A dynamic kill method is widely used intervention method. The dynamic kill procedure is performed by circulating a kill fluid (for example seawater) with density resulting in hydrostatic pressure less than the static pressure before the fluid is weighted. The rate at which is kill fluid pumped will generate sufficient frictional pressures in the blowing well and stop the influx of formation fluid. After the influx is stopped, a weighted mud will circulate to statically control the well. Dynamic kill method can also be performed in combination with capping what will give additional backpressure. It can also be performed either through relief well or through surface intervention. The decision of using relief well or a drill string in the wild well is based on the required kill rate as well as the likelihood of successfully snubbing a drill string into the well. Because of a small inner diameter of a drill string, significant pressure will be lost inside drill string at a high kill rate. For a relief well the annulus may be used to circulate down the kill fluid, which would result in less frictional pressure drop and less pumping power required for the same kill rate.
4.2.5. Gunk plugs
Gunk squeezes or plugs are last option if all mentioned methods fail. Gunk plug generally consists of 150 ppb bentonite and 150 ppb cement mixed with diesel oil and reacted with fresh water mud (Tarr, Flak, 2017). The plug is pumped in into the formation where forms a high viscosity pill that retards the fluid flow. The advantage of gunk squeezes is that it consists of additives not soluble in diesel oil and that prevents setting in drill pipe before placement is completed. An alternative to gunk plug is using fast-reacting cement.
Except for gunk plugs, there are several options if they are not effective. Isolation can be accomplished by bridging (natural or induced), plugging (lost circulation materials or soft plugs) or mechanical means (open hole packers, cased hole packers) (Tarr, Flak). Natural bridging controls most underground blowouts because exposed shales cannot withstand resulting pressure differentials. Plugging charged zone with lost circulation materials or barite plugs is not so effective in controlling significant underground flows. Enormous quantities of LCM must be pumped in attempt to plug off a charged zone. On the other hand, barite plugs can be effective only if hydrostatic control is regained long enough to let barite to settle. When it comes to mechanical means, packers were used to isolate the hole. Modern open hole packers that can handle high differential pressures (7000-9000 psi) were snubbed in and set near the flowing zone. A major difficulty in this case is that hydrostatic control or bridging/plugging isolation is still needed.
1. Examples of underground blowouts-case history
1) Relief well control of underground blowout in Bahrain
2) Algerian underground blowout
1.1 Relief well control of underground blowout in Bahrain
Producing wells in Middle East are faced with downhole corrosion problems as they mature. External and internal corrosion of production casing and tubing can lead to underground blowout as it was the case in well 159. Well 159 was drilled in 1959 in Bahrain and was completed within the shallow sandstone reservoir. Thirty-six years after the well was completed, a routine annulus survey was conducted and indicated tubing leak in 5” production casing. Several attempts were made to work the well over to get near the producing perforations after pulling part of the original completion. These conventional methods failed and attempt to kill and cement off flow using a snubbing unit failed as well.
In the interval within the shallow aquifer from 300′ to 700′, casing had shown signs of severe corrosion. The casing was parted and shifted below 681′ and the wellbore was no longer accessible from the surface below this depth. Blowout flow rate was estimated at 8 MMscfpd of gas. This caused significant safety problems (Flak L.H. et al, 1995): 1) sour gas venting out of the charged aquifer in offset well cellars; 2) subsurface charging and associated shallow gas hazard. Relief well 515 was drilled as a solution to kill underground blowout in well 159, plug and abandon well 159 and then serve as a replacement well. Few challenges have to be considered in pre-planning process (Flak L.H. et al, 1995):
1) Surface separation of blowout and relief wells limited by shallow reservoir depth to 85′;
2) No borehole surveys of any kind available on the blowout;
3) Safely drilling and casing of the shallow actively charged aquifer;
4) Maintain well control and borehole stability of the relief wellbore;
5) Near well intercept (<3') to assure blowout kill and well 159 abandonment; 6) Using relief well to replace blowout after the kill.