Drilling Mud Parameters
There will be many changes in the drilling mud as an overpressured zone is entered. The main effects on the mud due to abnormal pressures will be:
• Increasing gas cutting of mud
• Decrease in mud weight
• Increase in flowline temperature
Since these effects can only be measured when the mud is returned to surface they involve a time lag of several hours in the detection of the overpressured zone. During the time it takes to circulate bottoms up, the bit could have penetrated quite far into an overpressured zone.
Gas cutting of mud may happen in two ways:
• From shale cuttings — if gas is present in the shale being drilled the gas may be released into the annulus from the cuttings.
• Direct influx — this can happen if the overbalance is reduced too much, or due to
• Swabbing when pulling back the drillstring at connections.
Continuous gas monitoring of the mud is done by the mudlogger using gas chromatography. A degasser is usually installed as part of the mud processing equipment so that entrained gas is not re-cycled downhole or allowed to build up in the mud pits.
The mud weight measured at the flowline will be influenced by an influx of formation fluids. The presence of gas is readily identified due to the large decrease in density, but a water influx is more difficult to identify. Continuous measurement of mud weight may be done by using a radioactive densometer.
Under-compacted clays, with relatively high fluid content, have a higher temperature than other formations. By monitoring the flowline temperature therefore a decrease in temperature will be observed when drilling through normally pressured zones. This will be followed by an increase in temperature when the overpressured zones are encountered (Figure 16). The normal geothermal gradient is about 1 degree F/100 ft. It is reported that changes in flowline temperature up to 10 degree F/100 ft. have been detected when drilling overpressured zones.
When using this technique it must be remembered that other effects such as circulation rate, mud mixing, etc. can influence the mud temperature.