OilAuthority.com

Gun Zero

Gun zero is the reference depth alignment procedure in perforating operations that establishes the exact correspondence between the position of the perforating gun in the wellbore and the depth scale of the open-hole or cased-hole wireline logs, ensuring that the perforating shots are fired at the correct formation depth relative to the logged reservoir intervals and the established oil-water or gas-water contacts.

Key Takeaways

  • Gun zero is established by correlating a gamma ray or casing collar locator (CCL) log run with the perforating gun string to distinctive radioactive markers or casing collar signatures whose depths have been established on the open-hole or previous cased-hole logs, allowing the gun position to be confirmed to within a few centimetres of the intended interval.
  • Depth errors in perforating, if undetected, can result in perforations being placed outside the productive reservoir interval, below the oil-water contact, or into a non-reservoir tight zone — all of which reduce initial production rates and may cause early water breakthrough that is difficult to remediate without costly workover operations.
  • The casing collar locator (CCL) is the primary tool for gun zero in cased wells, detecting the electromagnetic signature of casing collar couplings at known depth spacings; its response is correlated to the CCL trace on the original cement bond log or casing tally to confirm gun depth.
  • In underbalanced perforating with tubing-conveyed guns or wireline-conveyed guns in wells with deviation, the additional challenge of cable stretch, wellbore friction, and differential sticking must be accounted for to maintain accurate gun depth through the perforating operation.
  • Radioactive markers (radioactive bullets or radioactive-tagged centralizers) are sometimes shot into the formation immediately before the main perforating run to provide a depth reference that can be confirmed on a subsequent gamma ray log, verifying that the perforation interval is where intended.

Fast Facts

Casing collars on standard API casing are typically spaced at the joint length of the casing pipe, either 9.1 metres (30 feet) or 12.2 metres (40 feet) for most production casing strings. A CCL log identifies each collar as a distinctive positive or negative spike, and counting collars from a surface-measured casing tally gives a depth reference accurate to within the measurement tolerance of the wireline cable depth measurement. The combination of CCL depth correlation and a gamma ray log run simultaneously with the gun provides the two independent depth checks needed to confidently place perforations in a specific reservoir interval — particularly important when thin beds (less than 1 metre) must be selectively perforated while avoiding thin interbedded shales or tight zones.

What Is Gun Zero?

Perforating is a critical step in well completion that determines which intervals in the casing wall are opened to the wellbore and therefore which formation layers will contribute to production. Because a perforated interval cannot be easily un-perforated, placing the shots at the correct depth is essential. Gun zero is the procedure by which the perforating engineer confirms the depth alignment between the physical gun position in the wellbore and the depth reference established on the formation evaluation logs before firing the charges.

The challenge of gun zero arises because wireline depth measurement has inherent uncertainties: cable stretch under the weight of the tool string, thermal expansion of the cable at high temperatures, cable slippage at the surface depth measurement wheel, and borehole friction in deviated or horizontal wells all introduce depth errors between the surface cable measurement and the actual position of the tool downhole. These errors are typically small (a few tenths of a per cent of total depth) but can translate to depth errors of 1 to 5 metres on a 3,000 metre well — enough to miss a thin productive zone or to perforate into an adjacent non-productive interval.

How Gun Zero Is Performed

The standard gun zero procedure in a cased hole uses the casing collar locator (CCL) sensor incorporated into the perforating gun string or run immediately above it. As the gun is lowered into the well, the CCL records a distinctive electromagnetic response each time it passes a casing collar — the collar couples the inner and outer casing pipe joints and has slightly different electromagnetic properties than the casing pipe body. These collar responses create a recognizable pattern of depth-spaced spikes on the CCL log that can be matched to the casing tally measured at surface during casing running.

The CCL log run during gun positioning is laid over the CCL log recorded during the cement bond log (CBL) run immediately after casing was cemented — the two CCL logs should show identical collar signatures at the same depths if both are correctly depth-referenced. Any systematic shift between the two CCL logs indicates a depth error that must be corrected before firing the gun.

A gamma ray sensor run with the gun provides a second independent depth reference by comparing the natural radioactivity curve against the openhole gamma ray log. Correlation of distinctive gamma ray markers (high-gamma radioactive shale beds, low-gamma clean sand beds) between the gun-conveyed GR and the openhole GR confirms that the gun is at the intended depth and that the depth scale of the perforating run matches the depth scale of the formation evaluation logs that defined the target interval.

Once the CCL and GR correlations confirm the gun is at the correct depth — within the tolerance specified in the perforating program, typically plus or minus 0.3 metres — the gun zero is established, the program is satisfied, and the charges are fired.

