collar log

A collar log (also called a CCL log or casing collar log) is the wireline log record produced by running a casing collar locator through a cased wellbore, displaying the electromagnetic voltage spike at each casing coupling collar as a distinctive deflection on the depth track alongside other log curves — most commonly the gamma ray channel in the standard GR-CCL combination — to create a physical depth reference grid tied to the casing running tally that is independent of cable-stretch or encoder errors and accurate to 0.05 to 0.15 m per collar, used to correlate and position all cased-hole tools including perforating guns, bridge plugs, packers, and recompletion equipment within their target formation intervals in WCSB wells. The collar log is the simplest and most universally run cased-hole log because it requires no radioactive sources, no active transmission system, and no formation contact; it produces a reliable signal in any fluid, any casing weight, and any well deviation up to approximately 70 degrees inclination, making it the standard pre-perforation survey tool in every WCSB producing basin from the shallow Mannville heavy oil formations at 300 to 800 m depth to the deep Devonian reef and Foothills gas wells at 3,000 to 6,000 m depth. In the Western Canada Sedimentary Basin, collar logs are run in three primary workflows: as a standalone GR-CCL log run before perforating or recompletion in cased wells where the original open-hole log suite defines the target intervals but the casing has shifted depth references from the original wireline measurements (depth shift of 0.3 to 2.5 m is common between open-hole and cased-hole depth references in WCSB wells due to cable-stretch differences at different temperatures and with different tool weights); as a continuous depth reference channel embedded in production logging or cased-hole formation evaluation tool strings (PLT, pulsed neutron, cement bond log) that records one CCL spike per collar traversed throughout every logging run so that every anomaly on the other channels can be assigned a collar-referenced depth rather than relying solely on the cable-measured depth; and as a CT-conveyed CCL channel in the BHA during WCSB Montney and Duvernay plug-and-perf horizontal completions, where the collar log transmitted in real time through a mud-pulse telemetry link or electromagnetic telemetry link to the surface control unit provides continuous confirmation that each plug is being set at the CCL-verified inter-stage boundary rather than relying on CT encoder depth alone.

• Collar log sensor physics, spike waveform characteristics, and signal processing in WCSB cased-hole logging operations: The collar log sensor outputs a continuous voltage trace as the tool moves through the wellbore; between collars, the voltage is near-zero as the magnetic flux through the uniform pipe body remains constant. At each collar, the coupling's additional steel mass increases the local magnetic flux density and then decreases it as the tool exits, producing a doublet waveform: a positive peak as the tool enters the coupling and a negative peak as it exits (or vice versa, depending on tool polarity). The doublet amplitude in millivolts is proportional to the steel mass contrast between the collar and adjacent pipe body, scaled by the magnet field strength and pickup coil sensitivity; API 5CT standard couplings for 5-1/2" production casing (28 kg/m N-80) produce typical CCL amplitudes of 80 to 250 mV at standard logging speeds of 15 to 30 m/min, while heavy-wall P-110 grade couplings produce 200 to 400 mV amplitudes that are easier to detect reliably in noise-contaminated surface acquisition systems. The doublet peak-to-peak distance on the depth scale (typically 0.3 to 0.6 m for standard API couplings) provides a quality check on logging speed consistency: if the doublet width varies significantly between adjacent collars, the tool speed changed during that collar traverse and the depth assignment for that collar position may have a larger-than-normal uncertainty.
• Joint tally correlation method and depth cross-checking in WCSB cased-hole collar log interpretation: Converting CCL spike positions to absolute measured depth requires the casing running tally — the field record documenting the pipe body length and coupling-to-coupling cumulative depth of every joint run during casing installation, typically accurate to 0.01 m per joint from pipe-body measurements with a calibrated steel tape before the joint is run. Starting from the casing shoe (whose depth is precisely known from the drilling rig drill-string tally), the CCL interpreter counts spikes upward from the shoe to the target zone, accumulating tally depth at each spike. This count-based depth is cross-checked against the CCL log's cable-measured depth at multiple reference collars to detect discrepancies indicating a missed spike (tool moved too fast past a collar) or a double-counted spike (surface noise or tool vibration artifact). WCSB well practice requires that the CCL tally-derived depth and the cable-measured depth agree within plus or minus 0.5 m at every reference collar through the target zone; a discrepancy larger than this triggers a second CCL pass at reduced logging speed to confirm the spike count before perforating or plug-setting operations proceed.
• GR-CCL combination log workflow for WCSB cased-hole recompletion and bypass perforation programs: The gamma ray-collar log combination is the standard pre-perforating log in WCSB Devonian carbonate, Cardium tight oil, and Mannville heavy oil recompletion programs because it simultaneously establishes the casing collar reference (CCL) and confirms which formation is opposite each casing interval (GR). The GR response in cased hole closely matches the open-hole GR in the same formation because formation gamma ray emission is unaffected by the presence of steel casing, allowing the completion engineer to overlay the cased-hole GR on the original open-hole log to verify formation top depths and identify the target interval's collar offset. In WCSB Devonian Nisku and Leduc reef recompletions at Pembina and Swan Hills, the GR-CCL combination log is run at 1:200 depth scale to resolve individual carbonate cycles 1 to 5 m thick in the reef section, with perforations placed at the GR-identified low-gamma (clean carbonate, high porosity) zones whose collar offset from the tally is confirmed by counting the CCL spikes from a reference below the reef crest; this approach routinely places WCSB perforations within 0.5 to 1 m of the designed top-perforation depth in intervals too thin to tolerate the 1 to 2 m cable-only depth uncertainty.
• Collar log in WCSB production logging and cased-hole formation evaluation as a continuous depth reference: In WCSB production logging tool strings (spinner flowmeter, temperature, pressure, and water holdup sensors), the CCL is run as a continuous track throughout the logging run, not just in a pre-perforating survey; every anomaly on the spinner or temperature curve (a thief zone accepting injected water, a flow crossflow between zones, a tubing leak) is depth-assigned by its position relative to the nearest CCL spike. This CCL-referenced depth is more reliable than cable-measured depth in WCSB production logging because production logging is typically run with the well flowing (open choke, live wellbore fluid) under conditions where cable speed may vary as the tool string is pumped uphole or pulled down against flow, making cable-measured depth less consistent than in the static conditions of a pre-perforating survey. Post-processing of WCSB production logs always includes a CCL depth shift correction that slides the entire log suite to match the CCL spikes to their known tally depths, removing any systematic offset introduced by logging speed variation or cable creep during the flow-rate changes that occur as the tool passes through different production zones.
• Deviated and horizontal collar log limitations and WCSB CT-conveyed solutions for Montney plug-and-perf depth control: The collar log's electromagnetic detection mechanism becomes less reliable above approximately 60 degrees inclination because the logging tool rests against the low side of the casing bore at a radial standoff from the casing wall that increases with well deviation and tool-to-casing ID ratio, reducing the effective magnetic coupling between the tool's permanent magnet and the casing steel collar. In WCSB Montney horizontal wells at 90 degrees inclination with 5-1/2" casing (ID 121 mm) and a 1-5/8" CCL tool (OD 41 mm), the tool's 40 mm radial standoff reduces collar spike amplitude by 30 to 60 percent compared to a vertically centered tool; this reduced amplitude is still detectable for standard API couplings but may be unreliable for flush-joint or premium-connection couplings where the collar-to-pipe-body mass contrast is already small. CT-conveyed collar logs in WCSB Montney plug-and-perf operations address this limitation by using the mechanical stiffness of the CT string to push the CCL sub against the low side of the casing (consistent standoff of zero to 10 mm) and by running the CCL sub at slow CT speeds (below 10 m/min) during the depth verification phase, with collar spike amplitude typically 50 to 80 percent of the upright-tool value rather than the 20 to 40 percent seen with cable-conveyed tools in horizontal orientations.

