Measured Depth

Measured depth is the total distance along the actual wellbore path from the surface reference point, commonly the rotary table (RT) or kelly bushing (KB), to any downhole location, serving as the primary depth reference for all logging tools, directional surveys, completion operations, and production reporting, and differing from true vertical depth (TVD) by the cosine integral of the wellbore inclination angle, with the difference growing as wellbore deviation increases from vertical toward horizontal.

Key Takeaways

Measured depth is the fundamental quantity actually controlled by field operations: drillers add pipe footage measured in MD, wireline operators play out cable measured in MD, and completion engineers land packers and perforation guns at specified MD intervals, making MD the only depth reference that can be acted upon in real time without post-processing.
Survey calculation methods including minimum curvature, radius of curvature, and the tangential method all convert the same MD and inclination/azimuth survey data into three-dimensional coordinates; minimum curvature is the industry-preferred method because it minimizes position error by assuming smooth arcs between survey stations rather than abrupt angle changes.
Tight tolerance on measured depth accuracy is critical for hydraulic fracture perforation placement in thin-pay horizontal wells: industry standard depth accuracy is plus or minus 0.1 percent of measured hole depth for wireline-conveyed logging runs and plus or minus 0.5 percent for pipe-conveyed logging runs due to the greater uncertainty from pipe weight and pipe stretch corrections versus cable stretch corrections.
The surface elevation datum for measured depth must be consistent across all datasets from a single well: rotary table elevation (RTE) is the standard for most wells and is recorded in the well header, but differences between multiple log runs from different service companies using slightly different datum points can introduce systematic depth offsets that require correction before log integration or perforation design.
Measured depth in directional wells must always be accompanied by TVD in regulatory filings and completion reports because regulatory bodies need TVD to verify formation isolation, injection zone depths relative to aquifers, and compliance with spacing and setback requirements that are defined in terms of vertical depth to formation tops.

Fast Facts

The minimum curvature method calculates incremental TVD between two survey stations separated by delta-MD using: delta-TVD = (delta-MD / 2) x (cos I1 + cos I2) x RF, where I1 and I2 are inclinations at the two stations and RF is the ratio factor = (2/DL) x tan(DL/2), with DL being the dogleg angle between survey points. For a 30-metre survey interval in a horizontal section at constant 90-degree inclination, delta-TVD = 0 and all MD increment is horizontal displacement. Modern MWD tools record survey data every 15-30 m (50-100 ft) along-hole, providing continuous trajectory reconstruction. Cumulative depth error from minimum curvature integration over a 5,000 m MD well is typically 1-3 m in TVD under normal survey frequency.

Tip: When integrating measured depth data from different sources (wireline logs, LWD logs, drill pipe tally, casing tally), always perform a gamma ray depth correlation before accepting the MD alignment. Even two wireline log runs in the same wellbore using the same service company equipment can have 1-3 metre MD offsets due to different cable weights, different sheave friction states, and different cable stretch correction parameters applied. Identify two or three distinctive gamma ray marker peaks, measure the depth shift between runs, and apply a constant or linearly varying shift to bring all datasets onto a common depth reference before cross-plotting or computing composite logs.

What Is Measured Depth

Measured depth is the most basic and physically direct of all depth measurements used in petroleum operations. It represents the length of the wellbore as actually drilled, the sum of all pipe joints or cable increments from surface to the point of interest, without any geometric correction for the wellbore's departure from vertical. The adjective "measured" distinguishes it from "true" depths that require mathematical computation to derive from the physical length measurement.

Every piece of downhole equipment is positioned in the wellbore using measured depth. When an operator says a packer is set at 4,200 metres, that depth is in MD. When a perforating engineer designs a 15-metre perforation interval from 3,850 to 3,865 metres, those limits are in MD. When a geologist reports that the top of the target formation was penetrated at 2,750 metres, the depth of bit at that moment as recorded in the driller's log is MD. This universality of MD as the operational depth reference makes it the default for wellbore data management, even in highly deviated wells where TVD may be far more geologically meaningful.

How Measured Depth Works

The physical basis of measured depth measurement is straightforward: it is the accumulated length of material run into the wellbore. For drill pipe, each joint is measured with a calibrated tape before it is added to the string. The cumulative tally of all joints, plus sub and tool lengths, gives the MD at the drill bit. For wireline logging, the cable is marked or encoded, and a precision depth counter on the logging unit measures cable movement as the tool travels down and up the wellbore. For pipe-conveyed logging tools, the drill pipe tally is used with corrections for temperature and tension.

Cable stretch is the primary source of depth uncertainty in wireline logging. As the cable descends, it bears the tool weight and its own weight, stretching elastically. A standard monoconductor cable at 5,000 m with a 500 kg toolstring may stretch 3-5 metres, applied as a correction to the raw counter reading. An incorrect elastic modulus in the logging unit's parameters introduces a systematic depth error across the entire run. Temperature-driven elongation is a secondary effect handled using the tool's downhole temperature measurement.