Gun Zero Across International Jurisdictions

Canada (AER / WCSB): Alberta Energy Regulator completion reporting requirements under Directive 059 require documentation of the perforating program including the perforation depth intervals referenced to KB, which implicitly requires that the gun zero procedure has been performed correctly to deliver the shots at the intended formation depths. WCSB unconventional completions in multistage horizontal wells — where 20 to 40 perforation clusters per lateral are required within specific reservoir-quality intervals — demand precise gun zero at each stage to correctly place clusters in the high-quality rock identified on formation evaluation logs.

United States (BSEE / API): API RP 19D (Measuring the Properties of Proppants Used in Hydraulic Fracturing and Gravel-Packing Operations) and API RP 43 (Recommended Practice for Evaluation of Well Perforators) provide technical guidance for perforating operations including depth control procedures. BSEE offshore perforating programs for Gulf of Mexico completions require that perforating depth be documented relative to the reference log suite, with CCL and GR correlation records submitted as part of the well completion package.

Norway (Sodir): Norwegian Petroleum Directorate / Sodir completion reporting requirements for NCS wells include depth references for all perforated intervals, requiring that the perforating depth documentation demonstrate correlation to open-hole logs through CCL and GR gun zero procedures. Equinor's completion engineering standards specify the required quality of depth correlation before perforating is authorized, with explicit requirements on the shift tolerance between the gun-conveyed log and the reference log suite.

Middle East (Saudi Aramco): Saudi Aramco's well completion procedures require a formal gun zero confirmation sign-off from the completion engineer before perforating charges are fired, based on CCL and gamma ray correlation meeting the specified depth accuracy requirements. In Arab Formation completions, where selective perforation of specific reservoir quality zones within tight stratigraphic intervals is the norm, gun zero accuracy is a direct determinant of initial well productivity and long-term sweep efficiency.

Gun zero is also called depth correlation for perforating, gun depth confirmation, or perforation depth calibration. Related terms include casing collar locator (CCL), perforating, perforation depth, gamma ray log, tubing-conveyed perforating (TCP), and wireline perforating. The term "tie-in" is used in some operations as a synonym for gun zero — confirming that the gun depth is tied to the reference log depth.

Tip: When performing gun zero in a highly deviated horizontal well, account for the wellbore friction that can cause the wireline cable to hang up at doglegs or in the horizontal section, creating depth discrepancies between the surface cable measurement and the actual gun position. If the CCL and GR correlations show a non-systematic scatter rather than a clean constant shift, the cable is sticking and slipping intermittently, making depth confirmation unreliable. In this case, use radioactive markers fired at a known reference depth (such as a strong gamma ray marker close to the perforating interval) to provide a directly measured in-situ depth reference that is independent of cable depth uncertainty.

FAQ

What happens if gun zero is not performed correctly?
If gun zero is not performed or is performed with excessive depth error, the perforating shots may be fired outside the intended reservoir interval. In a tight reservoir with a 1 to 2 metre net pay zone, a 1 to 2 metre depth error can place all shots in the adjacent shale rather than in the pay zone, resulting in a completion that looks mechanically successful (shots fired, wellbore integrity intact) but produces no hydrocarbons from the intended interval. This is not immediately detectable without a subsequent production log, and the well may be incorrectly characterized as a reservoir quality failure rather than a completion depth error, with consequential impact on field development decisions.

Can gun zero errors be corrected after perforating?
Once perforations are in place, they cannot be easily removed or relocated. If a depth error is discovered after perforating — from a subsequent production log, a downhole camera survey, or a comparison of radioactive bullet positions against the target log depth — the options are limited. The mis-perforated zone can be isolated with a mechanical bridge plug or with a cement squeeze, and a new set of perforations can be added at the correct depth in an additional run. This remedial perforation requires a workover operation and represents avoidable cost and deferred production. Prevention through careful gun zero procedure is far preferable to remediation.

Why Gun Zero Matters

The accuracy of perforation placement determines how effectively the completion connects the wellbore to the productive reservoir interval, and a properly executed gun zero is the technical foundation for that accuracy. In tight reservoirs, thin pays, and precision multi-stage horizontal completions, depth errors of even a fraction of a metre can result in shots placed in non-productive rock, contacts perforated rather than avoided, or clusters placed in poor reservoir quality intervals that do not respond to stimulation. Gun zero is the quality control step that ensures the investment in formation evaluation — the logs, core analysis, and reservoir characterization that defined the target interval — is correctly translated into the physical placement of the completion that must drain the reservoir over the well's producing life.

Browse the Oil and Gas Glossary