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Directional survey errors
The following are some of the most common sources of error in directional drilling.
The survey instrument’s performance depends on the package design elements, calibration performance, and quality control during operation. System performance will functionally depend on the borehole inclination, azimuth, geomagnetic-field vector, and geographical position. Because of the dependency on sensing Earth’s spin rate, the performance of gyro compassing tools is inversely proportional to the cosine of the latitude of wellbore location. The sensor systems’ performance generally degrades as the inclination increases, especially in an east/west direction at higher latitudes. For magnetic tools, high latitudes result in weaker horizontal components of Earth’s field. For two-axis gyro tools, the approach to east/west at high inclinations places the sensor axes increasingly parallel to Earth’s spin. With magnetic tools, errors increase at high east/west inclinations because of the progressive difficulty in compensating for the effect of drillstring magnetism.
Gyros suffer from the additional problem of time-related drift uncertainty. The time component may be significant for gyro systems, particularly in horizontal wells and possibly in east/west orientation. The survey duration inevitably extends beyond the average survey-duration period. Long survey duration means larger drift uncertainty and more exposure to the wellbore environment, which may potentially reduce the accuracy of directional data.
The ability of the tool to freefall into the well will decrease substantially at approximately 60°. Consequently, gyro performance degrades at 60°, and most gyros cannot be used to survey at greater than 70°.
The misalignment of the survey instrument with the wellbore results in errors in measuring wellbore-axis direction and inclination. (Note: inclination and azimuth are affected.) Sources of this kind of error are detailed in Table 1. Sensor-to-instrument error is independent of inclination, which is an important variable for both instrument to drillstring/casing and drillstring to wellbore. Misalignments have long been recognized as significant error sources in directional surveying.
Measured depth (MD) error
Sources of depth error depend on the type of survey system used. Drillpipe-conveyed tools (measurement while drilling (MWD), multishots, and single-shot) suffer from errors in the physical measurement of drillpipes and the differential effects of drillstring compression and stretch. Because of wellbore friction, drillstring compression and stretch are not easily calculated, particularly in inclined wells. Depth errors can account for the relatively large angular errors frequently observed when comparing overlapping, high-accuracy surveys in deviated wells.
Wireline survey tools generally have smaller depth errors than drillstring-conveyed tools, provided adequate quality-control measures have been taken. Errors on the order of 1/1,000 for gyroscopic tools and 2/1,000 for drillstring tools are commonly quoted. However, this may not apply to horizontal-well situations.
Magnetic interference may be defined as corruption of the geomagnetic field by a field from an external source. This can cause serious errors in measuring hole direction (azimuth). Potential sources of magnetic interference are:
- Adjacent wells
- Casing shoes
- Magnetic formations
- “Hot spots” in nonmagnetic drill collars
Although all the previous error sources may compromise the magnetic survey’s quality, drillstring (axial) interference is probably the most common and frequent cause of errors in hole direction. The drillstrings may be regarded as a steel-bar, dipole magnet. The normal approach for magnetic survey tools is to place the survey sensor within sufficient quantity of nonmagnetic drill collars in the bottomhole assembly (BHA). Azimuth measurement errors are minimized by virtue of their distance from the interference source. Magnetic interference diminishes proportionally with the inverse of the square of the distance from the source. The bar-dipole-magnet analogy is simplistic. There is evidence that downhole drillstring magnetism may be much more complex, even dynamic in nature. In practice, it may be hard to remove interference completely. The magnitude of the effect of magnetic interference depends on the strength of the interference field as well as the inclination and direction of the wellbore and its geographical latitude. Highly deviated wells drilled in an east/west direction are likely to suffer greater magnetic-interference errors, especially in higher latitudes. 
There are several techniques to correct the effects of magnetic interference. These tend to be proprietary, but at least two are based upon a common hypothesis. The corrupted sensor measurements can be replaced with values calculated from a model of the local geomagnetic parameters, which allows azimuth estimation without interference errors. The techniques have been proved to be sound, in theory. In practice, the available geomagnetic models are imperfect, resulting in potentially significant errors in the calculated azimuth. If good geomagnetic-field information is available, then these correction routines can provide accurate azimuth data. In some cases, the hole direction’s (azimuth’s) accuracy has approached gyro quality.
This can arise from hot spots or from close proximity to magnetic elements in the drillstring. Cross-axial magnetic interference can cause significant survey errors, especially when the well being surveyed is in an east/west direction or approximately horizontal. A few companies have devised means for dealing with this type of interference, and at least one company combines this with an axial interference correction. These techniques also rely on knowing the magnitude of the local magnetic field and the dip angle. As with axial interference corrections, performance can be affected significantly by the imperfections in commonly available geomagnetic models.
Wellbore position error
The survey errors described previously must be translated into positional errors so that geoscientists can assess the impact of those errors on their understanding of the subsurface model and behavior of the well when on production. In extreme cases, these errors, if not recognized, can result in a well missing its target completely. Thus, the wellbore position error is a multidisciplinary problem and should be considered during well planning. Also, drillstring magnetic interference affects hole-direction measurements most severely at high inclinations when the well is traveling close to east or west. Planning the drainage of a field with wells oriented not along east or west greatly improves the accuracy of the directional MWD surveys.
