Wellbore surveying has advanced significantly from the breakthrough understandings on errors in the 1980s to the application of aerospace technology with advanced error modeling and correction.
Unfortunately, a huge gap exists between modern wellbore survey capability and the application of these tools and techniques for proper wellbore positioning and subsurface asset modeling.
This gap causes oil and gas companies to carry large unnecessary risk and realize significant loss of asset value throughout the field and well life cycle.
Inadequate Practices Are Too Common
Concerns that should seriously worry C-suite managers in oil and gas companies are numerous.
- Many oil companies lack the required wellbore position uncertainty knowledge in their drilling programs to reliably intercept with a relief well in the event of a blowout, despite their blowout contingency plan (BOCP). The consequences could result in operator bankruptcy.
- Inadequate wellbore position uncertainty from poor wellbore surveying practices negatively impacts subsurface models. Many times, this imposes “ghost” faults, creates unreliable wellbore-to-wellbore formation and pressure correlations, both of which negatively impact reservoir development and delivery (thus calculations of 1P, 2P, and 3P reserves).
- Unrealistic drilling expectations of trying to hit the “bull’s-eye” or small cross sections for drilling targets with wellbore position uncertainties that far exceed the target dimensions—a technically impossible task.
- Borehole surveying that focuses on the calculation of ellipsoids of position uncertainty for wellbore collision avoidance during drilling and ignoring the application of these uncertainty ellipsoids through the reservoir for geology and reservoir modeling. Furthermore, many companies do not address the measured depth errors of drillstrings, wirelines, and drilling techniques, thereby increasing uncertainty of true vertical depths (TVD) which are not actually “true.” These depth errors are generated from many sources including, but not limited to, along-hole drag, pipe/wireline stretch, internal pipe pressure, inappropriate survey spacing, and, especially in geothermal drilling, borehole temperature.
- Drilling engineers typically lack the depth of education to apply the appropriate tools and practices to achieve the required position uncertainties and need the support of subject-matter experts (SMEs). These experts exist in some major oil companies and are usually participants in the Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA), also known as the SPE Wellbore Position Technical Section (WPTS). Oil and gas companies without these experts must rely on SMEs in their wellbore surveyor suppliers, necessitating a knowledgeable buyer capability.
- Traditional wellbore survey tabulations passed from surveyors to drilling and onto subsurface traditionally show the lateral position and TVD to two decimal places. This is despite the uncertainty of these positions being in the order of tens of feet (or meters). This practice misleads the end users of the wellbore position data into believing they are mapping with unrealistic certainty and would have earned me a “F” grade on a university lab report.
Oilfield Wellbore Survey Development Is Advanced
In the 1970s, senior management at a major oil and gas company had a wake-up call when they were told that relief wells in the world’s largest high-pressure gas field located in Europe were “highly likely to be unsuccessful at intercepting a blowout well.” The field was fully resurveyed using best gyros with best running practices to replace poorly run and maintained magnetic single-shot surveys. The outcome included a total revision of the subsurface map of the reservoir, including the elimination of ghost faults and transformation of its 3D shape.
Measurement-while-drilling (MWD) tools entered the market in the second half of the 1970s with amazing mechanical-orientation technology and the first implementation of mud-pulse telemetry. This was quickly followed by solid-state measurements and logging while drilling, transforming directional drilling. This enabled faster directional drilling and steering more-complex borehole paths.
Using missile navigation, dynamically tuned gyros from the aerospace sector were introduced into wireline wellbore-survey tools in the 1980s. Today, multiple service companies have replaced them with solid-state gyro technology. These advanced gyros have enabled improved measurement accuracy and the establishment of gyros as part of the drillstring steering assembly.
In 1995, industry experts formed an organization to share knowledge of best practices and to develop and publish uncertainty (i.e., error) models. This organization, the ISCWSA, was later aliased to the SPE as the WPTS. The original uncertainty models produced in 1980 in SPE 9223 by Wolff and de Wardt have undergone significant development in terms of modeling techniques to match newer instrument accuracy, running procedures, and quality-control criteria.
Furthermore, ISCWSA has developed advanced guidance on running error mitigation such as sag corrections, multistation analysis, and infield referencing for more-accurate magnetic-declination corrections.
In addition, methods have been published to correct for along-hole depths for drillpipe and wireline depth measurements. This has become especially important with the growth in geothermal drilling, in which higher temperatures cause even greater depth errors and correlation difficulties between wellbore log data and borehole position in the reservoir.
Recommended practices for wellbore surveying and positioning are being released as API RP 78. This will provide a more-robust foundation for companies to adopt good (i.e., fit-for-purpose) practices and reduce risk while driving higher asset value.
