last-shot detection, Oil & Gas Glossary

Last-shot detection is a surface or downhole monitoring technique used during tubing-conveyed perforating to confirm that every perforating gun in the string has fired, from the uppermost shot to the deepest, so the completion crew knows the planned interval is fully open before pulling out of the hole. The problem it solves is specific and costly. In tubing-conveyed perforating, long gun strings carrying hundreds of shaped charges are run on tubing or drill pipe and detonated at depth, often across several thousand feet of pay in one descent. Unlike wireline guns, which are short and quickly retrievable, a TCP string is committed to the well, and a misfire of one section is not visible from surface by any obvious signal. If the bottom gun fails to detonate while the upper guns fire, the operator can wrongly believe the whole interval is perforated, run completion equipment, and only discover the unperforated zone weeks later when production falls short of forecast, by which time remediation means a costly re-entry. Last-shot detection closes that gap by giving a positive, real-time confirmation that the detonation wave propagated through the entire string. Several physical principles are used. Pressure-based systems watch for the characteristic pressure transient the firing produces in the wellbore, since detonation and the inrush of formation fluid into the newly created tunnels generate a recordable signature at surface gauges or downhole memory gauges. Marker-based systems place a fluorescent or weak radioactive tracer in the firing head or in a marker sub so that, once the bottom gun fires, the released marker is detected at surface in returns or logged on a subsequent pass, confirming the detonation reached the last charge. Fiber-optic and acoustic systems sense the detonation event directly along the string. Depth correlation of TCP guns is commonly tied to a radioactive marker placed in a locator sub and read against a gamma-ray log, which both positions the guns and supports firing confirmation. In the Western Canadian Sedimentary Basin, where multi-stage horizontal completions in the Montney, Duvernay, and Cardium increasingly use long perforating intervals and where rig time runs into thousands of CAD per hour, reliable last-shot confirmation protects against the largest avoidable completion failure: a zone that was paid for, planned, and logged as perforated but never actually opened to flow. It feeds directly into the decision to proceed with stimulation, to set packers, or to make a confirmation run before committing further capital.

Key Takeaways

Confirms full-string detonation: Last-shot detection verifies that the detonation propagated from the top shot to the bottom shot of a TCP gun string, not just that firing was initiated at surface. This distinction matters because a detonation train can fail partway down, leaving lower guns intact while surface indications suggest a successful fire.
Pressure signature method: The most common surface technique reads the wellbore pressure transient that accompanies firing, the abrupt change as charges detonate and formation fluid surges into the new perforation tunnels. A clean, full-magnitude transient on a fast-sampling gauge indicates the guns fired as designed; a muted or absent signal flags a possible misfire requiring investigation.
Marker and tracer confirmation: Fluorescent or low-activity radioactive markers placed in the firing head or marker sub are released or activated only on detonation. Detection of the marker in returns at surface, or on a follow-up gamma-ray pass, gives independent physical proof that the bottom of the string fired, complementing the pressure record.
Depth tie via radioactive sub: TCP perforating depth is correlated by reading a radioactive marker in the locator sub against a gamma-ray log, the same correlation infrastructure that supports firing confirmation. Accurate depth control ensures the shots land on the target sand and not in shale or against a casing collar.
Protects high-value rig time: A single undetected misfire can cost a re-entry running well into six figures CAD plus deferred production. Real-time last-shot confirmation lets the crew decide on the spot whether to proceed, re-fire, or run a confirmation log, turning a hidden risk into a managed decision point.

Pressure-Transient and Memory-Gauge Detection

The dominant field method records the wellbore pressure response to firing on a high-frequency gauge, either at surface or as a downhole memory gauge recovered after the run. Successful detonation produces a sharp pressure spike or drop, depending on wellbore conditions, followed by the characteristic fill-up signature as reservoir fluid enters the tunnels. Analysts compare the measured transient against the modelled response for the planned shot count and charge size; a deficit suggests fewer charges fired than intended. On underbalanced TCP jobs, where the well is purposely run with lower wellbore than formation pressure, the immediate inflow surge after firing is especially clear, making the pressure method both the detection signal and a first indication of interval productivity.

Marker, Acoustic, and Fiber-Optic Systems

Where pressure data is ambiguous, independent confirmation comes from markers and direct sensing. Enhanced fluorescent or weak radioactive marker material loaded near the bottom gun is released on detonation and identified in returns or on a logging pass, proving the last shot fired. Fiber-optic strings and downhole acoustic sensors capture the detonation acoustically along the gun, time-stamping each gun section's fire so the operator can pinpoint exactly where a string stopped detonating. These systems convert a single pass-fail surface reading into a section-by-section diagnosis, which is valuable when a long Montney or Duvernay interval must be confirmed shot for shot before stimulation.

Fast Facts

The reliability problem last-shot detection addresses is not theoretical. Industry studies of tubing-conveyed perforating have documented misfire and partial-detonation rates high enough that operators in critical completions routinely budget a separate confirmation run. A single shaped charge in a modern deep-penetrating gun jets a copper liner at roughly 7,000 metres per second, creating a tunnel in milliseconds, yet the detonation train linking guns can be interrupted by a faulty booster or wet connection, which is precisely why a positive end-of-string signal, rather than an assumption, is treated as the standard of completion assurance.

Last-shot detection is meaningful only in the context of tubing-conveyed perforating, the technique of running guns on pipe rather than wireline, which is what makes a misfire hard to see. The shaped charges it confirms are the working element of perforating, the process of creating the flow path between reservoir and wellbore. Many TCP jobs are run as underbalanced perforating, where the post-fire inflow surge both cleans the tunnels and sharpens the pressure signature that last-shot detection relies on for confirmation.

Real-World WCSB Scenario: Confirming a 250-Metre Duvernay TCP Interval

A completion crew running a tubing-conveyed gun string across a 250-metre Duvernay interval near Fox Creek, Alberta, fires the guns underbalanced and watches a fast-sampling surface pressure gauge. The recorded transient comes in at roughly 80 percent of the modelled magnitude for the planned 320-shot string, a deficit consistent with one of the lower gun sections failing to detonate. Rather than pull out and risk leaving 50 metres of pay unperforated, the crew runs a quick gamma-ray correlation pass that reads the bottom marker sub as undischarged, confirming the lowest guns did not fire.

The operator re-runs a short TCP string on the unperforated section the following day. The CAD 95,000 confirmation pass and re-fire is a fraction of the deferred production and re-entry cost that an unnoticed misfire would have caused once the multi-million-dollar stimulation program had been pumped past the dead interval.