Measure Point

Key Takeaways

• The bit-to-sensor distance (BSD) or bit-to-measure-point distance is the critical BHA geometry parameter that connects the survey record (which references the measure point depth) to the actual bit position (which is what the driller needs to know for geological steering decisions and landing zone control): the BSD is calculated by summing the lengths of all BHA components between the bit and the MWD sensor package — the bit (including bit shank), the sub-surface motor (if a mud motor is used), the near-bit stabilizer or sub, any collar subs between the motor and the MWD, and the distance within the MWD collar to the sensor package; typical BSD values in conventional rotary BHAs range from 10 to 30 meters (approximately 30 to 100 feet), while in motor assemblies with extended sections between the motor and the MWD collar, the BSD can be 15 to 45 meters (50 to 150 feet); the uncertainty in the bit position equals the uncertainty in the BSD, which is a function of the measurement accuracy of each component length — component lengths are measured during BHA inspection on the rig floor to within centimeter accuracy, but pipe stretch under tension and thermal expansion of the BHA components add additional uncertainty that is typically 0.1-0.5 meters in total; in geological landing zone operations where the horizontal well must be positioned within a 1-2 meter target window within the reservoir, minimizing the BSD by placing the MWD sensor as close to the bit as technically possible improves the positional certainty of the bit relative to the formation tops identified on the real-time LWD measurements.
• Survey station spacing (the depth interval between successive survey measurements taken at the measure point) determines the resolution of the wellbore trajectory description and affects the uncertainty in the calculated wellbore position: surveys are typically taken at every connection (every 30 feet or 9 meters in conventional drill pipe), at every single joint in critical directional sections, or continuously in some advanced MWD systems that record survey data at a specified depth interval while drilling; the calculated wellbore trajectory between survey stations is estimated by interpolation using a mathematical method (minimum curvature, balanced tangential, or average angle method), with the minimum curvature method being the industry standard because it assumes the wellbore follows a circular arc between stations (which is the physically correct approximation for a continuously curving drilled path) rather than the less accurate straight-line segments assumed by the tangential method; the positional error in the calculated trajectory grows with the survey station spacing because the interpolation error between stations grows with the distance being interpolated, particularly in sections with high dogleg severity (high rate of change of inclination and azimuth) where the actual wellbore path deviates most from the arc assumed by the minimum curvature interpolation; tight survey station spacing (single-joint surveys in critical sections) improves positional accuracy at the cost of increased rig time for connections and data processing.
• Measure point depth assignment in the survey record is a source of systematic error when the field engineer uses the drill pipe tally (the measured depth of the drill bit, inferred from pipe count and string weight) rather than the MWD sensor depth as the survey station depth: the drill pipe tally provides the bit depth (the depth of the bottommost component of the drill string), which exceeds the measure point depth by exactly the bit-to-sensor distance; if the driller reports the survey depth as the bit depth rather than the measure point depth, every survey station in the record is deeper than the true measure point position by the BSD, creating a systematic depth shift in the wellbore trajectory that displaces the calculated trajectory toward greater depth relative to the formation tops; this error propagates into all geological conclusions drawn from the survey (landing zone depth predictions, reservoir thickness estimates, and inter-well spacing calculations), and can cause a horizontal well to be drilled at the wrong depth within the target formation if the geological markers used for steering are correlated to the erroneous survey depth; standard well surveying practice requires that survey depth be explicitly identified as the measure point depth (not the bit depth) in the survey report, and that the BSD be documented and applied to convert measure point depths to bit depths for drilling engineering and geological steering purposes.
• Multi-station analysis (MSA) of MWD surveys uses the magnetic field measurements from multiple survey stations within a single wellbore section to improve azimuth accuracy beyond the single-station measurement accuracy, exploiting the fact that the earth's magnetic field is consistent (within local anomaly tolerances) across the multiple stations and any inconsistency between stations indicates measurement error or magnetic interference: the MSA algorithm processes the combined magnetic field vector measurements from all stations simultaneously, fitting the best-consistent azimuth solution across all stations subject to the constraint that the measured magnetic field magnitude and inclination angle must match the International Geomagnetic Reference Field (IGRF) values for the survey location; the MSA output includes a quality factor (QF) that indicates the consistency of the station set and the probable error in the calculated azimuth, with high-quality MSA surveys achieving azimuth accuracy of 0.1-0.5 degrees compared to 1-2 degrees for uncorrected single-station surveys; the MSA processing requires that the measure point depths be accurately known for all stations (because the depth-versus-field-vector plot is used to identify outlier stations affected by magnetic interference or measurement error), making accurate measure point depth recording a prerequisite for MSA quality control; erroneous measure point depths introduce false depth-field correlations in the MSA that can cause the algorithm to include or exclude stations incorrectly, degrading rather than improving the azimuth solution.
• In completion engineering, the measure point concept extends to the positioning of perforating guns, packers, and completion tools relative to geological markers in the wellbore, where the working position of each completion tool (the depth at which it will be set or activated) must be related to the tool's measure point (the specific reference location on the tool used to define its working depth) to correctly position the tool in the intended formation interval: perforating gun depth control uses the top-of-gun reference as the measure point, with the gun positioned using wireline or coiled tubing depth counters that measure line movement from a known reference depth (a casing collar log tag or a previously correlated gamma ray marker); the gun position uncertainty (the uncertainty in the relationship between the measured depth on the wireline depth counter and the actual formation depth opposite the gun charge face) is a combination of wireline depth measurement accuracy, wellbore deviation effects on wireline depth measurement, and the uncertainty in the gamma ray correlation used to identify the formation marker at the completion depth; completion intervals that are positioned more than 1-2 feet from the intended location relative to the target zone boundaries risk perforating into adjacent formations (a shale or non-reservoir sand) rather than the productive reservoir interval, with consequences for both productivity and zone isolation.