Warning signals while drilling

a. An increase in penetration rate can often be a sign of drilling into a softer formation.

Pump pressure can also change because the drilling fluid is simply cleaning out the cuttings under the bit better than previously. These two examples are not of course signals that a kick is about to occur.

We discussed earlier also that penetration rate is also dependent upon differential pressure. An increase in penetration rate can also indicate, therefore, that differential pressure is known to be reduced and there is danger of taking a kick. An increase in penetration rate must always be noted and acted upon.

The bit must be picked up off bottom, the drilling fluid pumps must be stopped, and the well must be checked for flow.

A drilling break is a sudden change in penetration rate from a low to a higher value. This sudden change in penetration can vary considerably depending upon the actual formation type. In some cases a "break" can be from between 10 ft. to 50 ft./hour, in others maybe only 5 ft. to 10 ft./hour. In all cases where drilling is conducted in areas that are unknown or where high pressures are expected, after a relatively long period of slow drilling is followed by faster drilling, no more than between 2 to 4 feet should be drilled before the pumps are shut down and the well checked for flow. See Fig 41 A negative drilling break could also be a warning sign that a cap rock is being penetrated and possible higher pressure

is contained in the formation below the cap rock.

b. Background gas increases quite suddenly if the bit penetrates a zone of higher pore pressure. This background gas is not gas that intrudes into the wellbore from the formation but gas which is contained between the wellbore cuttings. If this gas has a high pore pressure it will expand considerably on the way up to the surface and may make up 50% of the drilling fluid volume. Such a situation is not so critical if it is properly treated.

This gas is removed from the drilling fluid at the surface with the help of a “Degasser”.

If the drilling fluid is not properly degassed before it is pumped back down the well the hydrostatic pressure in the well will be lowered and the chance for a kick to occur is possibly. On the other hand, gas in the return drilling fluid could mean that there actually exists an underbalance in the well and gas is intruding. In such a case this is a real kick situation and the necessary steps to contain it must be made immediately.

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c. The chloride content in drilling fluid, (fresh-water based drilling fluid) will normally increase when high-pressure zones are penetrated. However, this increase is impossible to notice when salt-water based drilling fluid, or drilling fluid with high chloride content, is in use. This indication therefore is not reliable enough on its own to be of any use to us in indicating high pressure zones.

d. The density of rock formation will nearly always be reduced if it is associated with a high pressure zone. This is because it will have a greater porosity. This is a good indicator if it is possible to examine different cuttings at the shale shaker, and with the help of S.P.M. (Pump speed) and annulus capacity decide from which depth they originate.

e. The size of the cuttings often suddenly change when a high-pressure zone is penetrated, they can become long and splintery in shape.

f. Changes in pump pressure are a direct result of changing resistance (friction) in the drilling fluid, if formation fluids or gas penetrate the wellbore and intermingle with the drilling fluid. However, it will only be a small part of the drilling fluid in the annulus that becomes affected in this way.

Pump pressure will normally fall if a kick occurs, as part of the drilling fluid in the wellbore becomes lighter through reduction in weight and viscosity. If the kick is a gas kick it is possible that the gas forces its way up the annulus of its own accord and pushes the drilling fluid ahead of it. This will cause large pressure fluctuations.

Fig 42 A reduction in pump pressure can in some cases give an increase in pump speed.

This occurs because of the decreased load on the pump. However, some rigs have drilling fluid pumps that are self-regulating, which is to say that regardless of what loads are imposed on the pumps they will automatically use less or more energy and maintain a constant pre-determined speed. See Fig 42

We should also be aware that pressure reductions can occur through reasons other than kick situations, such as washouts in the drill string etc.

g. Flow line (return drilling fluid) temperature often increases when a high pressure zone is penetrated, but although this has been observed in many places throughout the world it is not a particularly trustworthy indication on its own. Drilling fluid temperature

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can often increase when caustic soda and barite are added, well geometry can also cause temperature increases (higher drilling fluid velocities). A clear and uniform temperature increase that could possibly indicate a formation with a high pressure is best seen when shown graphically in detail.

h. Rotary table speed (R.P.M.) often increases when a high pressure zone is penetrated.

This is because the formation is breaking up easier and, therefore, offering less resistance to the bit.

i. Increasing flow at the flow-line is immediately the first signal that a kick is occurring.

This indication is called a “positive kick indicator” and require no flow check, but an immediately shut in of the well to minimise the size of the influx. See Fig 43

Fig 43

j. An increase in pit volume will always occur when fluid or gas enters the wellbore, because a proportional amount of drilling fluid is displaced out of the well and into the drilling fluid pits. Any unexplained pit gain is a sure sign of a kick and is also called a

“positive kick indicator” where the necessary precaution and steps must be carried out immediately.

WELL CONTROL MANUAL

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