Any invasion of formation fluid must result in the expulsion of mud from the well, and this shows up as an increase in surface volume in what is, normally a closed circulating system.
As is the case with flowrate, a gain in pit level may be hard, or impossible, to detect when a slow bleed-in of fluid occurs. It is also very easy for other factors to mask a change in pit level. Surface additions to the mud system, or surface withdrawals and dumpings, must be done with the Driller's knowledge. When a continuous addition is being made, for instance seawater ('giving the mud a drink'), the addition rate should be determined and monitored so that any further increase due to a kick can be detected.
The addition of significant amounts of material such as barite also changes the total mud volume. This should be pre-calculated, and again the Driller informed of the likely increase, and over what period such increase will occur.
The continuous use of de-sanders and/or de-silters and mud cleaners on the active system while drilling results in a slow continuous loss of mud. Experience with the particular equipment installed on a rig enables an estimate for the rate of loss to be calculated. Such a continuous loss easily masks small continuous gains. If the driller does not know the equipment is running, he will not be surprised that the mud level is steady, hence he must be notified whenever this equipment is switched on or off.
A continuously recording pit level monitor aids this process considerably, allowing the Driller to see at a glance if any change in pit level, or in the trend of pit level variation, is taking place; See Figure 2.
Visual observation of mud pit level, recorded at regular intervals with notes on additions and alterations made is a valuable direct reference to what is happening. The drill crew should be made aware of the importance of maintaining an accurate record of actual pit levels by direct observation and bringing any suspect variation immediately to the Driller's attention.
Floating vessels produce problems in accurate measurement of pit volume, as motion of the vessel varies the mud level at the tank sensors. The use of several floats, or other sensors per pit can reduce this problem to acceptable levels if properly located. The effects of heavy weather provide a considerable masking effect; See Figure 3.
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