Perforating Depth-Control Log

A perforating depth-control log is a wireline log run before a perforating job to confirm exactly where the perforating gun will fire relative to the geological zones the operator wants to perforate. The log compares the position of casing collars (the joints where two pieces of casing screw together) to a reference log such as a gamma ray, then aligns those casing collar positions to the depth scale of the original openhole logs that mapped the formation. The output tells the perforating engineer where to position the gun so that when it fires, the perforations land in the right rock. A few centimetres of error in this alignment can mean perforating the wrong zone, missing the target entirely, or punching holes into a water-bearing layer instead of an oil-bearing one.

Key Takeaways

A perforating depth-control log is run inside cased hole, with the perforating gun assembled in the toolstring, before the gun is fired. It uses a casing collar locator (CCL) plus a gamma ray sensor to tie the gun's position to known reference points in the well.
Casing collars are the joints where adjacent pieces of casing screw together. Each collar has a slight diameter increase that the CCL detects as a magnetic anomaly. The CCL log is essentially a series of pulses, one per collar, recording the position of each joint along the wellbore.
The gamma ray sensor records natural radioactivity from the formation behind the casing. This gamma ray response is matched to the gamma ray curve from the original openhole log run before the casing was set, giving an independent depth correlation.
Combining the CCL pattern with the cased-hole gamma ray gives two independent depth references. The perforating engineer uses both to position the gun within a few centimetres of the planned target depth before firing.
Without depth correlation, perforations can land outside the intended zone. The cost of a missed perforation set is significant: pulling the depleted gun, running a new gun, and re-perforating typically adds USD 50,000 to USD 200,000 in service charges and a day or more of rig time, plus the production loss from any wrongly perforated water zones that now have to be isolated.

Fast Facts

Modern perforating gun assemblies often combine the depth-control log with the perforating run on the same toolstring, eliminating the need for a separate logging run. The CCL and gamma ray sensors sit above the gun. The wireline operator records the depth-control log on the way down to the target depth, correlates to the openhole logs in real time, positions the gun, and fires. The whole sequence takes a few hours for a single perforation set in a vertical well, much less than the separate "log first, then run gun" workflow that used to be standard before about 2000.

What Depth Correlation Actually Solves

Imagine you have measured the height of a building on the outside, then walked inside and tried to figure out exactly which floor you are on. Your outside measurement tells you the building is 60 metres tall with 20 floors. Your inside survey records each step of the staircase. You can count steps and estimate which floor you are on, but small errors in your inside survey accumulate as you go up, and after 15 floors of estimation you might be a metre off.

Wells have the same problem. The original openhole logs measured the formation depths to within a few centimetres at every point. After the casing was set, the inside-the-casing depth measurements drift slightly because the wireline tool stretches and slips as it runs in and out, and because the depth measurement at surface accumulates small errors over thousands of metres of cable.

The depth-control log fixes this by tying the cased-hole position back to the openhole reference at multiple points along the wellbore. The CCL gives a discrete pulse at every casing joint (typically one every 12 metres). The gamma ray gives a continuous curve that can be matched to the openhole gamma ray. Between the two references, the depth uncertainty before perforating drops from a few metres to a few centimetres.

Where Depth Correlation Matters Most

The most demanding application is in horizontal wells where multiple perforation clusters must be placed precisely along the lateral. A multi-stage shale completion in the Permian Basin or the Montney Formation might call for 50 to 80 perforation clusters distributed across a 3,000-metre lateral. Each cluster has to land in the planned interval. A correlation error of a metre at one cluster might mean the cluster is in slightly less productive rock; an error of 5 metres could mean missing the target zone entirely.

Vertical wells with thin pay zones are the other demanding case. Some Devonian carbonate plays in Western Canada produce from pay sections only 2 to 4 metres thick. Perforating a 4-metre zone requires putting the gun within about a metre of the right depth. A depth-control log lets the perforating engineer hit that target reliably. Without one, the engineer is essentially shooting blind based on the original surface depth measurement, which is rarely accurate enough.

Older wells being recompleted bring their own challenges. A well drilled in the 1970s may not have a good openhole gamma ray on file, or the available logs may have been depth-shifted multiple times by previous owners. A modern depth-control log run in cased hole gives the recompletion engineer a fresh, reliable reference that can be tied to whatever historical log data still exists.

The perforating depth-control log is sometimes called a CCL/GR log, a perforating correlation log, or just a "correlation run." Related terms include casing collar locator (CCL, the magnetic sensor that detects the small diameter increase at each casing joint; produces a series of pulse signals as the tool passes each collar; the workhorse depth reference for cased-hole logging), gamma ray log (a measurement of natural formation radioactivity; works through casing because gamma rays penetrate steel; the reference curve that depth-control logs match to the original openhole gamma ray for absolute depth correlation), perforating gun (the assembly of shaped charges that fire when triggered to create perforations through the casing and cement into the formation; positioned using depth-control log correlation before firing), perforating (the operation of creating connections between the wellbore and the formation by firing shaped charges through the casing; depends critically on accurate depth control), and cased-hole logging (the family of wireline measurements run after casing is set; includes the depth-control log, production logging, cement bond evaluation, and various reservoir-monitoring logs).

Why Half a Metre Matters in a Three-Kilometre Lateral

A completion engineer is supervising a multi-stage perforating job on a horizontal well in northeast British Columbia. The well has 48 planned perforation clusters distributed across a 2,890-metre lateral in the Montney Formation. Stage 14 of 48 calls for clusters at 3,860 metres, 3,896 metres, and 3,932 metres measured depth, all within a planned 4-metre window of organic-rich silty rock.

The wireline crew runs the perforating gun string, which includes a CCL and gamma ray for depth control. The CCL pattern is matched against the cement bond log run earlier. The gamma ray curve is matched against the openhole gamma ray from the geosteering data. Both references agree to within 0.3 metres. The gun is positioned. The first cluster fires.

Without the depth correlation, the surface depth measurement on the wireline alone would have placed the gun about 1.4 metres deeper than its actual position. The 4-metre target window would have been a 2.6-metre window after subtracting the depth error. Two of the three planned clusters would have been outside the target. The two miss-positioned clusters would have produced perforations into less-productive rock, reducing stage 14's contribution to total well production by an estimated 25 to 30 percent.

The depth-control log added 35 minutes to the perforating run and about CAD 8,000 to the wireline service charge. The avoided production loss was worth something in the range of CAD 80,000 to CAD 150,000 in present value. Across the 48 stages of the completion, the cumulative value of accurate depth control is the difference between a routine, well-performing completion and one that under-performs the type curve. Few line items on the completion AFE pay back as reliably as the depth-control log.