Precision: Definition, Repeatability, and Logging Measurement Quality
What Is Precision?
Precision quantifies the closeness of agreement between repeated results from the same measurement procedure applied to identical material under prescribed conditions, per ISO 5725. In well logging, precision describes the repeatability of statistical measurements such as nuclear logs, and it is reported alongside accuracy and calibration records.
Key Takeaways
- Precision measures repeatability, the spread of results when the same test is run multiple times.
- Precision is distinct from accuracy, which measures closeness to the true value.
- Nuclear log precision degrades as logging speed rises and counting time per depth falls.
- ISO 5725 and API recommended practices govern precision reporting in oilfield service contracts.
- Operators worldwide require documented precision before accepting logs for reserves disclosure.
How Precision Works
Precision is expressed as a standard deviation, a coefficient of variation, or a 95% confidence interval around the mean of repeated measurements. ISO 5725 splits precision into repeatability, the variability obtained under the same operator and instrument within a short time, and reproducibility, the variability between different operators, instruments, or laboratories. API RP 86 applies these definitions to multiphase flow meter testing on production platforms.
For a nuclear log such as a density log or neutron log, precision is governed by counting statistics. The standard deviation of count rate is approximately the square root of the count rate, so doubling logging speed from 1,800 ft/hr (549 m/hr) to 3,600 ft/hr (1,097 m/hr) halves the counting time per depth and worsens precision by a factor of 1.41. Service companies publish precision specifications such as 0.015 g/cm³ at 1,800 ft/hr for the formation density measurement.
Precision Across International Jurisdictions
In Canada, AER Directive 017 sets precision and proving requirements for liquid hydrocarbon measurement at custody transfer, and AER Directive 047 covers gas measurement. Operators in the Montney and Athabasca document log precision in submissions to the AER for reserves under National Instrument 51-101. United States custody transfer follows API MPMS Chapter 4 and Chapter 21, with precision audited by independent measurement specialists.
Norwegian operators working under Sodir submit measurement plans aligned with NORSOK I-106 for fiscal metering on Johan Sverdrup, Troll, and Ekofisk. Australia's NOPSEMA accepts API and ISO precision frameworks for offshore production allocation in the Carnarvon Basin. Saudi Aramco SAES-J standards and ADNOC measurement specifications mirror API tolerances on Ghawar and Upper Zakum. The OIML R 117 standard governs precision in international fiscal metering wherever crude is sold across borders.
Fast Facts
A 0.25% precision improvement on a 200,000 bbl/d (31,800 m³/d) custody-transfer skid changes allocated revenue by roughly USD 14.6 million per year at USD 80/bbl, which is why North Sea operators retest fiscal meters every six months under Sodir oversight.
Precision in Nuclear and Acoustic Logs
Counting statistics dominate precision in gamma ray, density, neutron, and pulsed-neutron tools. Operators control precision by selecting logging speed, source strength, and stationary-station counting time. A repeat section of 60 m to 90 m (197 ft to 295 ft) is run at the end of every logged interval to demonstrate repeatability, with a typical density tolerance of 0.025 g/cm³ between main pass and repeat per Schlumberger and Halliburton field manuals.
Acoustic and resistivity tools are less statistical, so their precision specifications are tighter. Induction tools quote precision in millisiemens per metre at fixed temperatures. Sonic compressional slowness is typically repeatable within 1 microsecond per foot. Mismatched repeat sections are the first sign of tool eccentricity, mud invasion change, or instrument drift.
Tip: Always read the repeat section first when QC-ing a log. If the repeat agrees with the main pass within the published tolerance, the run is precise; if it does not, the precision specification has been violated and the log should be re-run before any reserves or completion decisions are made.
Precision Synonyms and Related Terminology
Precision is also known as:
- Repeatability — short-term precision under identical conditions
- Reproducibility — long-term precision across operators and instruments
- Random error — the statistical component that precision quantifies
Related terms: accuracy, calibration, well logging, density log
Frequently Asked Questions
What is the difference between precision and accuracy?
Precision describes the spread of repeated measurements around their own mean, while accuracy describes how close that mean lies to the true value. A measurement can be precise but inaccurate, for example a miscalibrated meter that reads consistently too high. ISO 5725 treats precision and trueness as separate components of total measurement uncertainty.
How does logging speed affect precision?
For nuclear logs, precision worsens with the square root of logging speed because counting time per depth falls proportionally. Doubling speed from 1,800 ft/hr (549 m/hr) to 3,600 ft/hr (1,097 m/hr) increases statistical noise by 41%. Operators trade precision for rig time, slowing down through pay zones and accelerating through shales where high precision is unnecessary.
Who sets precision standards in custody transfer?
API MPMS Chapter 4 and Chapter 21 govern most international custody-transfer measurement, with ISO 5168 and OIML R 117 also widely accepted. AER Directive 017 codifies these for Alberta, while NORSOK I-106 applies offshore Norway. Independent inspectors verify precision through proving runs at intervals defined in the contract.
Why Precision Matters in Oil and Gas
Precision underpins reserves disclosure, royalty calculation, fiscal metering, and completion design across every basin. A poorly repeatable nuclear log can shift reserves estimates by tens of millions of barrels; a drifting custody-transfer meter can divert millions of dollars between buyer and seller every month. Documented precision is the basis on which regulators in Canada, the United States, Norway, Australia, and the Middle East accept oilfield measurements as fit for commercial and statutory use.