Perforation Misfire: A Wireline Engineer’s Hard Lesson on Blasting Cap Orientation

Posted on by Crissa Fel Tawadan

In wireline perforating, precision is everything. Whether you’re setting up perforating guns for cased hole wireline or running pump down perforating operations, one small mistake can mean a failed detonation—or worse, an unintended one.

I learned this the hard way during a critical well perforation job.

The Setup: A Routine Perforating Run

We were preparing for a wireline perforating job on a high-pressure well, setting up A-140 blasting caps in tandem with perforating guns. The client expected a clean, efficient detonation, ensuring optimal wellbore connectivity.

We ran through our wireline service provider checklist:

  • Verified the firing sequence using Titan Control Fire switches.
  • Checked ground continuity with the logging cable.
  • Confirmed det cord integrity and proper charge loading.

Everything looked perfect. We armed the system, pumped down, and prepared to fire.

Then… nothing happened.

The Problem: A Misfire Due to Blasting Cap Orientation

Instead of a clean perforation sequence, we were dealing with a total misfire.

  • Surface equipment indicated the firing signal was sent.
  • No perforation events were detected downhole.
  • Pressure data showed zero communication between zones.

Something was wrong.

After pulling the toolstring back up, we found the issue—the A-140 blasting cap had been inserted backwards.

How Did This Happen?

The A-140 blasting cap is directionally sensitive—if installed incorrectly, it won’t properly initiate the prima cord.

  • The wires had been inserted first instead of the charge end.
  • The prima cord had doubled over inside the tandem, creating a fold that interfered with the detonation.
  • Extra scrap prima cord had been left inside the gun, adding to the interference.

These mistakes directly caused the misfire.

Fixing the Issue: Proper A-140 Blasting Cap Installation

To prevent this from happening again, we followed the correct procedure:

  • Inserted the A-140 cap with the wires last, ensuring the charge end was facing the prima cord tail.
  • Trimmed the prima cord to the correct length, avoiding any folds or doubling over inside the tandem.
  • Removed all unused or scrap prima cord—never place it back into the gun!

We also made a crucial change—switching from the A-140F to the A-140S.

  • The A-140S is shorter, making it easier to insert correctly.
  • It contains the same explosive load as the A-140F.
  • It won’t become fluid disabled until five minutes after wetting, reducing failure risks.

With everything correctly configured, we reran the guns, fired again, and this time, the perforations went off flawlessly.

Key Takeaways for Wireline Engineers

This experience reinforced several critical lessons about wireline perforating safety:

  • A-140 blasting caps must be installed in the correct orientation.
    • Wires go last, and the charge end faces the prima cord tail.
  • Never leave excess prima cord inside the gun.
    • Doubling over can cause interference and misfires.
  • Switch to the A-140S for easier installation.
    • It’s shorter, safer, and less prone to errors.
  • Test every gun assembly before deployment.
    • Use VeriFire or another testing system to check continuity before running in the hole.

Final Thoughts: Small Details Prevent Big Problems

In wireline perforating, even the smallest oversight can cause a misfire. That day, we avoided a major setback by catching the issue early—but it was a lesson I’ll never forget.