If secondary control cannot be properly maintained, emergency procedures can be implemented to prevent the loss of control. Two common procedures involve the use of barytes plugs and cement plugs.
1. Barytes plugs
A barytes plug is a slurry of barytes in fresh water or diesel oil which is displaced through the drillstring. If conditions allow, the string is then pulled up above it. The barytes settles out rapidly and should form an impermeable mass, capable of isolating the producing zone.
The effectiveness of a barytes plug derives from the high density and fine particle size of the material and its ability to form a tough impermeable barrier. It has the following advantages: it can be pumped through the bit and offers a reasonable chance of recovering the drillstring. In addition, the material is normally available and the plug can be drilled up easily if required.
To be effective the slurry must:
·be made with good quality barytes with a low clay content;
·have a high density. The density must be at least 0.15 psi/ft greater than the mud density;
·have a rapid settling rate;
·have a high water loss.
The main disadvantage is the risk of settlement and consequent plugging in the string if pumping has to be stopped before the slurry has been completely displaced.
Two common types of mix for barytes plugs may be used:
·barytes-water mix;
·barytes-diesel mix.
Before mixing up a plug, a pilot test should be made using the available materials.
Barytes-water mix
Sodium hexameta phosphate (2.00 kg/m3 (0.7 lb/bbl)) should be included as a thinner to keep the barytes particles separate. Alternatively, a mixture of lignosulphate (1.14 kg/m3 (0.4 lb/bbl)) and caustic soda (0.71 kg/m3 (0.25 lb/bbl)) may be used. Sodium hexameta phosphate should only be used when temperatures below 60°C are expected.
Barytes-diesel mix
A barytes-diesel mix plug is preferred in oil based or invert emulsion muds (alternatively a barytes-water mix can be used with a diesel spacer ahead and behind). An oil wetting agent should be added
Displacement of barytes plugs
Barytes plugs can be batch-mixed and displaced with rig pumps. Continuous agitation of the barytes pill is highly recommended. To mix and displace with a cementing unit may give more accurate volume control.
A minimum final plug length of 60 m (200 ft) and not less than 1.6 m3 (10 bbl) volume should be used to ensure a good seal and allow accurate displacement into the well bore.
2. Cement plugs
The use of cement plugs offers little chance of recovering the drillstring. It is also likely that after pumping cement through the string, it will become plugged, precluding any second attempt if the first should not succeed. Cement plugging should, therefore, be regarded as a last attempt.
A quantity of quick setting (accelerated) cement is pumped into the annulus via the drillstring. Quick setting cement reduces the possibility of gas cutting taking place. The cement is usually displaced until pump and choke pressures indicate that a bridge has formed.
If a cement plug is to be set with mud below it, consideration should be given to setting a slug of viscous mud below the zone to be plugged. This precaution is particularly useful in long or deviated holes or when the cement slurry is substantially heavier than the mud.