WCSB Cardium Recompletion Catching Two-Collar Depth Error

A Pembina Cardium recompletion operator ran a GR-CCL combination log before perforating a bypassed C sand interval at 1,880 m depth in a 1994-vintage producer. The cable-measured depth from the wireline truck indicated the target interval (12 m Cardium C sand identified on the original open-hole GR) was at 1,878 to 1,890 m, with the planned top perforation 2 m below collar 46 from the casing shoe tally. CCL interpretation confirmed collar 46 at cable depth 1,878 m, but the cumulative tally-derived depth for collar 46 was 1,877.3 m, and the tally showed only 45 collars rather than 46 to this depth: investigation revealed the CCL had detected a liner hanger at 1,420 m as a regular collar spike, adding a phantom collar to the count. Correcting the count, the true top perforation was 2 m below collar 45 at tally depth 1,838 m, placing it 40 m shallower than the cable reading suggested and squarely in the Cardium A sand (a water-bearing zone already producing from the existing completion). The CCL re-interpretation prevented perforating the wrong interval and allowed the operator to correctly place guns in the C sand with a redesigned collar offset, adding 38 bbl/d incremental oil on the subsequent well test.

Related Terms

Collar locator is the tool that produces the collar log; the CCL sensor (permanent magnet plus pickup coil) generates the voltage doublet at each coupling that is recorded as the collar log trace used for cased-hole depth correlation in WCSB perforating, recompletion, and production logging operations. Gamma ray log is combined with the collar log in the GR-CCL combination to provide formation lithology context for each collar position; the cased-hole GR matches the open-hole GR closely enough to correlate formation tops to collar offsets within 0.5-1 m in WCSB Devonian carbonate and Cretaceous sandstone recompletions. Perforating depth is specified as a collar offset (meters above or below a named collar number) rather than absolute measured depth to eliminate cable-stretch-dependent depth errors; the collar log provides the verified collar positions used to set that offset. Casing running tally documents the pipe body length and coupling depth of each joint installed; the tally is the ground truth against which CCL spike positions are counted and cross-checked to assign absolute tally depth to every collar detected on the log. Collar lock mechanically anchors a wireline or CT tool assembly at a CCL-verified collar position; the collar log run before the plug or gun deployment confirms which collar number to lock at, and the collar lock prevents the tool from shifting during the perforating or plug-set impulse load.