Depth Control: Base-Log Referencing, Cable Stretch Correction, and Log Curve Tie-In

Depth control is the discipline of ensuring that every measurement recorded in a borehole, whether from wireline, logging-while-drilling, coring, or perforating, is tied to a single consistent depth reference so that all data describe the same point in the rock. The reference is conventionally the base log, normally the deep resistivity curve recorded on the first comprehensive logging run, and every subsequent pass or tool string is shifted to match it so the gamma ray, density, neutron, sonic, and image logs all align to the same formation tops. The need for depth control arises because measured depth, the distance along the borehole from a fixed surface datum such as the kelly bushing or rotary table, is not measured perfectly. The primary measurement is made at surface by a calibrated wheel counting cable movement at the winch, but the steel wireline is elastic and stretches under its own weight and the tool weight, more at total depth than near surface, and it dilates with the high temperatures of deep wells. The tool may also stick and then jump, or move at a different rate than the cable is being reeled, so the depth counted at the winch can differ from the true position of the sensor downhole by several metres in a deep wireline well. Depth control corrects for these errors so that a perforating gun is set across the intended pay and not the shale above it, so that a core depth matches the log it will be calibrated against, and so that correlations between offset wells in a field are geologically valid. The practical workflow begins by establishing the base log on the deepest, most complete run, applying a stretch correction derived from cable tension and known tool weight, and then depth-matching every other curve to that base by correlating distinctive markers such as a tight carbonate stringer or a sharp resistivity kick. In the Western Canadian Sedimentary Basin, depth control is critical across the full stratigraphic column, from shallow Mannville coals to deep Montney and Duvernay targets at 2,500 to 4,000 m where cable stretch can exceed several metres and a misplaced perforation wastes an expensive frac stage. Casing collar locators provide an independent depth check in cased hole by detecting the steel collars at known joint lengths, letting completion crews tie perforating and cement-evaluation runs back to the open-hole base log. Modern logging-while-drilling adds another reference frame, driller's depth from the pipe tally, which must be reconciled with wireline depth because the two systems disagree by amounts that matter when picking landing points for a horizontal lateral. Good depth control underpins reserve mapping, net-pay calculation, and the AER Directive 059 well-data submissions that populate the public record, so an error propagates from the wellbore into the field model and the regulatory file.

Stretch Correction and Tension Logging

The raw depth a logging unit records is the cable paid out past a calibrated measuring wheel, but the cable behaves like a long spring. Under the combined weight of itself and a heavy tool string, the deepest section stretches most, and downhole temperature adds thermal elongation. Service companies apply a stretch correction computed from continuously logged cable head tension and the known weight and elastic modulus of the cable, pulling recorded depth toward true depth. On deep, hot WCSB wells the correction can be several metres, and because the stretch grows with depth it is non-linear, so the correction is applied as a depth-dependent function rather than a single offset. Without it, the deepest and most valuable pay would be systematically mis-depthed.

Tying Open-Hole and Cased-Hole Operations

Depth control must survive the transition from open hole to cased hole. The open-hole base log defines the formation tops, but perforating, cement evaluation, and production logging happen after casing is set, when the resistivity tools can no longer see the formation. The casing collar locator bridges the gap by detecting collars at known joint lengths and is run together with a gamma ray so the completion engineer can correlate the cased-hole gamma to the open-hole base and shift the perforating depths to match. This open-hole-to-cased-hole tie is what places a frac stage across the intended Montney interval rather than wasting CAD-scale proppant and pump time on barren rock.

Related Terms

Depth control ties together the data sources that describe a wellbore. Wireline is the cable system whose stretch and slip create the depth errors that depth control corrects. The resistivity log is conventionally chosen as the base curve because its sharp bed boundaries make a reliable reference. Measured depth is the along-hole distance that depth control aims to render accurate, and gamma ray log is the curve most often correlated between open hole and cased hole to carry the base depth through casing.

Real-World WCSB Scenario: Perforating a Duvernay Completion

A completion crew preparing a Duvernay well near Kaybob plans a 40-stage frac and must place each perforation cluster within a 30 m target interval picked on the open-hole resistivity base log at about 3,400 m. The cased-hole perforating string carries a gamma ray and casing collar locator, and the engineer correlates the cased-hole gamma to the open-hole base, finding a 1.8 m discrepancy caused by uncorrected cable stretch on the first wireline run. Left uncorrected, the deepest clusters would land in the bounding shale below the pay.

Applying the depth shift and re-tying every cluster to the base log costs an extra two hours of rig time, roughly CAD 18,000, but ensures all 40 stages perforate the engineered rock. Misplacing even three stages into non-reservoir would have wasted an estimated CAD 450,000 in proppant, pump charges, and lost stage productivity, so the disciplined depth tie is among the cheapest insurance on the entire completion.