Time-dependent current fluctuations in the Earth's ionosphere cause inaccuracies in wellbore directional surveying. These inaccuracies increase at higher latitudes, and although monitoring and correction are possible, they become less valid as the distance between the monitoring site and the rigsite increases, which is a particular problem for offshore drillsites. The characteristics of the ionosphere currents indicate that the most favorable location for monitoring stations is on the same geomagnetic latitude as the drillsite. Such an arrangement has been used to monitor and correct directional surveys at the Haltenbanken area of the Norwegian Sea over a period of approximately 2 years. Haltenbanken is approximately 200 km west of the Norwegian coast at latitude 65 N, where magnetic-storm activity can have a significant effect on directional surveying. A monitoring station was set up on the coast at the same geomagnetic latitude as Haltenbanken. To test the idea that magnetic disturbances are similar along constant magnetic latitude, an additional monitoring station was established 200 km east of the main station. The data broadly confirmed the hypothesis, although isolated events were observed when this was not the case. The challenges of surveying at offshore sites north of 62 N latitude are probably greater than the oil and gas industry is accustomed to-but such challenges will become more significant if the Arctic Ocean is opened to drilling operations. The technique described in this paper may contribute to safer and more-productive offshore operations at high latitudes.

Time-dependent current fluctuations in the Earth's ionosphere cause inaccuracies in wellbore directional surveying. These inaccuracies increase at higher latitudes and although monitoring and correction is possible, it becomes less valid as the distance between the monitoring site and the rigsite increases, a particular problem for offshore drill sites. The characteristics of the ionosphere currents indicate that the most favourable location for monitoring stations is on the same geomagnetic latitude as the drill site. Such an arrangement has been used to monitor and correct directional survey at the Haltenbanken area of the Norwegian Sea over a period of approximately two years. Haltenbanken is located about 200km west of the Norwegian coast at latitude 65° north, where magnetic storm activity can have a significant effect on directional surveying. A monitoring station was set up on the coast at the same geomagnetic latitude as Haltenbanken. To test the idea that magnetic disturbances are similar along constant magnetic latitude, an additional monitoring station was established 200km east of the main station. The data broadly confirmed the hypothesis, although isolated events were observed when this is not the case. The challenges of surveying at offshore sites north of 62° N latitude are probably greater than the oil and gas industry is accustomed to; challenges that will become more significant if the Arctic Ocean is opened to drilling operations. The technique described in this paper may contribute to safer and more productive offshore operations at high latitudes.

The authors are concerned that the claim of increased accuracy for MWD directional surveys processed using multistation analysis (MSA) may not always be valid, resulting in incorrect assumptions regarding the probability of well collision and target intersection. Very little information is available about the MSA methods currently in use. The only relevant published work identified by the authors is an SPE paper defining the minimum survey log conditions required to validate surveys against the ISCWSA MWD error model, but the requirements do not support assumptions of increased accuracy, post correction. It was therefore decided to determine if MSA can be used to reliably increase survey accuracy. The study also evaluated the validity of using the published minimum requirements to validate data against a typical MSA enhanced error model. The mathematical technique of consider covariance analysis was used to quantify post correction position uncertainty, and compare it to typical MSA error model assumptions. The results were validated using monte carlo simulation. The study showed that use of the published minimum data condition requirements to validate surveys against the increased accuracy claim typical of MSA error models is invalid. For the minimum requirements method to provide validation of such arbitrarily enhanced error models, new requirements must be defined specific to the MSA model. However, typical MSA error model assumptions are likely to result in requirements that can very rarely be met. Consider covariance analysis offers a reliable method of evaluating actual survey data against an arbitrarily enhanced error model on a case-by-case basis, which would allow more frequent use of the MSA error model.

After completing the drilling phase of the 8½″ section for a well in a giant mature field offshore Abu Dhabi, due to geomechanical challenges it was not possible to run the 7″ liner in a shale formation which was open for a long period of time due to rig repairs (top drive failure in open hole), exposing all reservoirs and compromising the field development strategy. After several unsuccessful attempts to run the liner and leaving a drilling BHA in the hole during one of the cleanout runs, it was decided to sidetrack around the fish to intersect the original 8½″ open hole section in order to recover the original hole and isolate the reservoir flow units from each other, which was critical for the field development since more than five reservoir layers were opened with water and oil bearings increasing the risk of damaging the reservoir integrity due to potential cross flow. Detailed measurement-while-drilling (MWD) survey analysis was conducted for the original hole in order to enhance surveys accuracy and minimize positional uncertainty. Typical survey management practices were implemented for Sag and Drilling String Interference; other techniques such as Dual Inclination, In-Field Referencing, and Multi Station Analysis were also applied. The implementation of these different survey management practices and their respective results are covered in detailed in the current article. Comprehensive planning was carried out, the sidetrack was accomplished and the original hole was successfully intersected at the first attempt. The advanced applied survey management techniques were crucial, particularly in the absence of magnetic ranging as the interval to intersect was open hole. The outcome of these corrections resulted in a shift of 8ft to the final well position, ensuring the correct direction and position for a successful attempt to intersect the well. This intersection was particularly challenging as the original hole had a 3D profile, thus it was critical to minimize both vertical and azimuthal uncertainties. Intersection was achieved with an RSS BHA, and the success of this intersection without magnetic ranging capability was only based on following a planned well trajectory that intersected the original hole surveys, clear validation of the accuracy of the surveys for both original and sidetrack holes. Achieving this challenging directional drilling goal allowed the completion of the well as per original plan, which was critical for the field development plan of these reservoirs. Based on the fact that there is very limited existing literature covering similar cases to the one presented, this current case represents a solid successful reference to be replicated in similar cases in the future covering these challenging applications of advanced survey management techniques.