14 – Flash X-Rays Can Take the Picture

Shaped charges for oilfield perforation services were initially developed using results of surface penetration tests. Designers relied on their intuition and examination of perforations in steel targets to improve their designs.

However, these examinations could not enable evaluation of the numerous factors that affected shaped charge performance. Even when testing charges in a laboratory under controlled fluid pressure and temperature conditions, it was difficult to determine if the outcome was a result of a specific design change.

Designers also knew that individual charges were never exactly identical, even if they were produced at the same time by the same manufacturer. Minor differences from charge-to-charge caused variations in penetration.

High-Speed Blasts

Visual observation of shaped charge explosions revealed insufficient information. The blasts were too rapid; a typical detonation sequence lasted only a few microseconds (1 microsecond equals 1 millionth of a second).

To appreciate the brevity of this measurement, consider this comparison: One microsecond is to one second, as one second is to two weeks.

Specialized instruments were needed to produce images of the detonation process. Designers turned to X-ray equipment that emitted a single pulse of X-rays lasting 0.1 to 0.2 microseconds. These short X-ray pulses prompted the name “Flash X-rays”.

When used with precise timing control, a Flash X-ray device produced a dense, high-resolution image (a shadowgraph) on X-ray film of the collapsing shaped charge and its jet (Figure 1).

However, oilfield services companies did not possess these instruments. Arrangements were made with specialized laboratories, institutions, and major companies (like E. I. DuPont de Nemours Company) to produce Flash X-ray imagery using their equipment.

Shadowgraphs were very useful for determining the nature of the jet.¹ They showed whether the jet was a continuous stream of liner particles or clusters of material (Figure 2). Deviated jets also displayed clearly in shadowgraphs, as did the size and shape of the jet tip.²

X-rays that produced a shadowgraph were not affected by the flash from the shaped charge detonation. The debris cloud that sometimes formed during the explosions also presented no barrier to the X-rays.

Multiple Flash X-ray instruments were frequently used simultaneously to produce shadowgraphs of a jet at slightly different time intervals or from different angles. This technique enabled calculation of the jet tip speed. It also displayed how the jet moved in three dimensions.

Some Secrets Are Revealed

Experiments with Flash X-ray imagery quickly revealed interesting and useful data. For example, the velocity of a typical jet was not uniform throughout its length; the tip travelled at approximately 30,000 ft/sec., while the rear portion moved at a slower 15,000 ft/sec. In addition, a jet could be a continuous rod of material or a series of small, closely packed pieces.³

Flash X-ray imagery also revealed that jets from highly ductile material, such as copper or aluminum, maintained their continuous structure much longer than those from brass or steel.⁴

Satisfaction with results from Flash X-ray imagery was so great that it motivated one author to extol its value in an article in The Petroleum Engineer.⁵ According to the author, the Flash X-ray device enabled detailed study of the jet stream and played an important role in the development of a new charge. The author also claimed that research using the Flash X-ray and field data enabled a jet stream to “be tailored to any specific job.”

References

1. Poulter, Thomas C. et al., Measurement of the Dynamic Characteristics of Perforating Shaped Charges by the Use of Ultra-High-Speed Photographic Techniques, Technical Paper at 1959 Fall Meeting of the Society of Petroleum Engineers. p. 258.↑
2. Walters, W. P. et al., Fundamentals of Shaped Charges, John Wiley & Sons, Inc., New York, NY. 1989. p. 338.↑
3. Poulter, Thomas C. et al., The Development of Shaped Charges for Oil Well Completion, Technical Paper at the 1956 Fall Meeting of the SPE Petroleum Branch. p. 14.↑
4. Pugh, E.M. et al., The Photography of High-Speed Metallic Jets, Technical Paper at the 1952 Meeting of the Meteoritical Society. p. 6.↑
5. Kastrop, J.E., “New Shaped Charge Packs a Bigger Wallop.” The Petroleum Engineer, January 1957. p. B-66.↑

Join the Discussion!

• Could you share any historical insights about the use of Flash X-ray equipment in the early days of oilfield shaped charge testing?
• Do you know of any early oilfield service companies or individuals who played a key role in adopting or promoting Flash X-ray technology for shaped charge development?