Improving the Quality of Ellipse of Uncertainty Calculations in Gyro Surveys to Reduce the Risk of Hazardous Events like Blow Outs or Missing Potential Production Through Incorrect Well Bore Placement


Wellbore position uncertainty estimation (error modeling) is a key element in safe and cost effective drilling. The quality of error modeling has improved a lot over the last 10 – 15 years through the publication of SPE papers like SPE-67616, 90408, 103734 and 105558. These papers taken together form a good framework for error modeling calculations with one exception. None of these papers supplies the drilling industry with realistic publicly documented uncertainty estimates for gyroscopic survey tools. It has been left to the gyro service providers themselves to supply the rest of the industry with the necessary gyro model inputs, mainly without any external review/audit process. This is not an ideal situation, and represents a potential safety hazard. The gyro service providers do not only compete on price, but also on survey accuracy. This paper is an attempt to start a process to close this safety gap by presenting the actual derivation of a brand new set of realistic uncertainty estimates for some of the existing gyroscopic tools based on real downhole data and guidelines given in the above mentioned SPE-papers.


The complexity of oil and gas well drilling has increased significantly over the last 100 years. Basic shallow vertical wells drilled in widely spaced grids have to a large extent been replaced by complicated wells such as extended reach, horizontal and multilateral wells, and so-called designer wells, which are often drilled out of closely placed template slots. This evolution would not have been possible without a broad range of technological improvements including the introduction of totally new disciplines such as directional surveying.

Directional surveying became a necessary drilling aid as soon as the drilling of deviated wells controlled through whip-stocks and depth dependent weight-on-bit programs became a reality. The whip-stock had to be mounted and oriented downhole, a process that required the measurement and display of its downhole horizontal orientation (azimuth); and the vertical orientation (inclination) had to be monitored to ensure correct tuning of weight-on-bit relative to the desired build/drop rates of the well trajectory. As a result of these demands, various instruments that utilize the Earth’s gravity, magnetic and spin fields to determine the inclination and azimuth at given locations within the wellbore were developed, and the directional surveying industry became a reality.

The main objective of the directional surveying industry has, over the years, changed from providing the drillers with inclination and azimuth information to supplying both drillers and reservoir scientists with 3D wellbore position data (north, east and vertical coordinates). This evolution has been driven by the simple fact that directional surveying instruments combined with pipe tally or wireline depth constitute a downhole 3D navigational system. Further, it would have been impossible to satisfy the operator’s resource and profit driven demand for more and more complicated wells and well clusters, without the availability of high quality wellbore position.