12 – Stop the Interference

Oilfield services companies knew that recovering oil and gas required numerous perforations in relatively thin formation layers. To obtain the maximum perforation of these layers, companies developed carriers housing multiple shaped charges placed within a few inches of each other.

Field experience with these carriers frequently showed disappointing results; the amount of oil or gas recovered was far less than anticipated. Something was affecting the perforation capability of these multi-charge units. Testing revealed the problem: a phenomenon called “interference”.

Charge Proximity Causes Problems

An explosive rope called “Primacord” detonated the shaped charges. The detonation speed of this rope was approximately 22,000 ft/sec.¹ When the exploding Primacord arrived at a charge, it detonated the charge explosive (usually RDX). The detonation rate of this explosive was faster than the Primacord—usually exceeding 25,000 ft/sec.

The charges detonated in sequence, and if they were closely packed the explosion wave from a charge interfered with the next charge (Figure 1).

Oilfield services companies performed testing to characterize the interference. In 1949, Val Forsyth, an engineer at the Lane Wells Company, announced results of his testing efforts in the company technical magazine, Tomorrow’s Tools Today.² According to Forsyth, interference could result in jet prevention, jet dispersion, or jet deflection.

The article did not specify if the Lane Wells Company solved the problem. Instead, Forsyth stated, “…the control of the problem is one for the individual service company.” He then pointed out that “sound engineering practices and thorough testing” should minimize the problem.

Patents Offer Solutions

The practice of not divulging specific details was understandable; companies did not want to give this information to competitors. Some ideas to prevent interference, though, were revealed in U.S. patents.

Patent 2,742,857, submitted by the Lane Wells Company, claimed that a carrier with shaped charges inside brittle (“frangible”) containers would prevent interference.³ As the carrier was lowered into the well, it filled with well fluid. When a shaped charge detonated, the container blocked the flying bits of the charge housing and the fluid cushioned the blasts, preventing interference with nearby charges (Figure 2). Fragments of the containers collected at the bottom of the carrier.

A different solution to interference was described in patent 2,926,603. It advocated placing thin metal shields over the detonator and the rear part of the charge explosive.⁴ According to the author, the shields produced a more precisely centered charge initiation and shielded the charges from the forces of adjacent exploding shaped charges (Figure 3).

Deflecting the Explosion Wave

Patent 2,818,808 described the use of “acoustical reflection plates” between charges.⁵ According to the author, these plates reflected the explosion pressure wave, enabling an adjacent shaped charge to detonate and form its jet (Figure 4).

Steel was proclaimed to be the best plate material because it had “high acoustical resistance”. In addition, the loosely attached plates provided a small amount of movement as each charge exploded.

The plates’ acoustical property and their movement helped prevent an “blast echo” off the plates that distorted the jet of a nearby shaped charge.

A Solution for Carrier Bulging

The content of another patent—2,837,995—was unique because it offered a solution to interference and different shaped charge issue: carrier bulging.⁶ As the shaped charges detonated, the blast shattered the casing and propelled the fragments at high speeds. The fragments impacted the interior wall of the carrier, causing it to bulge.

To solve this issue, the patent described shaped charges with casings having two partially encased portions located approximately ninety degrees apart (Figure 5).

The charges would be aligned in the carrier so the partially encased portions faced nearby charges and the uncased segments faced the carrier wall.

The encased portions would prevent interference with other charges, and the uncased segments would not propel enough material to damage the carrier wall.

Has Interference Been Eliminated?

Unofficial closure of the interference discussion was made by Val Forsyth, the Lane Wells Engineer, in his technical paper submitted to the 1950 Spring meeting of the American Petroleum Institute.⁷ Forsyth stated, “Considerable progress has been made toward the elimination of interference.” He confidently proclaimed that most service companies recognized the effects of carrier dimensions, charge positioning, and more design details. “They have taken corrective measures.”

References

1. McLemore, R., Application of the Shaped Charge Process to Petroleum Production, Technical Paper at the 1947 Annual Meeting of The American Petroleum Institute. p. 129.↑
2. Forsyth, Val L., “Koneshot Perforating,” Tomorrow’s Tools Today, 3Q 1949. p. 114.↑
3. Turechek, G. F., Gun Perforators, U.S. Patent Number 2,742,857. Filed January 12, 1950.↑
4. Lindsay, W.H., Well Perforator Shaped Charge, U.S. Patent Number 2,926,603. Filed March 12, 1952.↑
5. Dill, W.S., Jet Perforating Gun, U.S. Patent Number 2,818,808. Filed April 7, 1954.↑
6. Castel, J.H., Unsymmetrically Encased Explosive Shaped Charges, U.S. Patent Number 2,837,995. Filed December 25, 1952.↑
7. Forsyth, Val L., A Review of Gun Perforating Methods and Equipment, Technical Paper at the 1950 Spring Meeting of the American Petroleum Institute. p. 30.↑

Join the Discussion!

• Do you have any knowledge of alternative solutions that were explored to mitigate interference in shaped charges, beyond the patents mentioned in the blog?
• In your opinion, did the technological advancements in mitigating interference impact the overall efficiency and economics of oilfield perforation?