MD (Measured Depth)

MD (measured depth) is the distance along the actual wellbore trajectory from the surface reference datum, typically the rotary table (RT) or kelly bushing (KB) elevation, to any downhole point of interest, measured by summing the lengths of all drill pipe, casing, or wireline cable segments run into the wellbore and recorded as the primary depth reference on all logging, completion, and production data without correction for wellbore deviation from vertical.

Key Takeaways

In a vertical well, MD and true vertical depth (TVD) are nearly identical; in a deviated or horizontal well, MD always exceeds TVD and the difference can be hundreds to thousands of metres in highly deviated horizontal wells where the lateral section may extend 3,000 metres horizontally with a vertical target zone only 50 metres thick.
All wireline logging tools, LWD sensors, and perforation guns are positioned and reported in MD because that is the physical distance from surface that cable length and drill pipe tally control; conversion to TVD requires integration of directional survey data using minimum curvature or radius of curvature algorithms.
The surface datum for MD must be explicitly stated in all well records: rotary table elevation (RTE) is the standard for most oil and gas wells, but kelly bushing (KB), derrick floor (DF), and ground level (GL) are also used, and datum mismatch between different log runs or between old and new data can cause depth shift errors that misplace perforations or misalign log correlations.
MD is also called along-hole depth or measured hole depth (MHD) in some company and regulatory documents, and the distinction between MD and TVD must always be specified when reporting completion intervals, pay zone tops, and production casing settings to avoid ambiguity in well design and regulatory submissions.
Depth accuracy tolerances for perforation placement in horizontal wells are typically plus or minus 1-2 metres in MD, requiring careful depth correlation between the logging run (measured by cable depth counter), the completion run (measured by tubing tally), and the LWD survey (measured by drill pipe tally), with gamma ray or CCL correlations used to verify and adjust depth matches before perforating.

Fast Facts

For a horizontal well with a kickoff point at 2,000 m TVD, a build section reaching 90 degrees inclination, and a 3,000 m horizontal lateral, the total MD at the end of the lateral may exceed 6,000 m while the TVD remains approximately 2,000 m throughout the lateral. The MD at the kickoff point equals the TVD at that point (assuming a vertical well above KOP), but every metre of additional MD in the lateral adds zero TVD. Industry standard depth measurement accuracy for wireline logging is approximately 0.1 percent of hole depth, meaning a 5,000 m well has a depth uncertainty of approximately 5 m cumulative, which can be significant for correlation between offset wells without log-to-log depth matching.

Tip: When comparing log data between wells or when correlating a new well to an existing field, always verify whether depths are reported in MD or TVD before making stratigraphic or fluid contact correlations. Comparing MD depths in a vertical well to MD depths in a deviated well without converting both to a common TVD datum (such as mean sea level, MSL, or formation top TVD) will produce spurious apparent depth differences that may be misinterpreted as structure or stratigraphic thinning. Most well data systems store both MD and TVD, but the default display and reporting field is often MD.

What Is MD

MD stands for measured depth and represents the cumulative length of the wellbore from the surface reference point to any location along the well path. It is "measured" in the sense that it is the physical length of pipe, cable, or tubing run into the well, not a geometric calculation. When a driller adds a 30-metre drill collar to the string, MD increases by 30 metres regardless of whether the wellbore is vertical, inclined, or horizontal at that point. MD is therefore always greater than or equal to TVD, with equality only in a perfectly vertical well.

MD is the fundamental reference depth for all downhole operations because it is the only depth that can be directly measured in real time. Every other depth representation, including TVD, true stratigraphic thickness (TST), and true vertical depth subsea (TVDSS), must be calculated from MD by applying directional survey data. For this reason, wireline logs are depth-indexed in MD, perforation reports specify MD intervals, and completion designs reference MD when specifying packer depths, tubing lengths, and hydraulic fracture stage positions.

How MD Works

Surface depth measurement is typically tracked by a cable depth counter (also called a depth measuring system or DMS) on the wellsite logging unit for wireline operations, or by the driller's pipe tally for drillstring operations. The cable counter measures the length of wireline cable played out from the surface drum, indexed against wheel rotations calibrated to cable diameter. Systematic errors arise from cable stretch under tension (which increases with depth and tool weight), thermal expansion of the cable under downhole temperatures, and calibration drift of the wheel encoder. These errors are managed by applying a cable stretch correction based on the known elastic modulus of the cable construction, the cable tension measured at surface, and the temperature profile of the well.

Pipe tally depth for completion operations uses the measured length of each joint of tubing or casing as it is run in hole, accumulated in a tally book and cross-checked by the wellsite supervisor. Individual joint lengths are measured with a calibrated steel tape and corrected for temperature differential between surface measurement conditions and downhole thermal expansion. Steel has a thermal expansion coefficient of approximately 12 x 10^-6 per degree Celsius, meaning a 5,000 m tubing string will be approximately 1 metre longer at 100 degrees Celsius than at 20 degrees Celsius, a depth shift that is significant for precise perforation placement in a tight formation with a 5-metre net pay interval.