To quantify the effects of the instrument errors described previously on bottomhole location, Walstrom et al. introduced the concept of survey uncertainty by generating 2D ellipses of uncertainty. An ellipse is used because the greatest survey errors are usually azimuth errors rather than inclination ones. The ellipse is expressed as an ellipsoid with the long axis at right angles to the wellbore direction. These calculations were based on the assumption that most survey errors were random. It later became evident that the calculated ellipses were too small and generally would not overlap.
In 1981, Wolff and de Wardt introduced an alternative method of determining wellbore uncertainty by suggesting that most survey errors were systematic rather than random. This method, or similar ones that also use systematic error sources, has become the accepted method of computing error source. While work was done in this area during the late 1980s, there was little standardization in computational technique. This caused many problems within the industry. To address these issues, a number of individuals created the Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA) in 1995. The committee’s objective is to produce and maintain standards relating to wellbore-survey accuracy for the industry. ISCWSA published a paper describing in detail how errors in sensor bias, scale factor, and misalignment propagate into errors in measured inclination and azimuth. Readers interested in survey accuracy and error models should contact ISCWSA for more information.
- Russell, A.W. and Roesler, R.F. 1985. Reduction of Nonmagnetic Drill Collar Length Through Magnetic Azimuth Correction Technique. Presented at the SPE/IADC Drilling Conference, New Orleans, Louisiana, 5-8 March. SPE-13476-MS. http://dx.doi.org/10.2118/13476-MS.
- Cheatham, C.A., Shih, S., Churchwell, D.L. et al. 1992. Effects of Magnetic Interference on Directional Surveys in Horizontal Wells. Presented at the SPE/IADC Drilling Conference, New Orleans, Louisiana, 18-21 February. SPE-23852-MS. http://dx.doi.org/10.2118/23852-MS.
- Russell, J.P., Shiells, G., and Kerridge, D.J. 1995. Reduction of Well-Bore Positional Uncertainty Through Application of a New Geomagnetic In-Field Referencing Technique. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 22-25 October. SPE-30452-MS. http://dx.doi.org/10.2118/30452-MS.
- Walstrom, J.E., Brown, A.A., and Harvey, R.P. 1969. An Analysis of Uncertainty in Directional Surveying. J Pet Technol 21 (4): 515-523. SPE-2181-PA. http://dx.doi.org/10.2118/2181-PA.
- Wolff, C.J.M. and de Wardt, J.P. 1981. Borehole Position Uncertainty - Analysis of Measuring Methods and Derivation of Systematic Error Model. SPE Journal of Petroleum Technology 33 (12): 2338-2350. SPE-9223-PA. http://dx.doi.org/10.2118/9223-PA.
- Williamson, H.S. 1999. Accuracy Prediction for Directional MWD. Presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, 3–6 October. SPE-56702-MS. http://dx.doi.org/10.2118/56702-MS.
Noteworthy papers in OnePetro
Williamson, H.S. 2000. Accuracy Prediction for Directional Measurement While Drilling. SPE Drilling & Completion 15 (4): 221-233. SPE-67616-PA. https://doi.org/10.2118/67616-PA.
Sawaryn, S.J. and Thorogood, J.L. 2005. A Compendium of Directional Calculations Based on the Minimum Curvature Method. SPE Drilling & Completion 20 (1): 24-36. SPE-84246-PA. https://doi.org/10.2118/84246-PA.
Torkildsen, T., Havardstein, S.T., Weston, J.L. and Ekseth, R. 2008. Prediction of Wellbore Position Accuracy When Surveyed With Gyroscopic Tools. SPE Drilling & Completion 23 (1): 5-12. SPE-90408-PA. https://doi.org/10.2118/90408-PA.
Ekseth, R., Torkildsen, T., Brooks, A.G., Weston, J.L., Nyrnes, E., Wilson, H.F. and Kovalenko, K. 2006. The Reliability Problem Related to Directional Survey Data. Presented at the IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition, 13-15 November, Bangkok, Thailand. SPE-103734-MS. https://doi.org/10.2118/103734-MS.
Ekseth, R., Torkildsen, T., Brooks, A.G., Weston, J.L., Nyrnes, E., Wilson, H.F. and Kovalenko, K. 2007. High Integrity Wellbore Surveys: Methods for Eliminating Gross Errors. Presented at the SPE/IADC Drilling Conference, 20-22 February, Amsterdam, The Netherlands. SPE-105558-MS. https://doi.org/10.2118/105558-MS.
Ekseth, R., Torkildsen, T., Brooks, A.G., Weston, J.L., Nyrnes, E., Wilson, H.F. and Kovalenko, K. 2010. High-Integrity Wellbore Surveying. SPE Drilling & Completion 25 (4): 438-447. SPE-133417-PA. https://doi.org/10.2118/133417-PA.
Ledroz, A., Bang, J. and Weston, J. 2016. New Instrument Performance Models for Combined Wellbore Surveys: A Move Toward Optimal Use of Survey Information. SPE Drilling & Completion 31 (4): 307-316. SPE-178826-PA. https://doi.org/10.2118/178826-PA.
Grindrod, S.J., Clark, P.J., Lightfoot, J.D., Bergstrom, N. and Grant, L. S. 2016. OWSG Standard Survey Tool Error Model Set for Improved Quality and Implementation in Directional Survey Management. Presented at the IADC/SPE Drilling Conference and Exhibition, 1-3 March, Fort Worth, Texas, USA. SPE-178843-MS. https://doi.org/10.2118/178843-MS.