Alternative Methods

Even though capping is the most common method of approaching a surface intervention, there are other, specialized techniques that help blowout control experts do their job.

Stinger Operation

A stinger is “an open-bore sub with a taper on the bottom end.”³ Its operation is best understood with the help of Fig. 5.4, where two stingers are displayed: One in the tubing, and one in the casing valve. The full opening valve is open when the stinger is brought over the stream, and generally, high-pressure pipe and pumping equipment is attached to the stinger, and kill operations initiated once the stinger is in place.^{3, 11} As the force from the blowing well pushes the equipment upward, the stingers are run on Athey wagons, or track hoes, to get them in place, and then rigged and tied down.^{3, 11} This type of operation is suitable for wells that are possible to kill by bullheading, among other criterias.³

Fig. 5.4 – Dual stinger operation – Left: Stinging into casing valve, and pumping down stinger in tubing. Right: Post-sting and kill operation.¹¹

Junk Shots

Junk shots are designed to seal flanges, BOPs, or valves that are leaking. It involves injecting different material into the flow path, thus plugging the leak before it grows bigger. Material such as “shredded rope, rubber, nut hull, ball sealers and even golf balls,”⁴⁵ are used to help plug the hole. In a stinging operation, the seal between the stinger and the exit of the blowing fluids is un-even and not sealed properly. Junk shots help seal it off.⁴⁵

Freezing and Hot Tapping

As the word implies, freezing is actually a process of creating an ice plug of viscous bentonite and water, and acts as a temporary plug in order to replace or fix equipment.^13,

45 Dry ice is normally used as the cooling agent, as direct application of liquid nitrogen would be too cold, and could make the steel brittle.¹³ Freezing has not been used to control a blowout, but can be useful in other parts of a blowout operation.⁴⁵

Hot tapping a well involves entering equipment under pressure, and is achieved by

“drilling entry ports into the pressured equipment.”²⁵ It is a useful process when there are trapped pressures that hinder the normal operation of a valve. The pressured zone above or below it can be hot tapped, and the pressure bled off.^{13, 25} This type of equipment “has been used on blowouts to allow pumping into wellheads, tubulars or fire-frozen

valves.”⁴⁵

Plugs

As a last resort, gunk or fast-acting cement plugs can be used to plug the flow path of the well. Gunk is a mix of cement, bentonite, and diesel, which will react with water⁴⁶ and turn into a thick gum-like substance. This method is sometimes used in underground blowouts, as it can isolate formations from pressures. The placement of a plug is critical, however, as a misplacement of the plug might “plug off the well above the underground flow and isolate the surface from the problem.”⁴⁶ There are variants of the

above-mentioned gunk, such as salt gunk, which reacts with saltwater flows, or invert gunk, which reacts with oil flows or oil-based mud.⁴⁶

Fast-acting cement is another material that might be used in creating plugs that are able to hold pressure, yet easy to drill through. Like gunk, it would be pumped down the wellbore through the wellhead or capping stack, and hopefully set before blowing fluids are able to push it out of the hole. As with gunk plugs, good placement is very important, and such a plug might cause more problems than already present. It is often difficult to ever regain control of a well in which a gunk or cement plug has been set.⁸

Bridging

Bridging is a term describing when a formation is not able to withstand the pressure differential, and caves in to block off the hole. Studies have shown that bridging actually occurred in almost 40% of OCS blowouts, and in 16% of Texas blowouts for the period 1960-1996, illustrated in Fig. 5.5.⁴⁷ These natural bridging occurrences generally occur during the first 24 hours of the blowout, and the probability of bridging decreases as time goes by.⁴⁸ Inducing bridging is sometimes attempted, and can be accomplished by reducing the flowing bottomhole pressure (FBHP) by venting at the surface, and hoping to reduce the FBHP enough to drop below fracture pressure and force formation

failure.⁴⁶

Fig. 5.5 – Comparison of kill methods between OCS and Texas from 1960 – 1996.⁴⁷

5.1.4 Bullheading

As discussed before, bullheading aims at forcing the blowing fluids back into the hole, and potentially fracturing a formation. This is not advisable in a drilling situation, where a kick is to be handled, but the circumstances of a blowout are very different. As Fig. 5.5 shows, bullheading is a very common kill method in both onshore and offshore blowout containment. It is a very easy and cost-effective way of killing a well, when there is access to the well through the surface.³ At the end of a bullheading operation, the kill fluid has displaced all of the original mud and kick fluids, and hopefully the well is in hydrostatic equilibrium.

The dual stinger operation shown in Fig. 5.4 set up a dual bullhead kill. The 2 inch by 4 inch stinger in the tubing, and the 5 inch by 8 inch stinger in the annulus both delivered 13 lb/gal (ppg) down the well, and the RU-64 was brought back under hydrostatic control.¹¹