Directional survey data, collected by magnetic or gyroscopic measurement-while-drilling (MWD) tools at survey stations every 15-30 metres, records the inclination (deviation from vertical) and azimuth (compass direction) of the wellbore at each station. The minimum curvature method, which is the industry standard for converting survey data to Cartesian coordinates, assumes the wellbore follows a smooth circular arc between survey stations. Integrating the inclination angle along the MD between the surface and any point downhole gives the TVD at that point: TVD = integral of cos(inclination) d(MD). For a horizontal well with uniform 90-degree inclination, every unit of MD adds zero TVD, so the TVD relationship demonstrates clearly why the two quantities are independent.

Depth matching between different logging runs or between logging and completion data is a routine but important operation. When a gamma ray log is run on wireline and compared to the gamma ray sensor on a casing collar locator (CCL) run during perforating, systematic depth offsets of 1-5 metres are common due to the different depth measurement systems, cable stretch corrections, and pipe tallies used. Log-to-log correlation using distinctive formation markers (characteristic gamma ray peaks or density spikes) shifts one dataset to align with the other. This correlation depth shift is applied to all perforation depths before firing, ensuring perforations are placed in the correct formation interval rather than above or below the intended target.

MD Across International Jurisdictions

In Canada, the AER requires all well depth references in WCRs, reservoir pressure reports, and production reports to specify the datum and whether depths are MD or TVD. AER Directive 059 requires logging data submission in MD referenced to kelly bushing elevation. Montney horizontal well programs routinely drill laterals exceeding 3,000 m MD in the horizontal section, and the AER's electronic submission system requires MD-to-TVD conversion tables for deviated wells to support formation top correlations in the regional GIS database.

In the United States, BSEE requires both MD and TVD in the Well Activity Report for OCS wells under 30 CFR Part 250. The EPA's UIC program requires injection well completion intervals to be specified in MD and TVD for Class II wells, confirming injection zone isolation from protected groundwater using TVD of perforations relative to the base of underground sources of drinking water (USDW). The USGS National Petroleum Data System archives MD as the primary depth field.

In Norway, Sodir requires all DISKOS well data submissions to specify whether depths are MD or TVD with the reference datum clearly identified. Offshore NCS drill floor elevations vary with rig type and water depth, making datum specification critical. Formation tops in Sodir's lithostratigraphic database are stored as TVDSS to provide a common datum across wells drilled from different rig types.

In the Middle East, Saudi Aramco requires MD as the primary depth reference on all logs and completion records, with TVD from the directional survey database. ADNOC's Abu Dhabi data management system maintains separate MD and TVD tracks, with Thamama carbonate formation tops reported in TVDSS to support cross-field structural mapping without datum artifacts.

MD is also written as M/D, measured hole depth (MHD), or along-hole depth (AHD) in various company and regulatory documents. The complementary depth reference is true vertical depth (TVD), which is the vertical component of the wellbore trajectory. True vertical depth subsea (TVDSS) references TVD to mean sea level rather than the rig floor, enabling cross-well structural mapping. True stratigraphic thickness (TST) corrects for both wellbore deviation and formation dip to give the actual thickness of a formation layer perpendicular to its bedding planes. The kelly bushing (KB) is the standard surface datum for MD measurement in rotary drilling operations. Minimum curvature method is the standard algorithm for converting MD and directional survey data to TVD and spatial coordinates.

FAQ

Can MD ever be less than TVD?
No. MD is always greater than or equal to TVD. TVD is the vertical component of the wellbore trajectory, which is always less than or equal to the actual path length. Equality occurs only in a perfectly vertical wellbore (zero inclination throughout). In any deviated wellbore, the path along the wellbore is longer than its vertical projection, so MD exceeds TVD. The difference can be very large: a horizontal well with 4,000 m of lateral section at 90-degree inclination may have an MD of 6,000 m while its TVD is only 2,000 m.

Why is MD used as the primary depth reference rather than TVD?
MD is the primary depth reference because it is the only depth that can be directly and continuously measured in real time during drilling and logging operations. TVD requires directional survey data acquired at discrete intervals (typically 15-30 m) and a numerical integration calculation that introduces additional uncertainty. Every operational decision made during drilling, logging, and completion, including setting casing, positioning logging tools, landing packers, and firing perforations, is made based on MD because the driller controls pipe length in hole, not vertical depth. TVD is a derived, computed quantity used for geological and engineering calculations after the fact.

Why MD Matters

MD is the operational language of the wellbore. Every field operation from spud to abandonment uses MD as the primary reference for positioning equipment, recording data, and communicating between surface and downhole. Errors in MD, whether from cable stretch miscorrection, pipe tally mistakes, or datum mismatch, translate directly into misplaced perforations, incorrectly positioned packers, and misaligned log correlations that can result in perforating the wrong zone, missing the pay interval, or incorrectly reporting completion depths to regulators. In horizontal wells targeting thin pay zones of 5-10 metres TVD thickness, a 3-metre depth error at surface translates to perforations that miss the target entirely. The precision and reliability of MD measurement is therefore a direct determinant of completion quality, and the industry has invested substantially in depth measurement systems, cable stretch correction algorithms, and log-to-log depth matching workflows to minimize depth uncertainty across all phases of the well lifecycle.