Shot Detection: Surface Acoustic Sensing, TCP Gun Verification, and Misfire Diagnosis
Shot detection is the surface monitoring technique used to confirm that downhole perforating guns have actually fired, giving the completion crew a real-time answer at the wellhead instead of waiting on an indirect pressure response or a later flow test. The method matters most in tubing-conveyed perforating (TCP) operations, where the gun string is run on production tubing or drillpipe and detonated by a dropped bar, a pressure cycle, or an electronic firing head, and where there is no wireline cable carrying a return current to confirm detonation. A shot detection system places one or more sensors on the wellhead, the lubricator, or the surface flowline that register the sharp acoustic, vibration, or hydraulic shock signature produced when shaped charges detonate hundreds or thousands of metres below. When the charges fire, the gun body ruptures and the wellbore fluid column transmits a pressure pulse and a structural vibration up the tubing string; an accelerometer, a hydrophone, or a high-frequency pressure transducer captures that transient and the surface readout shows a clear spike against the background noise. The basic physics is straightforward: detonation of a 39 g RDX or HMX charge generates a shock front that couples into the steel and the fluid, and the travel time from the perforating interval to surface is governed by the acoustic velocity of the fluid, roughly 1,200 to 1,500 m/s in completion brine, so a gun at 2,800 m typically registers at surface within about two seconds. In a deep, deviated, or gas-filled well the signal attenuates and the surface indication can become inconclusive, which is exactly why operators in the Western Canadian Sedimentary Basin run dedicated digital shot detection rather than trusting a microphone taped to the tree. Verification is not a luxury. A TCP gun that fails to fire, or that suffers a low-order partial detonation, leaves the reservoir unperforated or only partially open, and an unfired live gun is a serious well-control and personnel hazard that must be handled, fished, and disposed of under strict explosives-handling procedures. Confirming a clean, full-order detonation lets the supervisor proceed to flow back, kill the well, or pull the spent gun string with confidence, and the recorded shot trace becomes part of the completion record alongside the perforating depth log and the charge inventory. Modern systems pair the surface sensor with a downhole bottom-shot indicator, a small secondary detonation initiated when the firing reaches the bottom of the gun string, so the crew sees both the initial and terminal events and can infer whether the entire gun column fired in sequence.
Key Takeaways
- Surface confirmation of firing: Shot detection answers the single most important post-perforating question, did the guns fire, using wellhead-mounted accelerometers, hydrophones, or fast pressure transducers that capture the detonation shock transmitted up the fluid column. In TCP work there is no wireline return signal, so this surface read is the primary verification before the crew flows the well or pulls the spent string.
- TCP is the primary use case: Because tubing-conveyed guns are fired hydraulically or mechanically rather than electrically, the operator cannot watch a firing current on a wireline panel. Shot detection fills that gap, and is standard on long perforating intervals, deviated horizontal Montney and Duvernay wells, and any job where an unfired live gun would create an unacceptable handling hazard.
- Travel time reveals depth and timing: The detonation pulse reaches surface at the fluid acoustic velocity, roughly 1,200 to 1,500 m/s in brine, so a gun at 2,800 m registers in about two seconds. Comparing the expected and observed arrival time helps confirm the firing occurred at the intended interval rather than at a shallow misfire point.
- Bottom-shot indicators close the loop: A secondary charge at the base of the gun string detonates last, so the surface trace should show both the main event and a terminal pulse. Seeing only one event, or a weak smeared signal, flags a possible low-order detonation or a gun section that failed to fire, prompting a remedial run.
- Safety and the completion record: Verified detonation governs whether crews can safely pull the gun string or must treat it as live explosives. The recorded shot trace, charge inventory, and perforating depth log together form the auditable completion file that operators retain under AER and BCER well-completion reporting expectations.
Sensor Types and Signal Interpretation at the Wellhead
Three sensor families dominate field practice. A piezoelectric accelerometer clamped to the tubing hanger or tree captures structural vibration; a hydrophone or fast pressure transducer plumbed into the wellhead reads the fluid pressure transient; and legacy jobs sometimes still use a simple geophone or microphone for a coarse audible check. Digital systems sample at several kilohertz and timestamp each event, so the analyst can distinguish a genuine full-order detonation, a sharp high-amplitude spike with fast rise time, from pump noise, slips setting, or a tubing bump. In a gas-charged or foam-filled well the pressure pulse attenuates sharply, so accelerometer data on the steel string often gives the cleaner record. Reading the trace correctly is the skill: amplitude alone is unreliable across different well geometries, so timing and waveform shape carry the diagnosis.
Misfire Diagnosis and Remedial Decisions
When the surface trace is ambiguous, the crew must decide whether to assume a misfire. A clear single spike at the predicted travel time with a matching bottom-shot pulse confirms a full detonation. A weak, smeared, or absent signal points to a low-order detonation, a failed firing head, or a gun that did not initiate, any of which leaves live charges downhole. The standard response is to wait the prescribed time, bleed pressure safely, and pull the string under explosives-handling protocol, treating every charge as live until visually confirmed spent at surface. In WCSB horizontal completions with multiple TCP intervals, a missed shot interval may be re-perforated with a follow-up wireline or coiled-tubing gun run rather than redressing the whole string, a decision that hinges directly on what the shot detection record shows.
Fast Facts
The acoustic velocity of completion brine sits near 1,450 m/s, only slightly faster than seawater, which is why a perforating event 3 km down arrives at the wellhead microphone in roughly two seconds rather than instantly. Operators learned the hard way that a taped-on microphone is unreliable in deep gas wells: the compressible gas column damps the pressure pulse so severely that early TCP jobs in the 1980s sometimes pulled guns believed unfired that had in fact detonated cleanly, and the reverse, both expensive mistakes that drove the move to multi-sensor digital detection.
Related Terms
Shot detection is inseparable from tubing-conveyed perforating, the deployment method whose lack of a wireline return signal created the need for surface verification in the first place. It depends on the behaviour of the perforating gun and its shaped charge detonation to generate a detectable shock, and the recorded event feeds directly into the broader well completion program, where confirmed perforation depth and gun condition determine the next step in bringing the reservoir on production.
Real-World WCSB Scenario: Verifying a Long Montney TCP Interval near Dawson Creek
An operator running a 180 m TCP gun string on 114 mm tubing across a Montney interval at 2,650 m near Dawson Creek, British Columbia, rigs up a dual-sensor shot detection package on the wellhead, an accelerometer on the tubing hanger and a fast pressure transducer on the kill line. The job carries a budget line of roughly CAD 18,000 to 25,000 for the detection service and crew time, trivial against the CAD 1.2 million perforating and completion cost. The bar is dropped, and the surface trace shows a sharp main spike at 1.9 seconds followed by the expected bottom-shot pulse at 2.1 seconds.
The matched timing confirms a clean full-order detonation across the entire gun column, so the supervisor proceeds to flow back without a remedial run. Had the bottom-shot pulse been absent, the crew would have treated the lower gun sections as live, pulled under explosives protocol, and scheduled a coiled-tubing re-perforation, adding two days and roughly CAD 140,000. The verified shot trace is filed with the BCER completion record.