6 – Slugs and Heat

Oilfield services companies faced significant challenges when designing their shaped charges, and no help was available from a prime source. The U.S. military still withheld its vast shaped charge knowledge that was obtained during wartime testing and research.

Consequently, oilfield services companies were forced to spend time and money to discover factors affecting the performance and reliability of shaped charges. In 1946, two significant factors appeared during their first testing efforts: slugs and wellbore heat.

Jets First and Then Slugs

Tests of oilfield shaped charges were revealing something that initially seemed to be only a nuisance but quickly became a potential problem. It was a small piece of metal that remained in a target after a shaped charge jet had penetrated it.

The basic mechanism of jet formation was well known. During the detonation of a lined shaped charge, the explosive shock wave collapsed the liner along its axis. The wave pressure was so great that the liner acted like a fluid and exited the charge as a high-velocity jet of particles.

As the shock wave passed over the liner, it forced most of the liner material into the jet. A small amount of liner material formed a projectile that followed the jet (Figure 1).

Designers labelled the projectile a slug or a “carrot” because its shape resembled this vegetable. Tests showed that a slug could lodge in the hole produced by the jet, blocking hydrocarbon passage to the wellbore.

Some shaped charge designers commented on slugs and blockage concerns in their patents. Clyde Davis and Walter Lawson, authors of patents 2,399,211¹ and 2,605,703², respectively, declared that slugs seemed to be an inevitable result of detonating lined shaped charges.

Are Slugs a Problem or Not?

Their reaction to this situation was more positive than might be expected. Davis considered slugs as potentially beneficial for increasing the penetration depth in the formation. Lawson acknowledged the presence of slugs and expressed hopeful confidence that “a more frangible, brittle metal for liners than heretofore employed may become available.”

Even Morris Muskat, leader of the design trio at Gulf Research and Development Company, was optimistic about the presence of slugs. In one of his patents (2,494,256)³, Muskat stated that his shaped charge design would produce a slug “sufficiently small to pass through the hole in the casing and come to rest deep in the formation where it has negligible effect on the flow of fluids through the hole.”

Robert McLemore, the well-published engineer at Well Explosives Inc., announced that designers could avoid slugs by selecting the correct metal for the liner. His article in the December 1946 issue of The Oil and Gas Journal stated that testing showed slug formation resulted from hard metal liners but not from soft metal.⁴ To prove his claim, he included extensive test results in the article.

Mathematics to the Rescue

During the debate about slugs, a mathematical theory explaining the jet formation process and slug formation appeared in the June 1948 issue of The Journal of Applied Physics.⁵

According to this theory, the tremendous force of the explosive blast wave collapsed the conical liner, separating it into an inner and an outer layer.

The inner layer became the jet that shot out of the charge at approximately 30,000 feet per second (ft/sec). The outer layer formed into a slug that followed the jet at the slower speed of 3,000 ft/sec. (Figure 2).

Cut, Detonate, and Measure

To verify the accuracy of the mathematics, the article’s authors conducted an experiment (Figure 3). They machined a series of cuts in a liner in planes parallel to the cone’s base. Each section was carefully weighed and colored.

The re-assembled sections were then inserted into a charge. The charge was detonated and directed at a large volume of water or sawdust to capture the jet materials.

The explosion resulted in a collection of small slugs. By weighing each slug particle and comparing it to the weight of its corresponding cone section, the authors found the contribution of each part of the cone to the slug.

This empirical approach showed that the upper part of the conical liner contributed the smallest amount of material to the slug. As the cone collapsed, contributions to the slug increased, with the base providing the most material.

Additional research by oilfield services companies generally corroborated these conclusions about slug formation. An article in the 1949 issue of the Lane Wells publication Tomorrow’s Tools Today stated that slugs could contain as much as 60 percent of the original liner material.⁶ The article author stated a common hope was that abrasion of the slug during its passage into the formation would erode it enough to prevent blockage of the hole.

It’s Hot in the Wellbore

Slugs were not the only challenge. High wellbore temperatures also complicated shaped charge development. Designers initially selected an explosive called Pentolite™ for these devices.⁷ Pentolite was insensitive to shock, had a high energy content, and exhibited a rapid detonation velocity of 4.6 miles per second—all desirable characteristics for penetrating through casing and into the formation.

Further research revealed a problem: The explosive’s melting point was approximately 180°F. Many wellbore temperatures were much hotter than this. Within minutes of lowering a shaped charge with Pentolite into a hot wellbore, the explosive fumed off as a gas, rendering the device useless.

Oilfield services companies turned to the E. I. DuPont de Nemours Company (DuPont) for a solution. In 1947, DuPont developed an explosive that could withstand 325°F for 24 hours. DuPont placed this explosive in their Standard Charge and sold it separately to oilfield service companies for use in charges of their own design.

References

1. Davis, C. O. et al., Method of Perforating Well Casings, U.S. Patent number 2,399,211. Filed March 19, 1942.↑
2. Lawson, W. E., Liner for Hollow Charges, U.S. Patent number 2,605,703. Filed July 6, 1944.↑
3. Muskat, M. et al., Apparatus for Perforating Well Casings and Well Walls, U.S. Patent number 2,494,256. Filed September 11, 1945.↑
4. McLemore, Robert, “Casing Perforating with Shaped Explosive Charges,” The Oil and Gas Journal, December 1945. p. 268.↑
5. Birkhoff, G. et al., “Explosives with Lined Cavities,” Journal of Applied Physics, Vol. 19, June 1948. p. 564.↑
6. Forsyth, Val, L., “Koneshot Perforating,” Tomorrow’s Tools Today, 3Q 1949. p. 5.↑
7. McLemore, Robert H., Application of the Shaped Charge Process to Petroleum Production, Technical Paper at the 27th Annual Meeting of the American Petroleum Institute. November 11, 1947.↑

Join the Discussion!

• Have you come across any firsthand accounts or additional sources about how oilfield services companies of that era addressed slug formation in shaped charges?
• What are your thoughts on the financial impact of shaped charge technology in the oil industry—both in terms of initial development costs and the long-term economic benefits?

I’d love to hear your perspective. Share your thoughts or answer a question in the comments below!