Survey calculation methods convert measured depth segments into three-dimensional position coordinates through integration of inclination and azimuth data. The minimum curvature method, which has been the industry standard since the 1980s, treats each borehole segment between consecutive survey stations as a smooth arc whose plane is defined by the survey direction vectors at the two endpoints. The ratio factor (RF) in the minimum curvature formula equals one when the wellbore path is straight and increases slightly as the arc curvature increases. The method conserves path length exactly (unlike the simpler tangential method, which does not) and provides the most accurate TVD and lateral displacement coordinates achievable from the survey data.

Depth matching between log runs and completion data uses distinctive log features for correlation. The gamma ray is the universal correlation tool because all tools see the same geological signal. The casing collar locator (CCL) provides magnetic reference points at known pipe tally depths, allowing wireline depths to be reconciled with completion depths before perforating. Depth shifts identified by CCL correlation are verified against at least two independent collar depths before application.

Measured Depth Across International Jurisdictions

In Canada, AER Directive 059 requires all depth references on WCRs, formation tops, and production logging surveys to specify MD or TVD with the reference datum, and mandates WITSML-format electronic log submissions with MD as the primary index. For Montney and Duvernay horizontal wells, industry standard practice is to express perforation intervals and fracture stage placement in MD along the lateral, with TVD referenced to geologic formation tops. CAPP recommended practices for depth measurement accuracy in horizontal wells are widely adopted across the WCSB.

In the United States, BSEE OCS regulations require all casing, perforation, and formation top depths in well reports to be specified with both MD and TVD with the datum identified. The SEC's reserves reporting rules under Regulation S-X require formation depths in TVD for investor comparability, though all TVD values are derived from MD by survey integration. API and SPE MWD accuracy guidelines provide the depth measurement standards used by major service companies.

In Norway, Sodir's DISKOS repository maintains MD as the primary log depth index, with the national WITSML 2.0 standard requiring MD reference datum and elevation in the well header. Petroleum Regulations require formation tops in appraisal well reports to be in TVDSS for direct comparison with seismic interpretation depths. Equinor's NCS exploration well standards specify cumulative MD error must not exceed 0.2 percent of total measured depth, verified by comparing cable counter, pipe tally, and CCL correlations.

In the Middle East, Saudi Aramco mandates MD as the primary depth reference for all wireline and LWD data in the EXPEC Data Center, with Arab-D formation tops stored as TVDSS for structural mapping using directional survey conversion. ADNOC requires MD accompanied by TVD in all well status reports to the Supreme Petroleum Council, with the drill floor elevation specified to three decimal places as the depth datum.

Measured depth is also referred to as along-hole depth (AHD), drilled depth, borehole depth, or simply well depth in common usage, though "well depth" is ambiguous because it sometimes refers to TVD in non-technical contexts. The abbreviated forms MD, M/D, and MHD (measured hole depth) appear in different company and regulatory documentation systems. Related depth references include true vertical depth (TVD), the vertical component of the wellbore trajectory from surface datum; TVDSS, TVD referenced below mean sea level; and true stratigraphic thickness (TST), the formation thickness corrected for both deviation and formation dip. The kelly bushing is the standard measurement datum for most wells. The minimum curvature method is the standard survey calculation tool that converts MD and survey data into three-dimensional position and TVD. The MD abbreviation is used interchangeably with "measured depth" in field operations.

FAQ

Why do two log runs in the same wellbore sometimes show different depths for the same formation?
Depth discrepancies between log runs arise from several sources: different cable stretch corrections applied by different operators or service companies, different tool weights (which change the cable tension profile), different surface cable lengths affecting the total weight hanging below any given point, and thermal expansion differences if the logs were run at different times during well temperature equilibration. Discrepancies of 1-5 metres between runs are common and are resolved by depth correlation using the gamma ray or CCL marker method, which identifies corresponding formation features or collar joints in both datasets and applies a depth shift to reconcile them. For precision operations such as perforating to an accuracy of 1-2 metres, a dedicated correlation log is always run before perforating to verify the current depth-to-casing relationship.

How does measured depth relate to reservoir navigation in geosteering?
Geosteering uses measured depth as the real-time coordinate while drilling a horizontal or deviated well, with TVD derived from MWD directional surveys updated at each survey station (every 15-30 m MD). The geosteerer's job is to maintain the wellbore within the target formation by adjusting the well trajectory based on measured depth position and real-time LWD formation evaluation. When the gamma ray or resistivity log shows the well is leaving the target zone, the geosteerer calculates what TVD change is needed at what MD interval to re-enter the zone, then directs the directional driller to adjust the toolface accordingly. The combination of MD as the operational progress reference and TVD as the geological position reference is essential to geosteering workflow.

Why Measured Depth Matters

Measured depth is the single reference that connects all wellbore datasets. Formation logs, directional surveys, casing tallies, perforation records, and pressure gauge placements all index on MD. Inconsistent MD references between datasets cause misplaced perforations, spurious log correlations, and incorrect formation depth reports. Horizontal completions with 20-40 fracture stages at precise MD intervals are particularly sensitive: a 5-metre systematic error across 40 stages misplaces significant stimulated volume relative to the geological target. Investment in depth measurement accuracy, cable stretch correction algorithms, and depth matching workflows directly improves completion efficiency and well economics.