Use Cases Reveal Extent of Inadequate Practices
Along the west coast of Africa, an operator was using inadequate practices including applying no corrections for the influence of bottomhole-assembly sag on inclination readings. My client retained the services of a well-known industry consultant who ran sag corrections through all their data. Injection of these revised calculated borehole positions into the subsurface model shifted data point TVDs such that the geologist realized correlation between wells without a vertical offset. He erased a ghost fault. The impact was huge as it eliminated the plan to drill an additional well to drain the reservoir across this ghost fault.
In offshore Asia, a simple review of the MWD-generated wellbore-survey tabulation provided by the survey company included the ISCWSA ellipsoid of uncertainty major, minor, and vertical axes. A discussion with the asset manager, a reservoir engineer by profession, made clear that his required vertical uncertainty was a magnitude less than that delivered by the MWD magnetic survey. This incongruity could be easily remedied by running an inertial grade gyro survey. Unfortunately, the wellbore of this long-reach appraisal well from a sixth-generation drillship had just been plugged back with cement plugs. Gone forever was the opportunity to significantly improve the correlation across widely dispersed wells, leaving subsurface modeling errors in place through the field-development planning cycle.
In multiple experiences, BOCP showed wellbore position uncertainty requirements for ranging to interception, which were ignored in developing the wellbore survey program, a very bad practice since it exposes the oil and gas company to significant reputation and company risk. This is strange since the BOCP position uncertainty requirement is usually similar to that for competent subsurface modeling. The same high-accuracy survey choices achieve both operator goals—one managing risk and the other driving value.
My interactions with academia have exposed a lack of recognition of the current state of wellbore-surveying technology and practices. In some cases, dated drilling books from the 1990s are being used as education sources for a profession that has advanced significantly since that time.
ISCWSA along with SPE WPTS have published two e-books that are updated with newest industry insights. These are freely available from the group’s website here. The ISCWSA also provides an online course which can be followed at a student’s own pace.
Dogleg severity (DLS) is no longer suited to quantify tortuosity in wellbores, cased and uncased. DLS was introduced to the industry in a 1961 landmark paper SPE 1543 by Lubinski to describe the twistiness of drilled boreholes to calculate torque and drag from drillstring to borehole-wall contact.
The DLS calculation typically used 90-ft spaced survey differences and is calculated per 100 ft. The current ability to perform continuous wellbore surveys and analyze the wellbore path in 1-ft increments has revealed significant tortuosity between the DLS stations.
A new computational method can show the effective diameter, typically smaller than the cased-well diameter, and in some cases significantly smaller or zero for the insertion of long tools. In multiple cases, this new method has revealed significant side loadings on downhole completion equipment placed in theoretically straight sections, especially electrical submersible pumps (ESPs). These unwanted side loadings cause premature ESP failure. This new tortuosity method can identify sections of borehole with low or zero side loadings for longer equipment life through best placement in the continuously surveyed wellbore.
The Future
Asset managers need to appreciate their loss of value and business risk from inadequate surveying and positioning practices. Subsurface geoscientists need to become educated such that they demand the wellbore position uncertainty they require for good modeling. Simply avoiding collisions with other wells is not adequate to meet the wellbore geological objectives.
Drilling engineers must access the resources that make them knowledgeable buyers of wellbore surveying, and educational institutions must upgrade their teaching to the highest level of available expertise.
Editors note: This article is based on an SPE Energy Stream webinar De Wardt delivered 14 July 2025, which is available to view here.
For Further Reading
SPE 9223 Borehole Position Uncertainty—Analysis of Measuring Methods and Derivation of Systematic Error Model by C.J.M. Wolff and J.P. de Wardt.
AADE Introduction to API RP 78, Wellbore Surveying and Positioning by J.D. Lightfoot, W. Tank, B. Coco, American Association of Drilling Engineers, 2023.
SPE 1543 Maximum Permissible Dog-Legs in Rotary Boreholes by Arthur Lubinski.
John de Wardt, SPE, is founder and president of De Wardt and Company, based in Elkhart Lake, Wisconsin. He provides management consulting services to upstream oil and gas and geothermal companies and previously held management and engineering positions with Halliburton, drilling contractor Forasol/Foramer, and Shell International.
De Wardt’s work experience spans more than 80 companies across 37 countries, significantly enhancing operational performance in wells, services, and manufacturing. He pioneered the application of Lean Drilling in 1995, which uses lean manufacturing principles in drilling, completion, and oilfield manufacturing.
De Wardt has published 35 SPE papers on drilling business models, drilling systems automation, well delivery process, drilling and completion performance, borehole position uncertainty. He holds a BS in mechanical engineering degree from the University of Newcastle upon Tyne in the UK, is a Chartered Engineer and Fellow of the Institution of Mechanical Engineers, and a Distinguished Member of SPE.
