Permanent Datum

Key Takeaways

• Kelly bushing (KB) datum as the permanent datum for drilling and wireline logging operations provides the reference point from which all measured depths on the drilling log, wireline logs, and formation evaluation data are measured, with the KB elevation above mean sea level (KB AMSL) recorded on every log header to allow conversion of log depths to true subsurface depths below sea level (depth below sea level = KB AMSL minus TVD): during active drilling, the KB datum is physically present as the top of the kelly bushing or rotary table through which the drill string passes, and all drill pipe measurements (drill collar depth, bit depth, casing shoe depth) are made from this reference point; when the drilling rig moves off the well, the physical KB no longer exists, but the KB datum elevation remains the reference point for all subsequent wireline log and completion measurements, as recorded on the log header and in the well completion report; the consistent use of the KB datum from initial drilling through the entire producing life of the well enables direct depth correlation between wireline logs from different logging runs, between logs from different service companies, and between the well's formation tops (picked from the logs in depth referenced to KB) and the subsurface mapping of formation elevations on the field development map; any failure to maintain consistent datum usage (for example, erroneously using tubing hanger elevation rather than KB elevation for a production log run depth reference) creates artificial depth shifts in the log data that appear as formation tops offsets or completion depth mismatches when the data is loaded into the reservoir model.
• Pressure datum correction is the application of the permanent datum elevation to convert measured wellbore pressures (which are measured at the depth of the pressure gauge, typically expressed as the pressure at gauge depth in psi) to datum-referenced pressures that represent the equivalent pressure at the datum elevation, enabling reservoir pressures from different wells and different gauges to be compared on a consistent basis: the datum correction converts the pressure measured at gauge depth to the pressure that would be measured at the permanent datum elevation by applying the hydrostatic head correction p_datum = p_gauge + rho * g * (h_gauge - h_datum), where rho is the fluid density, g is gravitational acceleration, h_gauge is the true vertical depth of the gauge below the datum, and h_datum is the TVD of the datum reference (zero for measurements at the datum itself); in reservoir pressure surveillance, the datum-corrected pressures from all observation wells, producing wells, and injection wells are plotted on a single pressure-depth plot referenced to the common datum (typically a reference level within or near the reservoir, such as the oil-water contact depth or a specific formation marker depth) to assess the spatial distribution of reservoir pressure and the rate of pressure depletion; the choice of datum for pressure referencing in a field development context is often a specific subsurface elevation (the datum structure depth or the fluid contact depth) rather than the surface KB elevation, requiring that all wellbore pressure measurements be corrected from the individual well KB datums to the common field datum before cross-well pressure comparison.
• Permanent datum documentation in well records and regulatory filings requires that the datum type, elevation, survey tie, and coordinate system be explicitly recorded and maintained throughout the well's life to support future engineers who may not have access to the original drillers who established the datum: in North America, well records filed with state and provincial regulators (Texas RRC, Alberta AER, Colorado OGCC) require that the KB elevation and ground level elevation be reported in feet above sea level referenced to the North American Vertical Datum of 1988 (NAVD 88) or an earlier predecessor datum (NGVD 29 in some jurisdictions), with the horizontal coordinates reported in the applicable state plane coordinate system or in geographic coordinates (latitude and longitude) referenced to the North American Datum of 1983 (NAD 83); the formal survey tie requires that the KB elevation be determined by a licensed surveyor using differential leveling or GPS surveying from a known benchmark (typically a USGS or NGS benchmark, or a nearby first- or second-order survey monument) rather than by the drilling contractor using uncalibrated instruments, because errors in the KB elevation propagate directly into all depth references derived from it and can create well-to-well depth discrepancies in geological correlations and reservoir modeling; in older wells drilled before GPS survey became standard, the KB elevation may have been measured by the drilling contractor using barometric altimetry or tape-and-transit methods, and these elevation values may carry uncertainties of 1 to 10 feet that must be recognized when correlating these wells with newer wells measured to centimeter accuracy by GPS.
• Offshore well datum conventions differ from onshore conventions in that the physical reference point at the wellhead changes with the water level (tidal variation) and with changes in the platform structure (subsidence, jack-up leg settlement), requiring that offshore well datums be defined with respect to a fixed reference that is independent of sea level variations: the Lowest Astronomical Tide (LAT) datum and Mean Sea Level (MSL) datum are commonly used for offshore well measurements in the North Sea and Gulf of Mexico, with the platform deck elevation above LAT or MSL recorded in the well completion report along with the air gap (the distance from mean sea level to the platform deck) and the water depth to mudline; for subsea wells (where the wellhead is on the seabed rather than on a platform), the datum is typically referenced to mean sea level with the mudline depth below sea level providing the correction from the surface datum to the wellhead elevation; in fields experiencing seafloor subsidence from reservoir compaction (such as the Ekofisk field in the Norwegian North Sea, which has subsided several meters since production began), the datum correction must account for the changing seafloor elevation over the producing life of the field to maintain consistent depth referencing across different vintages of well measurements.
• Permanent datum use in production logging and reservoir monitoring requires that the gauge depth in a production log (usually referenced to the tubing hanger or wellhead flange) be corrected to the KB or field datum before the log data is used for reservoir mapping or cross-well comparison: production logging tools (spinner flowmeters, gradiometers, distributed temperature sensing fiber) record their measurements as a function of measured depth in the wellbore from the surface, and the conversion from measured depth to true vertical depth and from the wellhead reference to the KB or sea level datum requires the well deviation survey (the directional survey that records the inclination and azimuth of the wellbore at intervals from surface to total depth) as well as the datum elevation correction; in horizontal wells where the wellbore may extend a mile or more horizontally while changing total vertical depth by only a few hundred feet, the conversion from measured depth to TVD and the datum correction are particularly important because a measurement reported as measured depth without datum correction appears at a very different location in the subsurface from the same measurement reported as TVD below datum; the integration of production log data into the reservoir simulation model requires that all downhole measurements be converted to the model's coordinate system (typically TVD below sea level or TVD below a field datum structure level) using the well's directional survey and datum elevation data, making the quality and documentation of the permanent datum a prerequisite for accurate reservoir model history matching.