mud should prevent a further influx of formation fluid when drilling ahead. The well will now be back under primary control.

secondary well control

Primary control of the well may be lost (i.e. the borehole pressure becomes less than the formation pressure) in two ways. The first is if the formation pressure in a zone which is penetrated is higher than that predicted by the reservoir engineers or geologist. In this case the drilling engineer would have programmed a mud weight that was too low and therefore the bottomhole pressure would be less than the formation pressure (Figure 1). The second is if the pressure due to the colom of mud decreases for some reason, and the bottom hole pressures drops below the formation pressure. Since the bottom hole pressure is a product of the mud density and the height of the colom of mud. The pressure at the bottom of the borehole can therefore only decrease if either the mud density or the height of the colom of mud decreases (Figures 3 and 4).

There are a number of ways in which the density of the mud (mudweight) and/or the height of the colom of mud can fall during normal drilling operations.

loss of primary control

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Primary control over the well is maintained by ensuring that the pressure due to the colom of mud in the borehole is greater than the pressure in the formations being drilled i.e. maintaining a positive differential pressure or overbalance on the formation pressures. (Figure 1)

primary well control

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